Crossing under very similar conditions shows, therefore, that the
difference between the sexual cells is smaller and thus the result of
crossing is only slightly superior to that given by self-fertilisation.
Is, then, the favourable result of crossing with a foreign stock to be
attributed to the fact that this belongs to another systematic entity
or to the fact that the plants, though belonging to the same entity
were exposed to different conditions? This is a point on which further
researches must be taken into account, especially since the analysis of
the systematic entities has been much more thorough than formerly. (In
the case of garden plants, as Darwin to a large extent claimed, it
is not easy to say whether two individuals really belong to the same
variety, as they are usually of hybrid origin. In some instances
(Petunia, Iberis) the fresh stock employed by Darwin possessed flowers
differing in colour from those of the plant crossed with it.) We
know that most of Linneaus's species are compound species, frequently
consisting of a very large number of smaller or elementary species
formerly included under the comprehensive term varieties. Hybridisation
has in most cases affected our garden and cultivated plants so that they
do not represent pure species but a mixture of species.
But this consideration has no essential bearing on Darwin's point of
view, according to which the nature of the sexual cells is influenced by
external conditions. Even individuals growing close to one another are
only apparently exposed to identical conditions. Their sexual cells may
therefore be differently influenced and thus give favourable results
on crossing, as "the benefits which so generally follow from a cross
between two plants apparently depend on the two differing somewhat in
constitution or character." As a matter of fact we are familiar with a
large number of cases in which the condition of the reproductive organs
is influenced by external conditions. Darwin has himself demonstrated
this for self-sterile plants, that is plants in which self-fertilisation
produces no result. This self-sterility is affected by climatic
conditions: thus in Brazil Eschscholzia californica is absolutely
sterile to the pollen of its own flowers; the descendants of Brazilian
plants in Darwin's cultures were partially self-fertile in one
generation and in a second generation still more so. If one has any
doubt in this case whether it is a question of the condition of
the style and stigma, which possibly prevents the entrance of the
pollen-tube or even its development, rather than that of the actual
sexual cells, in other cases there is no doubt that an influence is
exerted on the latter.
Janczewski (Janczewski, "Sur les antheres steriles des Groseilliers",
"Bull. de l'acad. des sciences de Cracovie", June, 1908.) has recently
shown that species of Ribes cultivated under unnatural conditions
frequently produce a mixed (i.e. partly useless) or completely sterile
pollen, precisely as happens with hybrids. There are, therefore,
substantial reasons for the conclusion that conditions of life exert an
influence on the sexual cells. "Thus the proposition that the benefit
from cross-fertilisation depends on the plants which are crossed
having been subjected during previous generations to somewhat different
conditions, or to their having varied from some unknown cause as if they
had been thus subjected, is securely fortified on all sides." ("Cross
and Self fertilisation" (1st edition), page 444.)
We thus obtain an insight into the significance of sexuality. If an
occasional and slight alteration in the conditions under which plants
and animals live is beneficial (Reasons for this are given by Darwin
in "Variation under Domestication" (2nd edition), Vol. II. page
127.), crossing between organisms which have been exposed to different
conditions becomes still more advantageous. The entire constitution
is in this way influenced from the beginning, at a time when the whole
organisation is in a highly plastic state. The total life-energy, so
to speak, is increased, a gain which is not produced by asexual
reproduction or by the union of sexual cells of plants which have lived
under the same or only slightly different conditions. All the
wonderful arrangements for cross-fertilisation now appear to be useful
adaptations. Darwin was, however, far from giving undue prominence to
this point of view, though this has been to some extent done by others.
He particularly emphasised the following consideration:--"But we should
always keep in mind that two somewhat opposed ends have to be gained;
the first and more important one being the production of seeds by
any means, and the second, cross-fertilisation." ("Cross and Self
fertilisation" (1st edition), page 371.) Just as in some orchids and
cleistogamic flowers self-pollination regularly occurs, so it may also
occur in other cases. Darwin showed that Pisum sativum and Lathyrus
odoratus belong to plants in which self-pollination is regularly
effected, and that this accounts for the constancy of certain sorts of
these plants, while a variety of form is produced by crossing. Indeed
among his culture plants were some which derived no benefit from
crossing. Thus in the sixth self-fertilised generation of his Ipomoea
cultures the "Hero" made its appearance, a form slightly exceeding its
crossed companion in height; this was in the highest degree self-fertile
and handed on its characteristics to both children and grandchildren.
