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Mendel's Paper in English


Experiments in Plant Hybridization (1865)
by Gregor Mendel
Read at the meetings of February 8th, and March 8th, 1865

[1] Introductory Remarks
Experience of artificial fertilization, such as is effected with
ornamental plants in order to
obtain new
variations in color, has led to
the
experiments
which
will
here
be
discussed.
The
striking
regularity
with
which
the
same
hybrid
forms
always
reappeared
whenever
fertilization
took
place between the same species induced further experiments to be
undertaken, the object
of
which was
to
follow up the
developments of the
hybrids in their progeny.
To this object numerous careful observers, such as Kölreuter, Gärtner,
Herbert, Lecoq, Wichura and others, have devoted a part of their lives
with inexhaustible perseverance. Gärtner especially in his work
Die
Bastarderzeugung im Pflanzenreiche
, has recorded very valuable
observations; and quite recently Wichura published the results of some
profound
investigations
into
the
hybrids
of
the
Willow.
That, so
far,
no
generally applicable law governing the formation and development of
hybrids has been successfully formulated can hardly be wondered at by
anyone who is acquainted
with the extent
of the task,
and can appreciate
the difficulties with which experiments of this class have to contend.
A final decision can only be arrived at when we shall have before us the
results of detailed experiments make on plants belonging to the most
diverse orders.
Those who survey the work done in this department will arrive at the
conviction
that
among
all
the
numerous
experiments
made,
not
one
has
been
carried out to such an extent and in such a way as to make it possible
to determine the number of different forms under which the offspring of
the hybrids appear, or to arrange these forms with certainty according
to their separate generations, or definitely to ascertain their
statistical relations.
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It
requires
indeed
some
courage
to
undertake
a
labor
of
such
far-reaching
extent; this appears, however, to be the only right way by which we can
finally reach the solution of a question the importance of which cannot
be overestimated in connection with the history of the evolution of
organic forms.
The
paper
now
presented
records
the
results
of
such
a
detailed
experiment.
This experiment was practically confined to a small plant group, and is
now,
after
eight
years'
pursuit,
concluded
in
all
essentials.
Whether
the
plan upon which the separate experiments were conducted and carried out
was the best suited to attain the desired end is left to the friendly
decision of the reader.
[2] Selection of the Experimental Plants
The value and utility
of any experiment
are determined by
the fitness of
the material to the purpose for which it is used, and thus in the case
before
us
it
cannot
be
immaterial
what
plants
are
subjected
to
experiment
and in what manner such experiment is conducted.
The
selection
of
the
plant
group
which
shall
serve
for
experiments
of
this
kind must be made with all possible care if it be desired to avoid from
the outset every risk of questionable results.
The experimental plants must necessarily:
1.

Possess constant differentiating characteristics.
2.

The hybrids of such plants must, during the flowering period, be
protected from the influence of all foreign pollen, or be easily
capable of such protection.
The hybrids and their offspring should suffer no marked disturbance in
their fertility in the successive generations.
Accidental impregnation by foreign pollen, if it occurred during the
experiments and were not recognized, would lead to entirely erroneous
conclusions.
Reduced
fertility
or
entire
sterility
of
certain
forms,
such
as
occurs
in
the
offspring
of
many
hybrids,
would
render
the
experiments
very difficult or entirely frustrate them. In order to discover the
relations in which the hybrid forms stand towards each other and also
towards their progenitors it appears to be necessary that all member of
the series developed in each successive generations should be,
without
exception
, subjected to observation.
第

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At the very outset special attention was devoted to the
Leguminosae
on
account of their peculiar floral structure. Experiments which were made
with
several
members
of
this
family
led
to
the
result
that
the
genus
Pisum

was found to possess the necessary qualifications.
Some
thoroughly
distinct
forms
of
this
genus
possess
characters
which
are
constant, and easily and certainly recognizable, and when their hybrids
are mutually crossed they yield perfectly fertile progeny. Furthermore,
a disturbance through foreign pollen cannot easily occur, since the
fertilizing organs are closely packed inside the keel and the anthers
burst
within
the
bud,
so
that
the
stigma
becomes
covered
with
pollen
even
before the flower opens. This circumstance is especially important. As
additional advantages worth mentioning, there may be cited the easy
culture of these plants in the open ground and in pots, and also their
relatively
short
period
of
growth.
Artificial
fertilization
is
certainly
a
somewhat
elaborate
process,
but
nearly
always
succeeds.
For
this
purpose
the bud is opened
before it
is
perfectly developed, the keel is removed,
and
each
stamen
carefully
extracted
by
means
of
forceps,
after which
the
stigma can at once be dusted over with the foreign pollen.
In all, 34 more or less distinct varieties of Peas were obtained from
several seedsmen and subjected to a two year's trial. In the case of one
variety there were noticed, among a larger number of plants all alike,
a few forms which were markedly different. These, however, did not vary
in
the
following
year,
and
agreed
entirely
with
another
variety
obtained
from the same seedsman; the seeds were therefore doubtless merely
accidentally mixed. All the other varieties yielded perfectly constant
and
similar
offspring;
at
any
rate,
no
essential
difference
was
observed
during two trial years. For fertilization 22 of these were selected and
cultivated during the whole period of the experiments. They remained
constant without any exception.
Their systematic classification is difficult and uncertain. If we adopt
the strictest definition of a species, according to which only those
individuals belong to a species which under precisely the same
circumstances display precisely similar characters, no two of these
varieties could be referred to one species. According to the opinion of
experts,
however,
the
majority
belong
to
the
species
Pisum
sativum;
while
the
rest
are
regarded
and
classed,
some
as
sub-species
of
P.
sativum,
and
some as independent species, such as P. quadratum, P. saccharatum, and
P. umbellatum. The positions, however, which may be assigned to them in
a classificatory system are quite immaterial for the purposes of the
experiments
in
question.
It
has
so
far
been
found
to
be
just
as
impossible
to draw a sharp line between the hybrids of species and varieties as
between species and varieties themselves.
第

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[3] Division and Arrangement of the Experiments
If two plants which differ constantly in one or several characters be
crossed, numerous experiments have demonstrated that the common
characters are transmitted unchanged to the hybrids and their progeny;
but
each
pair
of
differentiating
characters,
on
the
other
hand,
unite
in
the hybrid to form a new character, which in the progeny of the hybrid
is usually variable. The object of the experiment was to observe these
variations in the case of each pair of differentiating characters, and
to
deduce
the
law
according
to
which
they
appear
in
successive
generations.
The experiment resolves itself therefore into just as many separate
experiments are there are constantly differentiating characters
presented in the experimental plants.
The various forms of Peas selected for crossing showed differences in
length and color of the stem; in the size and form of the leaves; in the
position,
color, size of
the flowers; in the length
of the flower
stalk;
in
the
color,
form,
and
size
of
the
pods;
in
the
form
and
size
of
the
seeds;
and in the color
of the
seed-coats
and of the albumen [cotyledons]. Some
of the characters noted
do not permit
of a
sharp and certain
separation,
since the difference is of a
difficult to define. Such characters could not be utilized for the
separate experiments; these could only be applied to characters which
stand out clearly and definitely in the plants. Lastly, the result must
show
whether
they,
in
their
entirety,
observe
a
regular
behavior
in
their
hybrid
unions,
and
whether
from
these
facts
any
conclusion
can
be
reached
regarding those characters which possess a subordinate significance in
the type.
The characters which were selected for experiment relate:
1.

To the
difference in the form of the ripe seeds.
These are either
round or roundish, the depressions, if any, occur on the surface,
being always only shallow; or they are irregularly angular and
deeply wrinkled (P. quadratum).
2.

To
the
difference
in
the
color
of
the
seed
albumen
(endosperm).

