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 feet to 12 feet high, and yellow coloured 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 Colour of Form of
combinations the unripe pods. the ripe pods.
1 long green inflated
2 " " constricted
3 " yellow inflated
4 " " constricted
5 short green inflated
6 " " constricted
7 " yellow inflated
8 " " constricted
The green colour 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 racemes 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 racemes, 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 coloured. Their fertility was very
limited; from seventeen plants, which together developed many hundreds
of flowers, only forty-nine seeds in all were obtained. These were of
medium size, and were flecked and splashed similarly to those of _Ph.
multiflorus_, while the ground colour was not materially different. The
next year forty-four plants were raised from these seeds, of which only
thirty-one 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
characters 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 colour in the flowers and seed of the hybrids. In
_Pisum_ it is known that the characters of the flower- and seed-colour
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[43]. It is otherwise
in the experiment we are considering. The white flowers and the
seed-colour of _Ph. nanus_ appeared, it is true, at once in the first
generation [_from_ the hybrids] in one fairly fertile example, but the
remaining thirty plants developed flower colours which were of various
grades of purple-red to pale violet. The colouring 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 fifteen plants
only were well-developed seeds obtained. The greatest disposition to
infertility was seen in the forms with preponderantly red flowers,
since out of sixteen of these only four yielded ripe seed. Three of
these had a similar seed pattern to _Ph. multiflorus_, but with a more
or less pale ground colour; the fourth plant yielded only one seed of
plain brown tint. The forms with preponderantly violet coloured flowers
had dark brown, black-brown, and quite black seeds.
[43] [This is the only passage where Mendel can be construed as
asserting universal dominance for _Pisum_; and even here, having
regard to the rest of the paper, it is clearly unfair to represent
him as predicating more than he had seen in his own experiments.
Moreover in flower and seed-coat colour (which is here meant), using
his characters dominance must be almost universal, if not quite.]
The experiment was continued through two more generations under
similar unfavourable circumstances, since even among the offspring of
fairly fertile plants there were still some which were less fertile
or even quite sterile. Other flower- and seed-colours than those
cited did not subsequently present themselves. The forms which in the
first generation [bred from the hybrids] 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 does _Pisum_.
With regard to the colour 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 colouring a whole series of colours
results, from purple to pale violet and white, the circumstance is a
striking one that among thirty-one flowering plants only one received
the recessive character of the white colour, 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 colour of
the flowers and seeds of _Ph. multiflorus_ is a combination of two
or more entirely independent colours, which individually act like
any other constant character in the plant. If the flower colour A
were a combination of the individual characters _A_{1} + _A_{2} +
... which produce the total impression of a purple colouration, then
by fertilisation with the differentiating character, white colour,
_a_, there would be produced the hybrid unions _A_{1}_a_ + _A_{2}_a_
+ ... and so would it be with the corresponding colouring of the
seed-coats[44]. According to the above assumption, each of these hybrid
colour 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 perfect
colour-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} + 2_A_{1}_a_ + _a_
_A_{2} + 2_A_{2}_a_ + _a_.
[44] [It appears to me clear that this expression is incorrectly
given, and the argument regarding compound characters is consequently
not legitimately developed. The original compound character should
be represented as _A_{1}_A_{2}_A_{3} ... which when fertilised by
_a_{1} gives _A_{1}_A_{2}_A_{3} ... a as the hybrid of the first
generation. Mendel practically tells us these were all alike,
and there is nothing to suggest that they were diverse. When on
self-fertilisation, they break up, they will produce the gametes he
specifies; but they may also produce _A_{1}_A_{1} and _A_{2}_A_{2},
_A_{1}_A_{2}_a_, &c., thereby introducing terms of a nature different
from any indicated by him. That this point is one of the highest
significance, both practical and theoretical, is evident at once.]
The members of this series can enter into nine different combinations,
and each of these denotes another colour[45]--
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_.
[45] [It seems very doubtful if the zygotes are correctly represented
by the terms _A_{1}aA_{2}a_, _A_{2}aa_, _A_{1}aa_; for in the hybrids
_A_{1}a_, &c. the allelomorphs _A_{1}_ and _a_, &c. should by
hypothesis be separated in the gametes.]