Similar forms were found in Mimulus luteus and Nicotiana (In Pisum
sativum also the crossing of two individuals of the same variety
produced no advantage; Darwin attributed this to the fact that the
plants had for several generations been self-fertilised and in each
generation cultivated under almost the same conditions. Tschermak
("Ueber kunstliche Kreuzung an Pisum sativum") afterwards recorded the
same result; but he found on crossing different varieties that usually
there was no superiority as regards height over the products of
self-fertilisation, while Darwin found a greater height represented by
the ratios 100:75 and 100:60.), types which, after self-fertilisation,
have an enhanced power of seed-production and of attaining a greater
height than the plants of the corresponding generation which are crossed
together and self-fertilised and grown under the same conditions.
"Some observations made on other plants lead me to suspect that
self-fertilisation is in some respects beneficial; although the benefit
thus derived is as a rule very small compared with that from a cross
with a distinct plant." ("Cross and Self fertilisation", page 350.) We
are as ignorant of the reason why plants behave differently when
crossed and self-fertilised as we are in regard to the nature of the
differentiation of the sexual cells, which determines whether a union of
the sexual cells will prove favourable or unfavourable.
It is impossible to discuss the different results of
cross-fertilisation; one point must, however, be emphasised, because
Darwin attached considerable importance to it. It is inevitable that
pollen of different kinds must reach the stigma. It was known that
pollen of the same "species" is dominant over the pollen of another
species, that, in other words, it is prepotent. Even if the pollen of
the same species reaches the stigma rather later than that of another
species, the latter does not effect fertilisation.
Darwin showed that the fertilising power of the pollen of another
variety or of another individual is greater than that of the plant's
own pollen. ("Cross and Self fertilisation", page 391.) This has
been demonstrated in the case of Mimulus luteus (for the fixed
white-flowering variety) and Iberis umbellata with pollen of another
variety, and observations on cultivated plants, such as cabbage,
horseradish, etc. gave similar results. It is, however, especially
remarkable that pollen of another individual of the same variety may be
prepotent over the plant's own pollen. This results from the superiority
of plants crossed in this manner over self-fertilised plants. "Scarcely
any result from my experiments has surprised me so much as this of the
prepotency of pollen from a distinct individual over each plant's own
pollen, as proved by the greater constitutional vigour of the crossed
seedlings." (Ibid. page 397.) Similarly, in self-fertile plants the
flowers of which have not been deprived of the male organs, pollen
brought to the stigma by the wind or by insects from another plant
effects fertilisation, even if the plant's own pollen has reached the
stigma somewhat earlier.
Have the results of his experimental investigations modified the point
of view from which Darwin entered on his researches, or not? In the
first place the question is, whether or not the opinion expressed in
the Orchid book that there is "Something injurious" connected with
self-fertilisation, has been confirmed. We can, at all events, affirm
that Darwin adhered in essentials to his original position; but
self-fertilisation afterwards assumed a greater importance than it
formerly possessed. Darwin emphasised the fact that "the difference
between the self-fertilised and crossed plants raised by me cannot be
attributed to the superiority of the crossed, but to the inferiority
of the self-fertilised seedlings, due to the injurious effects of
self-fertilisation." (Ibid. page 437.) But he had no doubt that in
favourable circumstances self-fertilised plants were able to persist for
several generations without crossing. An occasional crossing appears to
be useful but not indispensable in all cases; its sporadic occurrence
in plants in which self-pollination habitually occurs is not excluded.
Self-fertilisation is for the most part relatively and not absolutely
injurious and always better than no fertilisation. "Nature abhors
perpetual self-fertilisation" (It is incorrect to say, as a writer has
lately said, that the aphorism expressed by Darwin in 1859 and 1862,
"Nature abhors perpetual self-fertilisation," is not repeated in his
later works. The sentence is repeated in "Cross and Self fertilisation"
(page 8), with the addition, "If the word perpetual had been omitted,
the aphorism would have been false. As it stands, I believe that it is
true, though perhaps rather too strongly expressed.") is, however, a
pregnant expression of the fact that cross-fertilisation is exceedingly
widespread and has been shown in the majority of cases to be beneficial,
and that in those plants in which we find self-pollination regularly
occurring cross-pollination may occasionally take place.