The
albumen
of
the
ripe
seeds
is
either
pale
yellow,
bright
yellow and
orange
colored,
or
it
possesses
a
more
or less
intense
green
tint.
This
difference
of
color
is
easily
seen
in
the
seeds
as
their
coats
are transparent.
3.

To
the
difference
in
the
color
of
the
seed-coat.

This
is
either
white,
with which character white flowers are constantly correlated; or
it is gray, gray-brown, leather-brown, with or without violet
spotting,
in
which
case
the
color
of
the
standards
is
violet,
that
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of the wings purple, and the stem in the axils of the leaves is of
a reddish tint. The gray seed-coats become dark brown in boiling
water.
4.

To the
difference in the form of the ripe pods.
These are either
simply inflated, not contracted in places; or they are deeply
constricted between the seeds and more or less wrinkled (P.
saccharatum).
5.

To the
difference in the color of the unripe pods.

They are either
light
to
dark
green,
or
vividly
yellow,
in
which
coloring
the
stalks,
leaf-veins, and calyx participate.*
6.

To the
difference in the position of the flowers.
They are either
axial, that is, distributed along the main stem; or they are
terminal,
that
is,
bunched
at
the
top
of
the
stem
and
arranged
almost
in a false umbel; in this case the upper part of the stem is more
or less widened in section (P. umbellatum).
7.

To
the
difference
in
the
length
of
the
stem.

The
length
of
the
stem
is
very
various
in
some
forms;
it
is,
however,
a
constant
character
for each, in so far that healthy plants, grown in the same soil,
are only subject to unimportant variations in this character. In
experiments
with
this
character,
in
order
to
be
able
to
discriminate
with
certainty,
the
long
axis
of
6
to
7
ft.
was
always
crossed
with
the short one of 3/4 ft. to 1 and 1/2 ft.
Each two of the differentiating characters enumerated above were united
by cross-fertilization. There were made for the







1st trial
2nd trial
3rd trial
4th trial
5th trial
6th trial
7th trial
60 fertilizations on 15 plants.
58 fertilizations on 10 plants.
35 fertilizations on 10 plants.
40 fertilizations on 10 plants.
23 fertilizations on 5 plants.
34 fertilizations on 10 plants.
37 fertilizations on 10 plants.
*One
species
possesses
a
beautifully
brownish-red
colored
pod,
which
when
ripening turns to violet and blue. Trials with this character were only
begun last year.

From
a
larger
number
of
plants
of
the
same
variety
only
the
most
vigorous
were chosen for fertilization. Weakly plants always afford uncertain
results,
because
even
in
the
first
generation
of
hybrids,
and still
more
so in the subsequent ones,
many
of the
offspring either entirely
fail to
flower or only form a few and inferior seeds.
第

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Furthermore, in all the experiments reciprocal crossings were effected
in such a way that each of the two varieties which in one set of
fertilizations served as seed-bearer in the other set was used as the
pollen plant.
The plants were grown in garden beds, a few also in pots, and were
maintained
in
their
natural
upright
position
by
means
of
sticks,
branches
of
trees,
and
strings
stretched
between.
For
each
experiment
a number
of
pot
plants
were
placed
during
the
blooming
period
in
a
greenhouse,
to
serve
as
control
plants
for
the
main
experiment
in
the
open
as
regards
possible
disturbance by insects. Among the insects which visit Peas the beetle
Bruchus
pisi

might
be
detrimental
to
the
experiments
should
it appear
in
numbers.
The
female
of
this
species
is
known
to
lay
the
eggs
in
the
flower,
and
in
so
doing
opens
the
keel;
upon
the
tarsi
of
one
specimen,
which
was
caught
in
a
flower,
some
pollen
grains
could
clearly
be
seen
under
a
lens.
Mention must also be made
of
a circumstance
which possibly might
lead to
the
introduction
of
foreign
pollen.
It
occurs,
for
instance,
in
some
rare
cases
that
certain
parts
of
an
otherwise
normally
developed
flower
wither,
resulting in a partial exposure of the fertilizing organs. A defective
development
of
the
keel
has
also
been
observed,
owing
to
which
the
stigma
and anthers remained partially covered. It also sometimes happens that
the pollen does not reach full perfection. In this event there occurs a
gradual lengthening of the pistil during the blooming period, until the
stigmatic tip protrudes at the point of the keel. This remarkable
appearance has also been observed in hybrids of Phaseolus and Lathyrus.
The
risk
of
false
impregnation
by
foreign
pollen
is,
however,
a
very
slight
one
with
Pisum,
and
is
quite
incapable
of disturbing
the
general
result.
Among more than 10,000 plants which were carefully examined there were
only
a
very
few
cases
where
an
indubitable
false
impregnation
had
occurred.
Since in the greenhouse such a case was never remarked, it may well be
supposed
that
Bruchus
pisi
,
and
possibly
also
the
described
abnormalities
in the floral structure, were to blame.
[4] The Forms of the Hybrids
Experiments
which
in
previous
years
were
made
with
ornamental
plants
have
already affording evidence that the hybrids, as a rule, are not exactly
intermediate
between
the
parental
species.
With
some
of
the
more
striking
characters,
those,
for
instance,
which
relate
to
the
form
and
size
of
the
leaves, the pubescence of the several parts, etc., the intermediate,
indeed, is nearly always to be seen; in other cases, however, one of the
two
parental
characters
is
so
preponderant
that
it
is
difficult,
or
quite
impossible, to detect the other in the hybrid.
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This is precisely the case with the Pea hybrids. In the case of each of
the
7
crosses
the
hybrid-character
resembles
that
of
one
of
the
parental
forms
so
closely
that
the
other
either
escapes
observation
completely
or
cannot be detected with certainty. This circumstance is of great
importance in the determination and classification of the forms under
which
the
offspring
of
the
hybrids
appear.
Henceforth
in
this
paper
those
characters which are transmitted entire, or almost unchanged in the
hybridization, and therefore in themselves constitute the characters of
the
hybrid,
are
termed
the
dominant
,
and
those
which
become
latent
in
the
process
recessive
.
The
expression

has
been
chosen
because
the
characters thereby designated withdraw or entirely disappear in the
hybrids, but nevertheless reappear unchanged in their progeny, as will
be demonstrated later on.
It was furthermore shown by the whole of the experiments that it is
perfectly immaterial whether the dominant character belongs to the seed
plant or to the pollen plant; the form of the hybrid remains identical
in
both
cases.
Thi
s
interesting
fact
was
also
emphasized
by
Gärtner,
with
the remark that even the most practiced expert is not in a position to
determine in a hybrid which of the two parental species was the seed or
the pollen plant.
Of
the
differentiating
characters
which
were
used
in
the
experiments
the
following are dominant:
1.

The round or roundish form of the seed with or without shallow
depressions.
2.

The yellow coloring of the seed albumen [cotyledons].
3.

The gray, gray- brown, or leather brown color of the seed-coat, in
association
with
violet-red
blossoms
and
reddish
spots
in
the
leaf
axils.
4.

The simply inflated form of the pod.
5.

The green coloring of the unripe pod in association with the same
color of the stems, the leaf-veins and the calyx.
6.

The distribution of the flowers along the stem.
7.