The figures prescribed for the separate combinations also indicate how
many plants with the corresponding colouring belong to the series.
Since the total is sixteen, the whole of the colours are on the average
distributed over each sixteen plants, but, as the series itself
indicates, in unequal proportions.
Should the colour development really happen in this way, we could offer
an explanation of the case above described, viz. that the white flowers
and seed-coat colour only appeared once among thirty-one plants of the
first generation. This colouring appears only once in the series, and
could therefore also only be developed once in the average in each
sixteen, and with three colour characters only once even in sixty-four
plants.
It must, however, 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 colour in hybrids by
similar experiments, since it is probable that in this way we might
learn the significance of the extraordinary variety in the colouring of
our ornamental flowers.
So far, little at present is known with certainty beyond the fact that
the colour of the flowers in most ornamental plants is an extremely
variable character. The opinion has often been expressed that the
stability of the species is greatly disturbed or entirely upset by
cultivation, and consequently there is an inclination to regard the
development of cultivated forms as a matter of chance devoid of rules;
the colouring of ornamental plants is indeed usually cited as an
example of great instability. It is, however, not clear why the simple
transference into garden soil should result in such a thorough and
persistent revolution in the plant organism. No one will seriously
maintain that in the open country the development of plants is ruled
by other laws than in the garden bed. Here, as there, changes of type
must take place if the conditions of life be altered, and the species
possesses the capacity of fitting itself to its new environment. It is
willingly granted that by cultivation the origination of new varieties
is favoured, and that by man’s labour many varieties are acquired
which, under natural conditions, would be lost; but nothing justifies
the assumption that the tendency to the formation of varieties is so
extraordinarily increased that the species speedily lose all stability,
and their offspring diverge into an endless series of extremely
variable forms. Were the change in the conditions of vegetation the
sole cause of variability we might expect that those cultivated plants
which are grown for centuries under almost identical conditions would
again attain constancy. That, as is well known, is not the case,
since it is precisely under such circumstances that not only the
most varied but also the most variable forms are found. It is only
the _Leguminosæ_, like _Pisum_, _Phaseolus_, _Lens_, whose organs of
fertilisation are protected by the keel, which constitute a noteworthy
exception. Even here there have arisen numerous varieties during a
cultural period of more than 1000 years; these maintain, however, under
unchanging environments a stability as great as that of species growing
wild.
It is more than probable that as regards the variability of cultivated
plants there exists a factor which so far has received little
attention. Various experiments force us to the conclusion that our
cultivated plants, with few exceptions, are _members of various hybrid
series_, whose further development in conformity with law is changed
and hindered by frequent crossings _inter se_. The circumstance must
not be overlooked that cultivated plants are mostly grown in great
numbers and close together, affording the most favourable conditions
for reciprocal fertilisation between the varieties present and the
species itself. The probability of this is supported by the fact
that among the great array of variable forms solitary examples are
always found, which in one character or another remain constant, if
only foreign influence be carefully excluded. These forms develop
precisely as do those which are known to be members of the compound
hybrid series. Also with the most susceptible of all characters, that
of colour, it cannot escape the careful observer that in the separate
forms the inclination to vary is displayed in very different degrees.
Among plants which arise from _one_ spontaneous fertilisation there
are often some whose offspring vary widely in the constitution and
arrangement of the colours, while others furnish forms of little
deviation, and among a greater number solitary examples occur which
transmit the colour of the flowers unchanged to their offspring. The
cultivated species of _Dianthus_ afford an instructive example of
this. A white-flowered example of _Dianthus caryophyllus_, which itself
was derived from a white-flowered variety, was shut up during its
blooming period in a greenhouse; the numerous seeds obtained therefrom
yielded plants entirely white-flowered like itself. A similar result
was obtained from a subspecies, with red flowers somewhat flushed with
violet, and one with flowers white, striped with red. Many others, on
the other hand, which were similarly protected, yielded progeny which
were more or less variously coloured and marked.
Whoever studies the colouration which results in ornamental plants
from similar fertilisation can hardly escape the conviction that here
also the development follows a definite law which possibly finds
its expression _in the combination of several independent colour
characters_.