The greater length of stem.
With regard to this last character it must be stated that the longer of
the two parental stems is usually exceeded by the hybrid, a fact which
is
possibly
only
attributable
to
the
greater
luxuriance
which
appears
in
all
parts
of
plants
when
stems
of
very
different
lengths
are
crossed.
Thus,
for
instance,
in
repeated
experiments,
stems
of
1
ft.
and
6
ft.
in
length
yielded without exception hybrids which varied in length between 6 ft.
and 7 [and] 1/2 ft.
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The
hybrid
seeds
in
the
experiments
with
seed-coat
are
often
more
spotted,
and the spots sometimes coalesce into small bluish-violet patches. The
spotting also frequently appears even when it is absent as a parental
character.
The hybrid forms of the
seed-shape
and of the
[color of the] albumen
are
developed immediately after the artificial fertilization by the mere
influence of the foreign pollen. They can, therefore, be observed even
in
the
first
year
of
experiment,
whilst
all
the
other
characters
naturally
only
appear
in
the
following
year
in
such
plants
as
have
been
raised
from
the crossed seed.
[5] The First Generation From the Hybrids
In
this
generation
there
reappear,
together
with
the
dominant

characters,
also
the
recessive
ones
with
their
peculiarities
fully
developed,
and
this
occurs in the definitely expressed average proportion of 3:1, so that
among each 4 plants of this generation 3 display the dominant character
and one the recessive. This relates without exception to all the
characters which were investigated in the experiments. The angular
wrinkled
form
of
the
seed,
the
green
color
of
the
albumen,
the
while
color
of the seed-coats and the flowers, the constrictions of the pods, the
yellow color of the unripe pod, of the stalk, of the calyx, and of the
leaf
venation,
the
umbel-like
form
of
the
inflorescence,
and
the
dwarfed
stem, all reappear in the numerical proportion given, without any
essential alteration.
Transitional forms were not observed in any
experiment.

Since
the
hybrids
resulting
from
reciprocal
crosses
are
formed
alike
and
present no appreciable difference in their subsequent development,
consequently these results can be reckoned together in each experiment.
The
relative
numbers
which
were
obtained
for
each
pair
of
differentiating
characters are as follows:
Expt.
1.
Form
of
seed.
--
From
253
hybrids
7324
seeds
were
obtained
in the second trial year. Among them were 5474 round or roundish
ones
and
1850
angular
wrinkled
ones.
Therefrom
the
ratio
2.96:1
is
deduced.


Expt. 2. Color of albumen. -- 258 plants yielded 8023 seeds, 6022
yellow, and 2001 green; their ratio, therefore, is as 3.01:1.


In these two experiments
each pod yielded usually
both kinds of
seed. In
well-developed
pods
which
contained
on
the
average
6
to
9
seeds,
it
often
happened
that
all
the
seeds
were
round
(Expt.
1)
or
all
yellow
(Expt.
2);
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on
the
other
hand
there
were
never
observed
more
than
5
wrinkled
or
5
green
ones on one pod. It appears to make no difference whether the pods are
developed early or later in the hybrid or whether they spring from the
main axis or from a lateral one. In some few plants only a few seeds
developed in the first formed pods, and these possessed exclusively one
of
the
two
characters,
but
in
the
subsequently
developed
pods
the
normal
proportions were maintained nevertheless.
As in separate pods, so did the distribution of the characters vary in
separate
plants.
By
way
of
illustration
the
first
10
individuals
from
both
series of experiments may serve.

Experiment 1


Experiment 2

Form of Seed

Color of Albumen
Plants Round Angular
Yellow Green

1 45 12 25 11

2 27 8

32 7

3 24 7

14 5

4 19 10

70 27

5 32 11

24 13

6 26 6

20 6

7 88 24

32 13

8 22 10 44 9

9 28 6

50 14
10 25 7

44 18
As extremes in the
distribution of
the
two seed characters
in one plant,
there
were
observed
in
Expt.
1
an
instance
of
43
round
and
only
2
angular,
and
another
of
14
round
and
15
angular
seeds.
In
Expt.
2
there was
a
case
of
32
yellow
and
only
1
green
seed,
but
also
one
of
20
yellow
and
19
green.
These
two
experiments
are
important
for
the
determination
of
the
average
ratios, because with a smaller number of experimental plants they show
that
very
considerable
fluctuations
may
occur.
In
counting
the
seeds,
also,
especially
in
Expt.
2,
some
care
is
requisite,
since
in
some
of
the
seeds
of many plants the green color of the albumen is less developed, and at
first may be easily overlooked. The cause of this partial disappearance
of
the
green
coloring
has
no
connection
with
the
hybrid-character
of
the
plants, as it likewise occurs in the parental variety. This peculiarity
is
also
confined
to
the
individual
and
is
not
inherited
by
the
offspring.
In
luxuriant
plants
this
appearance
was
frequently
noted.
Seeds
which
are
damaged
by
insects
during
their
development
often
vary
in
color
and
form,
but with a little practice in sorting, errors are easily avoided. It is
almost
superfluous
to
mention
that
the
pods
must
remain
on
the
plants
until
they are thoroughly ripened
and have become
dried, since it
is only then
that the shape and color of the seed are fully developed.
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









Expt. 3. Color of the seed-coats. -- Among 929 plants, 705 bore
violet-red
flowers
and
gray-brown
seed-coats;
224
had
white
flowers
and white seed- coats, giving the proportion 3.15:1.
Expt. 4. Form of pods. -- Of 1181 plants, 882 had them simply
inflated, and in 299 they were constricted. Resulting ratio,
2.95:1.
Expt. 5. Color of the unripe pods. -- The number of trial plants
was 580, of which 428 had green pods and 152 yellow ones.
Consequently these stand in the ratio of 2.82:1.
Expt. 6. Position of flowers. -- Among 858 cases 651 had
inflorescences axial and 207 terminal. Ratio, 3.14:1.
Expt. 7. Length of stem. -- Out of 1064 plants, in 787 cases the
stem was long, and in 277 short. Hence a mutual ratio of 2.84:1.
In this experiment the dwarfed plants were carefully lifted and
transferred to a special bed. This precaution was necessary, as
otherwise
they
would
have
perished
through
being
overgrown
by
their
tall
relatives.
Even
in
their
quite
young state
they
can
be
easily
picked out by their compact growth and thick dark-green foliage.
If now the results of the whole of the experiments be brought together,
there is found, as between the number of forms with the dominant and
recessive characters, an average ratio of 2.98:1, or 3:1.
The
dominant
character
can
have
here
a
double
signification
;
namely,
that
of a parental character, or a hybrid- character. In which of the two
significations it appears in each separate case can only be determined
by the following generation. As a parental character it must pass over
unchanged to the whole of the offspring; as a hybrid- character, on the
other
hand,
it
must
maintain
the
same
behavior
as
in
the
first
generation.
[6] The Second Generation From the Hybrids
Those
forms
which
in
the
first
generation
exhibit
the
recessive
character
do not further vary in the second generation as regards this character;
they remain constant in their offspring.
It is otherwise with those which possess the dominant character in the
first generation. Of these
two
-thirds yield offspring which display the
dominant and recessive characters in the proportion of 3:1, and thereby
show exactly the same ratio as the hybrid forms, while only
one
-third
remains with the dominant character constant.
The separate experiments yielded the following results:
第

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Expt.
1.
Among
565
plants
which
were
raised
from
round
seeds
of
the
first generation, 193 yielded round seeds only, and remained
therefore
constant
in
this
character;
372,
however,
gave
both
round
and wrinkled seeds, in the proportion of 3:1. The number of the
hybrids, therefore, as compared with the constants is 1.93:1.


Expt. 2. Of 519 plants which were raised from seeds whose albumen
was
of
yellow
color
in
the
first
generation,
166
yielded
exclusively
yellow,
while
353
yielded
yellow
and
green
seeds
in
the
proportion
of 3:1. There resulted, therefore, a division into hybrid and
constant forms in the proportion of 2.13:1.


For each separate trial in the following experiments 100 plants were
selected
which
displayed
the
dominant
character
in
the
first
generation,
and
in
order
to
ascertain
the
significance
of
this,
ten
seeds
of
each
were
cultivated.










Expt.
3.
The
offspring
of
36
plants
yielded
exclusively
gray-brown
seed-coats,
while
of
the
offspring
of
64
plants
some
had
gray-brown
and some had white.
Expt.
4.
The
offspring
of
29
plants
had
only
simply
inflated
pods;
of the offspring of 71, on the other hand, some had inflated and
some constricted.
Expt. 5. The offspring of 40 plants had only green pods; of the
offspring of 60 plants some had green, some yellow ones.
Expt. 6.
The
offspring of
33
plants had only axial flowers; of the
offspring
of
67,
on
the
other
hand,
some
had
axial
and
some
terminal
flowers.
Expt.
7.
The
offspring
of
28
plants
inherited
the
long
axis,
of
those
of 72 plants some the long and some the short axis.
In
each
of
these
experiments
a
certain
number
of
the
plants
came
constant
with the dominant character. For the determination of the proportion in
which
the
separation
of
the
forms
with
the
constantly
persistent
character
results, the two first experiments are especially important, since in
these a larger number of plants can be compared. The ratios 1.93:1 and
2.13:1 gave together almost exactly the average ratio of 2:1. The sixth
experiment
gave
a
quite
concordant
results;
in
the
others
the
ratio
varies
more or less, as was only to be expected in view of the smaller number
of 100 trial plants. Experiment 5, which shows the greatest departure,
was
repeated,
and
then
in
lieu
of
the
ratio
of
60:40,
that
of
65:35
resulted.
The
average
ratio
of 2:1
appears,
therefore,
as
fixed
with
certainty.

It
is
therefore
demonstrated
that,
of
those
forms
which
posses
the
dominant
character
in
the
first
generation,
two-thirds
have
the
hybrid-character,
while one-third remains constant with the dominant character.
第

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The
ratio
of
3:1,
in
accordance
with
which
the
distribution
of
the
dominant
and
recessive
characters
results
in
the
first
generation,
resolves
itself
therefore
in all experiments into the ratio of 2:1:1,
if the dominant
character be differentiated according to its significance as a
hybrid-character or as a parental one. Since the members of the first
generation spring directly
from
the seed of the
hybrids,
it is now clear
that
the
hybrids
form
seeds
having
one
or
other
of
the
two
differentiating
characters, and of these one-half develop again the hybrid form, while
the
other
half
yield
plants
which
remain
constant
and
receive
the
dominant
or the recessive characters in equal numbers.

[7] The Subsequent Generations From the Hybrids
The
proportions
in
which
the
descendants
of
the
hybrids
develop
and
split
up in the first and second generations presumably hold good for all
subsequent
progeny.
Experiments
1
and
2
have
already
been
carried
through
6 generations, 3 and 7 through 5, and 4, 5, and 6 through 4, these
experiments
being
continued
from
the
third
generation
with
a
small
number
of plants, and no departure from the rule has been perceptible. The
offspring
of
the
hybrids
separated
in
each
generation
in
the
ratio
of
2:1:1
into hybrids and constant forms.
If
A

be
taken
as
denoting
one
of
the
two
constant
characters,
for
instance
the dominant,
a
the recessive, and
Aa
the hybrid form in which both are
conjoined, the expression



A + 2Aa + a

shows the terms in the series for the progeny of the hybrids of two
differentiating characters.
The
observation
made
by
Gärtner,
Kölreuter,
and
others,
that
hybrids
are
inclined to revert to the parental forms, is also confirmed by the
experiments described. It is seen that the number of the hybrids which
arise
from
one
fertilization,
as
compared
with
the
number
of
forms
which
become constant, and their progeny from generation to generation, is
continually diminishing, but that nevertheless they could not entirely
disappear. If an average equality of fertility in all plants in all
generations be assumed, and if, furthermore, each hybrid forms seed of
which
one-half
yields
hybrids
again,
while
the
other
half
is
constant
to
both characters in equal proportions, the ratio of numbers for the
offspring in each generation is seen by the following summary, in which
A

and
a

denote
again
the
two
parental
characters,
and
Aa

the
hybrid
forms.
For brevity's sake it may be assumed that each plant in each generation
furnishes only 4 seeds.
第

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Ratios
Generation
A Aa a
A : Aa : a
----------------------------------------- -----------
1

1 2 1
1 : 2 : 1
2
6 4 6
3 : 2 : 3
3
28 8 28 7 : 2 : 7
4 120 16 120 15 : 2 : 15
5 496 32 496 31 : 2 : 31
.

..........
........




n n
n



2 - 1 : 2 : 2 - 1
In the tenth generation, for instance, 2^
n
- 1 = 1023. There result,
therefore, in each 2048 plants which arise in this generation 1023 with
the constant dominant character, 1023 with the recessive character, and
only two hybrids.
[8] The Offspring of Hybrids in Which Several Differentiating
Characters are Associated.
In the experiments above described plants were used which differed only
on one essential character. The next task consisted in ascertaining
whether the law of development discovered in these applied to each pair
of
differentiating
characters
when
several
diverse
characters
are
united
in the hybrid by crossing. As regards the form of the hybrids in these
cases,
the
experiments
showed
throughout
that
this
invariably
more
nearly
approaches to that one of the two parental plants which possesses the
greater number of dominant characters. If, for instance, the seed plant
has a short stem, terminal white flowers, and simply inflated pods; the
pollen plant, on the other hand, a long stem, violet-red flowers
distributed along the stem, and constricted pods; the hybrid resembles
the seed parent only in the form of the pod; in the other characters it
agrees
with
the
pollen
parent.
Should
one
of
the
two
parental
types
possess
only dominant characters, then the hybrid is scarcely or not at all
distinguishable from it.
Two experiments were made with a considerable number of plants. In the
first
experiment
the
parental
plants
differed
in
the
form
of
the
seed
and
in the color of the albumen; in the second in the form of the seed, in
the
color
of
the
albumen,
and
in
the
color
of
the
seed-coats.
Experiments
with
seed
characters
give
the
result
in
the
simplest
and
most
certain
way.
第

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In order to facilitate study of the data in these experiments, the
different
characters
of
the
seed
plant
will
be
indicated
by
A
,
B
,
C
,
those
of the pollen plant by
a
,
b
,
c
, and the hybrid forms of the characters
by
Aa
,
Bb
, and
Cc
.
First Experiment:
AB
Seed parents,
abc
Pollen parents,



A
form round
a
form wrinkled



B
albumen yellow
b
albumen green
The fertilized seeds appeared round and yellow like those of the seed
parents. The plants raised therefrom yielded seeds of four sorts, which
frequently
presented
themselves
in
one
pod.
In
all,
556
seeds
were
yielded
by 15 plants, and of these there were:
315 round and yellow,


101 wrinkled and yellow,


108 round and green,


32 wrinkled and green.


All were sown the following year. 11 of the round yellow seeds did not
yield plants, and 3 plants did not form seeds. Among the rest:
38 had round yellow seeds ........
AB



65 round yellow and green seeds..........
ABb



60 round yellow and wrinkled yellow seeds........
AaB



138 round yellow and green, wrinkled yellow
and green seeds...... .....
AaBb



From the wrinkled yellow seeds 96 resulting plants bore seed, of which:
28 had only wrinkled yellow seeds................
aB



68 wrinkled yellow and green seeds .............
aBb



From 108 round green seeds 102 resulting plants fruited, of which:
35 had only round green seeds ...............
Ab



67 round and wrinkled green seeds ..........
Aab



The wrinkled green seeds yielded 30 plants which bore seeds all of like
character; they remained constant
ab
.
The
offspring
of
the
hybrids
appeared
therefore
under
9
different
forms,
some of them in very unequal numbers. When these are collected and
coordinated we find:
第

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页



38 plants with the sign AB


35


28


30


65


68


60


67

138
The
whole
of
the
forms
may
be
classed
into
3
essentially
different
groups.
The
first
includes
those
with
the
signs
AB
,
Ab
,
aB
,
and
ab

:
they
possess
only constant characters and do not vary again in the next generation.
Each of these forms is represented on the average 33 times. The second
group includes the signs
ABb
,
aBb
,
AaB
,
Aab
: these are constant in one
character and hybrid in another,
and vary in the next
generation only as
regards the hybrid-character. Each of these appears on any average 65
times. The form
AaBb

occurs 138 times : it is hybrid in both characters,
and behaves exactly as do the hybrids from which it is derived.
If the numbers in which the forms belonging to these classes appear be
compared, the ratios of 1:2:4 are unmistakably evident. The numbers 33,
65, 138 present very fair
approximations to the
ratio numbers of 33,
66,
132.
The development series consists, therefore, of 9 classes, of which 4
appear
therein
always
once
and
are
constant
in
both
characters;
the
forms
AB
,
ab
,
resemble
the
parental
forms,
the
two
others
present
combinations
between the conjoined characters
A
,
a
,
B
,
b
, which combinations are
likewise possibly constant. Four classes appear always twice, and are
constant
in
one
character
and
hybrid
in
the
other.
One
class
appears
four
times, and is hybrid in both characters. Consequently, the offspring of
the hybrids, if two kinds of differentiating characters are combined
therein, are represented by the expression

AB + Ab + aB + ab + 2ABb + 2aBb + 2AaB + 2Aab + 4AaBb

This expression is indisputably a combination series in which the two
expressions for the characters
A
and
a
,
B
and
b
are combined. We arrive
at
the
full
number
of
the
classes
of
the
series
by
the
combination
of
the
expressions:



A + 2Aa + a




B + 2Bb + b

Second Experiment:
ABC
Seed parents,


abc
Pollen parents,



A
form round
a
form wrinkled



B
albumen yellow
b
albumen green



C
seed-coat gray-brown
c
seed-coat white
第

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This experiment was made in precisely the same way as the previous one.
Among all the experiments
it demanded the most
time and trouble.
From 24
hybrids 687 seeds were obtained in all: these were all either spotted,
gray-brown
or
gray-green,
round
or
wrinkled.
From
these
in
the
following
year
639
plants
fruited,
and
as
further
investigation
showed,
there
were
among them:


8 plants ABC
22 plants ABCc
45 plants ABbCc

14
17

9
25

11
20
40

8
15

10
18
48

10
19

7
24



14
78



18



20



16
The whole expression contains 27 terms. Of these 8 are constant in all
characters, and each appears on the
average 10 times; 12 are constant in
two characters, and hybrid in the third; each appears on the average 19
times; 6 are constant in one character and hybrid in the other two; each
appears on the average 43 times. One form appears 78 times and is hybrid
in all of the characters. The ratios 10:19:43:78 agree so closely with
the
ratios
10:20:40:80,
or
1:2:4:8
that
this
last
undoubtedly
represents
the true value.
The development of the hybrids when the original parents differ in 3
characters results therefore according to the following expression:

ABC + ABc + AbC + Abc + aBC + aBc + abC + abc +

2ABCc + 2AbCc + 2aBCc + 2abCc + 2ABbC + 2ABbc +

2aBbC + 2aBbc + 2AaBC + 2AaBc + 2AabC + 2Aabc +

4ABbCc + 4aBbCc + 4AaBCc + 4AabCc + 4AaBbC +

4AaBbc + 8AaBbCc.
Here also is involved a combination series in which the expressions for
the characters
A
and
a
,
B
and
b
,
C
and
c
, are united. The expressions



A + 2Aa + a




B + 2Bb + b




C + 2Cc + c

give
all
the
classes
of
the
series.
The
constant
combinations
which
occur
therein agree with all combinations which are possible between the
第

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页

characters
A
,
B
,
C
,
a
,
b
,
c
; two thereof,
ABC
and
abc
, resemble the two
original parental stocks.
In addition, further experiments were made with a smaller number of
experimental
plants
in
which
the
remaining
characters
by
twos
and
threes
were
united
as
hybrids:
all
yielded
approximately
the
same
results.
There
is therefore no doubt that for the whole of the characters involved in
the experiments the principle applies
that the offspring of the hybrids
in which several essentially different characters are combined exhibit
the
terms
of
a
series
of
combinations,
in which
the
developmental
series
for
each
pair
of
differentiating
characters
are
united.

It
is
demonstrated
at the same time that
the relation of each pair of different characters
in
hybrid
union
is
independent
of
the
other
differences
in
the
two
original
parental stocks
.
If
n
represent the number of the differentiating characters in the two
original
stocks,
3^
n

gives
the
number
of
terms
of
the
combination
series,
4^
n
the number of individuals which belong to the series, and 2^
n
the
number of unions which remain constant. The series therefore contains,
if the original stocks differ in four characters, 3^
4
= 81 classes, 4^
4

= 256 individuals, and 2^
4
= 16 constant forms: or, which is the same,
among each 256 offspring of the hybrids are 81 different combinations,
16 of which are constant.
All constant combinations which in Peas are possible by the combination
of the said 7 differentiating characters were actually obtained by
repeated crossing. Their number is given by 2^
7
= 128. Thereby is
simultaneously given the practical proof
that the constant characters
which
appear
in
the
several
varieties
of
a
group
of
plants
may
be
obtained
in all the associations which are possible according to the laws of
combination, by means of repeated artificial fertilization.

As
regards
the
flowering
time
of
the
hybrids,
the
experiments
are
not
yet
concluded.
It
can,
however,
already
be
stated
that
the
time
stands
almost
exactly between those of the seed and pollen parents, and that the
constitution of the hybrids with respect to this character probably
follows
the
rule
ascertained
in
the
case
of
the
other
characters.
The
forms
which are selected for experiments of this class must have a difference
of at least 20 days from the middle flowering period of one to that of
the
other;
furthermore,
the
seeds
when
sown
must
all
be
placed
at
the
same
depth in the earth, so that they may germinate simultaneously. Also,
during the whole flowering period, the more important variations in
temperature must be taken into account, and the partial hastening or
delaying of the flowering which may result therefrom. It is clear that
第

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页

this experiment presents many difficulties to be overcome and
necessitates great attention.
If
we
endeavor
to
collate
in
a
brief
form
the
results
arrived
at,
we
find
that those differentiating characters, which admit of easy and certain
recognition
in
the
experimental
plants,
all
behave
exactly
alike
in
their
hybrid associations.
The offspring of the hybrids of each pair of
differentiating characters are, one-half, hybrid again, while the other
half
are
constant
in equal
proportions
having
the
characters
of
the
seed
and pollen parents respectively. If several differentiating characters
are
combined
by
cross-fertilization
in
a
hybrid,
the
resulting
offspring
form the terms of a combination series in which the combination series
for each pair of differentiating characters are united.
The
uniformity
of
behavior
shown
by
the
whole
of
the
characters
submitted
to experiment permits, and fully justifies, the acceptance of the
principle that a similar relation exists in the other characters which
appear
less
sharply
defined
in
plants,
and
therefore
could
not
be
included
in the separate experiments. An experiment with peduncles of different
lengths gave on the whole a fairly satisfactory results, although the
differentiation
and
serial
arrangement
of
the
forms
could
not
be
effected
with that certainty which is indispensable for correct experiment.
[9] The Reproductive Cells of the Hybrids
The results of the previously described experiments led to further
experiments,
the
results
of
which
appear
fitted
to
afford
some
conclusions
as regards the composition of the egg and pollen cells of hybrids. An
important clue is afforded in
Pisum
by the circumstance that among the
progeny
of
the
hybrids
constant
forms
appear,
and
that
this
occurs,
too,
in respect of all combinations of the associated characters. So far as
experience
goes,
we
find
it
in
every
case
confirmed
that
constant
progeny
can only be formed when the egg cells and the fertilizing pollen are of
like
character,
so
that
both
are
provided
with
the
material
for
creating
quite similar individuals, as is the case with the normal fertilization
of pure species. We must therefore regard it as certain that exactly
similar factors must be at work also in the production of the constant
forms
in
the
hybrid
plants.
Since
the
various
constant
forms
are
produced
in
one
plant, or even in
one
flower of a plant, the conclusion appears
logical
that
in
the
ovaries
of
the
hybrids
there
are
formed
as
many
sorts
of egg cells, and in the anthers as many sorts of pollen cells, as there
are possible constant combination forms, and that these egg and pollen
cells agree in their internal compositions with those of the separate
forms.
第

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In point of fact it is possible to demonstrate theoretically that this
hypothesis would fully suffice to account for the development of the
hybrids in the separate
generations, if we
might at the
same time assume
that
the
various
kinds
of
egg
and
pollen
cells
were
formed
in
the
hybrids
on the average in equal numbers.
In order to bring these assumptions to an experimental proof, the
following experiments were designed. Two forms which were constantly
different
in
the
form
of
the
seed
and
the
color
of
the
albumen
were
united
by fertilization.
If the differentiating characters are again indicated as
A
,
B
,
a
,
b
, we
have:

AB
Seed parents,
ab
Pollen parents,



A
form round
a
form wrinkled



B
albumen yellow
b
albumen green


The artificially fertilized seeds were sown together with several seeds
of both original stocks, and the most vigorous examples were chosen for
the reciprocal crossing. There were fertilized:
1.

The hybrids with the pollen of
AB

2.

The hybrids with the pollen of
ab

3.

AB
with the pollen of the hybrids.
4.

ab
with the pollen of the hybrids.
For
each
of
these
4
experiments
the
whole
of
the
flowers
on
3
plants
were
fertilized. If the above theory be correct, there must be developed on
the
hybrids
egg
and
pollen
cells
of
the
forms
AB
,
Ab
,
aB
,
ab
,
and
there
would
be combined:
1.

The egg cells
AB
,
Ab
,
aB
,
ab
with the pollen cells
AB
.
2.

The egg cells
AB
,
Ab
,
aB
,
ab
with the pollen cells
ab
.
3.

The egg cells
AB
with the pollen cells
AB
,
Ab
,
aB
, and
ab
.
4.

The egg cells
ab
with the pollen cells
AB
,
Ab
,
aB
, and
ab
.
From
each
of
these
experiments
there
could
then
result
only
the
following
forms:
1.

AB
,
ABb
,
AaB
,
AaBb

2.

AaBb
,
Aab
,
aBb
,
ab

3.

AB
,
ABb
,
AaB
,
AaBb

4.

AaBb
,
Aab
,
aBb
,
ab

第

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页

If, furthermore, the several forms of the egg and pollen cells of the
hybrids were produced on an average in equal numbers, then in each
experiment
the
said
4
combinations
should
stand
in
the
same
ratio
to
each
other. A perfect agreement in the numerical relations was, however, not
to be expected since in each fertilization, even in normal cases, some
egg cells remain undeveloped or subsequently die, and many even of the
well- formed seeds fail to germinate when sown. The above assumption is
also limited in so far that while it demands the formation of an equal
number of the various sorts of egg and pollen cells, it does not require
that this should apply to each separate hybrid with mathematical
exactness.
The
first
and
second

experiments
had
primarily
the
object
of
proving
the
composition of the hybrid egg cells, while the
third and fourth

experiments were to decide that of the pollen cells. As is shown by the
above demonstration the first and third experiments and the second and
fourth experiments should produce precisely the same combinations, and
even
in
the
second
year
the
result
should
be
partially
visible
in
the
form
and color of the artificially fertilized seed. In the first and third
experiments the dominant characters of form and color,
A
and
B
, appear
in each union, and are also partly constant and partly in hybrid union
with
the
recessive
characters
a

and
b
,
for
which
reason
they
must
impress
their
peculiarity
upon
the
whole
of
the
seeds.
all
seeds
should
therefore
appear round and yellow, if the theory be justified. In the second and
fourth experiments, on the other hand, one union is hybrid in form and
in color, and consequently the seeds are round and yellow; another is
hybrid
in
form,
but
constant
in
the
recessive
character
of
color,
whence
the seeds are round and green; the third is constant in the recessive
character
of
form
but
hybrid
in
color,
consequently
the
seeds
are
wrinkled
and
yellow;
the
fourth
is
constant
in
both
recessive
characters,
so
that
the seeds are wrinkled and green. In both these experiments there were
consequently
four
sorts
of
seed
to
be
expected;
namely,
round
and
yellow,
round and green, wrinkled and yellow, wrinkled and green.
The crop fulfilled these expectations perfectly. There were obtained in
the
1st Experiment, 98 exclusively round yellow seeds;


3rd Experiment, 94 exclusively round yellow seeds.


In
the
2nd
Experiment,
31
round
and
yellow,
26
round
and
green,
27
wrinkled
and yellow, 26 wrinkled and green seeds.
In
the
4th
Experiment,
24
round
and
yellow,
25
round
and
green,
22
wrinkled
and yellow, 27 wrinkled and green seeds.
第

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There could scarcely be now any doubt of the success of the experiment;
the
next
generation
must
afford
the
final
proof.
From
the
seed
sown
there
resulted for the
first
experiment
90 plants,
and for the
third 87 plants
which fruited: these yielded for the
1st Exp. 3rd Exp.
20
25
round yellow seeds ..................... AB
23
19
round yellow and green seeds ............. ABb
25
22
round and wrinkled yellow seeds .......... AaB
22
21
round and wrinkled green and yellow seeds.. AaBb
In the second and fourth experiments the round and yellow seeds yielded
plants with round and wrinkled yellow and green seeds,
AaBb
.
From
the
round
green seeds
plants
resulted
with
round
and
wrinkled
green
seeds,
Aab
.
The
wrinkled
yellow
seeds
gave
plants
with
wrinkled
yellow
and
green
seeds,
aBb
.
From
the
wrinkled
green
seeds
plants
were
raised
which
yielded
again
only
wrinkled and green seeds,
ab
.
Although
in
these
two
experiments
likewise
some
seeds
did
not
germinate,
the figures arrived at already in the previous year were not affected
thereby,
since
each
kind
of
seed
gave
plants
which,
as
regards
their
seed,
were like each other and different from the others. There resulted
therefore from the

2nd. Exp. 4th Exp.

31

24
plants of the form AaBb

26

25
plants of the form Aab

27

22
plants of the form aBb
26

27
plants of the form ab
In
all
the
experiments,
therefore,
there
appeared
all
the
forms
which
the
proposed theory demands, and they came in nearly equal numbers.
In
a
further
experiment
the
characters
of
flower-color
and
length
of
stem

were experimented upon,
and
selection was
so
made that in
the third year
of
the
experiment
each
character
ought
to
appear
in
half

of
all
the
plants
if the above theory were correct.
A
,
B
,
a
,
b
serve again as indicating the
various characters.
A, violet-red flowers;


a, white flowers;


B, axis long;


第

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57
页



A, axis short.
The form
Ab
was fertilized with
ab
, which produced the hybrid
Aab
.
Furthermore,
aB
was also fertilized with
ab
, whence the hybrid
aBb
. In
the second year, for further fertilization, the hybrid
Aab
was used as
seed parent, and hybrid
aBb
as pollen parent.
Seed parent,Aab;


Pollen parentaBb;


Possible egg cells, Ab,ab;


Pollen cells, aB, ab.


From the fertilization between the possible egg and pollen cells four
combinations should result, namely:


AaBb
+
aBb
+
Aab
+
ab

From
this
it
is
perceived
that,
according
to
the
above
theory,
in
the
third
year of the experiment out of all the plants
half should have violet-red flowers (Aa)
Classes 1, 3









1, 2




From 45 fertilizations of the second year 187 seeds resulted, of which
only 166 reached the flowering stage in the third year. Among these the
separate classes appeared in the numbers following:
Class Flower color Stem
---------------------------------------------

1
violet-red
long
47 times

2
white

long
40

3
violet-red
short 38

4
white

short 41
There subsequently appeared
the violet-red flower color (
Aa
) in 85 plants,


the white flower-color (
a
) in 81 plants,


the long stem (
Bb
) in 87 plants,


the short stem (
b
) in 79 plants.


The theory adduced is therefore satisfactorily confirmed in this
experiment also.
For
the
characters
of
form
of
pod
,
color
of
pod
,
and
position
of
flowers
,
experiments
were
also
made
on
a
small
scale
and
results
obtained
in
perfect
第

22
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57
页

agreement.
All
combinations,
which
were
possible
through
the
union
of
the
differentiating characters duly appeared, and in nearly equal numbers.
Experimentally, therefore, the theory is confirmed that
the pea hybrids
form
egg
and
pollen
cells
which,
in
their
constitution,
represent
in
equal
numbers all constant forms which result from the combination of the
characters united in fertilization
.
The difference of the forms among the progeny of the hybrids, as well as
the respective ratios of the numbers in which they are observed, find a
sufficient
explanation
in
the
principle
above
deduced.
The
simplest
case
is afforded by the developmental series of
each pair of differentiating
characters
.
This
series
is
represented
by
the
expression
A+2Aa+a
,
in
which
A
and
a
signify the forms with constant differentiating characters, and
Aa
the hybrid form of both. It includes in 3 different classes 4
individuals. In the formation
of these, pollen
and egg cells
of the form
A

and
a

take
part
on
the
average
equally
in
the
fertilization;
hence
each
form
[occurs]
twice,
since
four
individuals
are
formed.
There
participate
consequently in the fertilization
the pollen cells
A
+
A
+
a
+
a
,
o

the egg cells
A
+
A
+
a
+
a
.
o

It remains, therefore, purely a matter of chance which of the two sorts
of pollen will become united with each separate egg cell. According,
however,
to
the
law
of
probability,
it
will
always
happen,
on
the
average
of
many
cases,
that
each
pollen
form
A

and
a

will
unite
equally
often
with
each egg cell form
A
and
a
, consequently one of the two pollen cells
A

in
the
fertilization
will
meet
with
the
egg
cell
A

and
the
other
with
the
egg
cell
a
,
and
so
likewise
one
pollen
cell
a

will
unite
with
an
egg
cell
A
, and the other with the egg cell
a
.

Pollen cells
A
A a
a



|
/ |



|
X
|



|
/
|

Egg cells
A A a a
The result of the fertilization may be made clear by putting the signs
for the conjoined egg and pollen cells in the form of fractions, those
for the pollen cells above and those for the egg cells below the line.
We then have


A
A
a a

----- + ----- + ----- + -----


A
a
A a
In the first and fourth term the egg and pollen cells are of like kind,
consequently the product of their union must be constant, namely
A
and
第

23
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57
页

a
;
in
the
second
and
third,
on
the
other
hand,
there
again
results
a
union
of the two differentiating characters of the stocks, consequently the
forms
resulting
from
these
fertilizations
are
identical
with
those
of
the
hybrid from which they sprang.
There occurs accordingly a repeated
hybridization.

This
explains
the
striking
fact
that
the
hybrids
are
able
to produce, besides the two parental forms, offspring which are like
themselves;


A
a

----- and -----


a
A
both give the same union
Aa
, since, as already remarked above, it makes
no
difference
in
the
result
of
fertilization
to
which
of
the
two
characters
the pollen or egg cells belong. We may write then

A
A
a
a
--- + --- + --- + --- = A + 2Aa + a

A
a
A
a
This represents the average result of the self-fertilization of the
hybrids when two differentiating characters are united in them. In
individual
flowers
and
in
individual
plants,
however,
the
ratios
in
which
the forms of the series are produced may suffer not inconsiderable
fluctuations. Apart from the fact that the numbers in which both sorts
of egg cells occur in the seed vessels can only be regarded as equal on
the average, it remains purely a matter of chance which of the two sorts
of pollen may fertilize each separate egg cell. For this reason the
separate values must necessarily be subject to fluctuations, and there
are
even
extreme
cases
possible,
as
were
described
earlier
in
connection
with
the
experiments
on
the
forms
of
the
seed
and
the
color
of
the
albumen.
The true ratios of the numbers can only be ascertained by an average
deduced from the sum of as many single values as possible; the greater
the number the more are merely chance effects eliminated.
The
developmental
series
for
hybrids
in
which
two
kinds
of
differentiating
characters
are
united
contains
among
16
individuals
9
different
forms,
AB
+ Ab + aB + ab + 2ABb + 2aBb + 2AaB + 2Aab + 4AaBb
. Between the
differentiating characters of the original stocks
Aa
and
Bb
4 constant
combinations are possible, and consequently the hybrids produce the
corresponding 4 forms of egg and pollen cells
AB
,
Ab
,
aB
,
ab
, and each of
these will on the average figure 4 times in the fertilization, since 16
individuals are included in the series. Therefore, the participators in
the fertilization are
Pollen cells: AB+AB+AB+AB+Ab+Ab+Ab+Ab+aB+aB+aB+aB+ab+ab+ab+ab.


Egg cells: AB+AB+AB+AB+Ab+Ab+Ab+Ab+aB+aB+aB+aB+ab+ab+ab+ab.


第

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57
页

In the process of fertilization each pollen form unites on an average
equally
often
with
each
egg
cell
form,
so
that
each
of
the
4
pollen
cells
AB

unites
once
with
one
of
the
forms
of
egg
cell
AB
,
Ab

aBab
.
In
precisely
the same way the rest of the pollen cells of the forms
AbaBab
unite with
all the other egg cells. We obtain therefore

AB
AB AB AB Ab
Ab
Ab
Ab

---- + ---- + ---- + ---- + ---- + ---- + ---- + ----

AB
Ab aB ab AB Ab aB ab



aB aB
aB
aB ab ab ab
ab

+ ---- + ---- + ---- + ---- + ---- + ---- + ---- + ----

AB Ab aB ab AB Ab aB ab
or


AB + ABb + AaB + AaBb + ABb + Ab + AaBb + Aab + AaB + AaBb
+ aB + aBb + AaBb + Aab + aBb + ab


=
AB + Ab + aB + ab + 2ABb + 2aBb + 2AaB + 2Aab + 4AaBb

In precisely similar fashion is the developmental series of hybrids
exhibited when
three kinds of differentiating characters
are conjoined
in them. The hybrids form 8 various kinds of egg and pollen cells:
ABC
,
ABc
,
AbC
,
Abc
,
aBC
,
aBc
,
abC
,
abc
;
and
each
pollen
form
unites
itself
again
on the average once with each form of egg cell.
The law of combination of different characters which governs the
development
of
the
hybrids
finds
therefore
its
foundation
and
explanation

in
the
principle
enunciated,
that
the
hybrids
produce
egg
cells
and
pollen
cells which in equal numbers represent all constant forms which result
from the combinations of the characters brought together in
fertilization.
[10] Experiments with Hybrids of Other Species of Plants
It
must
be
the
object
of
further
experiments
to
ascertain
whether
the
law
of
development
discovered
for
Pisum
applies
also
to
the
hybrids
of
other
plants.
To
this
end
several
experiments
were
recently
commenced.
Two
minor
experiments
with
species
of
Phaseolus
have
been
completed,
and
may
be
here
mentioned.
An experiment with Phaseolus vulgaris and Phaseolus nanus gave results
in
perfect
agreement.
Ph.
nanus
had
together
with
the
dwarf
axis,
simply
inflated,
green
pods.
Ph.
vulgaris
had,
on
the
other
hand,
an
axis
10
ft.
第

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57
页

to
12
ft.
high,
and
yellow
colored
pods,
constricted
when
ripe.
The
ratios
of the numbers in which the different forms appeared in the separate
generations were the same as with Pisum. Also the development of the
constant
combinations
resulted
according
to
the
law
of
simple
combination
of characters, exactly as in the case of Pisum. There were obtained

Constant Axis
Color of the
Form of the
combinations
unripe pods ripe pods
------------------------------------- --------------------

1
long
green
inflated

2

constricted

3

yellow
inflated

4


5
short
green
inflated

6


constricted

7

yellow
inflated

8



constricted
The green color of the pod, the inflated forms, and the long axis were,
as in Pisum, dominant characters.
Another
experiment
with
two
very
different
species
of
Phaseolus
had
only
a
partial
result.
Phaseolus
nanus,
L,
served
as
seed
parent
,
a perfectly
constant species, with white flowers in short recemes and small white
seeds in straight, inflated, smooth pods; as
pollen parent
was used Ph.
multiflorus, W, with tall winding stem, purple-red flowers in very long
recemes, rough, sickle-shaped crooked pods, and large seeds which bore
black flecks and splashes on a peach-blood-red ground.
The hybrids had the greatest similarity to the pollen parent, but the
flowers
appeared
less
intensely
colored.
Their
fertility
was
very
limited;
from 17 plants, which together developed many hundreds of flowers, only
49
seeds
in
all
were
obtained.
These
were
of
medium
size,
and
were
flecked
and
splashed
similarly
to
those
of
Ph.
multiflorus,
while
the
ground
color
was not materially different. The next year 44 plants were raised from
these
seeds,
of
which
only
31
reached
the
flowering
stage.
The
characters
of
Ph.
nanus,
which
had
been
altogether
latent
in
the
hybrids,
reappeared
in various combinations; their ratio, however, with relation to the
dominant
plants
was
necessarily
very
fluctuating
owing
to
the
small
number
of
trial
plants.
With
certain
characters,
as
in
those
of
the
axis
and
the
form
of
pod,
it
was,
however,
as
in
the
case
of
Pisum,
almost
exactly
1:3.
Insignificant as the results of this experiment may be as regards the
determination
of
the
relative
numbers
in
which
the
various
forms
appeared,
it presents, on the
other hand,
the
phenomenon of a
remarkable change of
color
in the flowers and seed of the hybrids. In Pisum it is known that
第

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57
页

the
characters
of
the
flower-
and
seed-color
present
themselves
unchanged
in
the
first
and
further
generations,
and
that
the
offspring
of
the
hybrids
display
exclusively
the
one
or
the
other
of
the
characters
of
the
original
stocks. It is otherwise in the experiment we are considering. The white
flowers and the seed-color of Ph. nanus appeared, it is true, at once in
the
first
generation in
one
fairly
fertile
example,
but
the
remaining
30
plants
developed
flower-colors
which
were
of
various
grades
of
purple-red
to
pale
violet.
The
coloring
of
the
seed-coat
was
no
less
varied
than
that
of the flowers. No plant could rank as fully fertile; many produced no
fruit
at
all;
others only
yielded
fruits
from
the
flowers
last produced,
which did not ripen. From 15 plants only were well- developed seeds
obtained. The greatest disposition to infertility was seen in the forms
with preponderantly red
flowers, since out
of 16 of these only
4 yielded
ripe seed. Three of
these had
a
similar seed pattern to Ph. multiflorus,
but with a more or less pale ground color; the fourth plant yielded only
one
seed
of
plain
brown
tint.
The
forms
with
preponderantly
violet-colored
flowers had dark brown, black-brown, and quite black seeds.
The experiment was continued through two more generations under similar
unfavorable circumstances, since even among the offspring of fairly
fertile
plants
there
came
again
some
which
were
less
fertile
and
even
quite
sterile. Other flower- and seed-colors than those cited did not
subsequently
present
themselves.
The
forms
which
in
the
first
generation
contained one or more of the recessive characters remained, as regards
these, constant without exception. Also of those plants which possessed
violet flowers and brown or black seed, some did not vary again in these
respects
in
the
next
generation;
the
majority,
however,
yielded
together
with offspring exactly like themselves, some which displayed white
flowers and white seed-coats. The red flowering plants remained so
slightly
fertile
that
nothing
can
be
said
with
certainty
as
regards
their
further development.
Despite the many disturbing factors with which the observations had to
contend,
it
is
nevertheless
seen
by
this
experiment
that
the
development
of the hybrids, with regard to those characters which concern the form
of
the
plants,
follows
the
same
laws
as
in
Pisum.
With
regard
to
the
color
characters, it certainly appears difficult to perceive a substantial
agreement. Apart from the fact that from the union of a white and a
purple- red
coloring
a
whole
series
of
colors
results,
from
purple
to
pale
violet and white, the circumstance is a striking one that among 31
flowering plants only one received the recessive character of the white
color, while in Pisum this occurs on the average in every fourth plant.
Even these enigmatical results, however, might probably be explained by
the law governing Pisum if we might assume that the color of the flowers
第

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57
页

and seeds of Ph. multiflorus is a combination of two or more entirely
independent colors, which individually act like any other constant
character in the plant. If the flower-color
A
were a combination of the
individual characters
A(1) + A(2) + .....
which produce the total
impression of a purple coloration, then by fertilization with the
differentiating character, white color,
a
, there would be produced the
hybrid unions
A(1)a + A(2)a + .....
and so would it be with the
corresponding coloring of the seed-coats. According to the above
assumptions,
each
of
these
hybrid
color
unions
would
be
independent,
and
would
consequently
develop
quite
independently
from
the
others.
It
is
then
easily
seen
that
from
the
combination
of
the
separate
developmental
series
a complete color-series must result. If, for instance,
A = A(1) + A(2)
,
then the hybrids
A(1)a
and
A(2)a
form the developmental series:


A(1) + 2A(1)a + a



A(2) + 2A(2)a + a

The members of this series can enter into nine different combinations,
and each of these denotes another color:


1 A(1)A(2)
2 A(1)aA(2)
1 A(2)a


2 A(1)A(2)a
4 A(1)aA(2)a
2 A(2)aa


1 A(1)a

2 A(1)aa
1 aa
The figures prescribed for the separate combinations also indicate how
many plants with the corresponding coloring belong to the series. Since
the total is 16, the whole of the colors are on the average distributed
over each 16 plants, but, as the series itself indicated, in unequal
proportions.
Should the color development really happen in this way, we could offer
an explanation of the case above described, namely that of the white
flowers
and
seed-coat
color
only
appeared
once
among
31
plants
of
the
first
generation. This coloring appears only once in the series, and could
therefore
also
only
be
developed
once
in
the
average
in
each
16,
and
with
three color characters only once even in 64 plants.
It must, nevertheless, not be forgotten that the explanation here
attempted is based on a mere hypothesis, only supported by the very
imperfect
result
of
the
experiment
just
described.
It
would,
however,
be
well worth while to follow up the development of color in hybrids by
similar
experiments,
since
it
is
probable
that
in
this
way
we
might
learn
the significance of the extraordinary variety in the
coloring of our
ornamental flowers
.
So far, little at present is known with certainty beyond the fact that
the
color
of
the
flowers
in
most
ornamental
plants
is
an
extremely
variable
character.
The
opinion
has
often
been
expressed
that
the
stability
of
the
第

28
页

共

57
页

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