An attempt has been made to express in brief the main results of
Darwin's work on the biology of flowers. We have seen that his object
was to elucidate important general questions, particularly the question
of the significance of sexual reproduction.
It remains to consider what influence his work has had on botanical
science. That this influence has been very considerable, is shown by
a glance at the literature on the biology of flowers published since
Darwin wrote. Before the book on orchids was published there was nothing
but the old and almost forgotten works of Kolreuter and Sprengel with
the exception of a few scattered references. Darwin's investigations
gave the first stimulus to the development of an extensive literature on
floral biology. In Knuth's "Handbuch der Blutenbiologie" ("Handbook
of Flower Pollination", Oxford, 1906) as many as 3792 papers on this
subject are enumerated as having been published before January 1, 1904.
These describe not only the different mechanisms of flowers, but deal
also with a series of remarkable adaptations in the pollinating insects.
As a fertilising rain quickly calls into existence the most varied
assortment of plants on a barren steppe, so activity now reigns in a
field which men formerly left deserted. This development of the biology
of flowers is of importance not only on theoretical grounds but also
from a practical point of view. The rational breeding of plants is
possible only if the flower-biology of the plants in question (i.e. the
question of the possibility of self-pollination, self-sterility, etc.)
is accurately known. And it is also essential for plant-breeders that
they should have "the power of fixing each fleeting variety of colour,
if they will fertilise the flowers of the desired kind with their own
pollen for half-a-dozen generations, and grow the seedlings under the
same conditions." ("Cross and Self fertilisation" (1st edition), page
460.)
But the influence of Darwin on floral biology was not confined to the
development of this branch of Botany. Darwin's activity in this domain
has brought about (as Asa Gray correctly pointed out) the revival of
teleology in Botany and Zoology. Attempts were now made to determine,
not only in the case of flowers but also in vegetative organs, in what
relation the form and function of organs stand to one another and
to what extent their morphological characters exhibit adaptation to
environment. A branch of Botany, which has since been called Ecology
(not a very happy term) has been stimulated to vigorous growth by floral
biology.
While the influence of the work on the biology of flowers was
extraordinarily great, it could not fail to elicit opinions at variance
with Darwin's conclusions. The opposition was based partly on reasons
valueless as counterarguments, partly on problems which have still to
be solved; to some extent also on that tendency against teleological
conceptions which has recently become current. This opposing trend
of thought is due to the fact that many biologists are content with
teleological explanations, unsupported by proof; it is also closely
connected with the fact that many authors estimate the importance of
natural selection less highly than Darwin did. We may describe
the objections which are based on the widespread occurrence of
self-fertilisation and geitonogamy as of little importance. Darwin did
not deny the occurrence of self-fertilisation, even for a long series
of generations; his law states only that "Nature abhors PERPETUAL
self-fertilisation." (It is impossible (as has been attempted) to
express Darwin's point of view in a single sentence, such as H.
Muller's statement of the "Knight-Darwin law." The conditions of life in
organisms are so various and complex that laws, such as are formulated
in physics and chemistry, can hardly be conceived.) An exception to
this rule would therefore occur only in the case of plants in which the
possibility of cross-pollination is excluded. Some of the plants with
cleistogamous flowers might afford examples of such cases. We have
already seen, however, that such a case has not as yet been shown to
occur. Burck believed that he had found an instance in certain
tropical plants (Anonaceae, Myrmecodia) of the complete exclusion of
cross-fertilisation. The flowers of these plants, in which, however,--in
contrast to the cleistogamous flowers--the corolla is well developed,
remain closed and fruit is produced.
Loew (E. Loew, "Bemerkungen zu Burck... ", "Biolog. Centralbl." XXVI.
(1906).) has shown that cases occur in which cross-fertilisation may be
effected even in these "cleistopetalous" flowers: humming birds visit
the permanently closed flowers of certain species of Nidularium and
transport the pollen. The fact that the formation of hybrids may occur
as the result of this shows that pollination may be accomplished.
The existence of plants for which self-pollination is of greater
importance than it is for others is by no means contradictory to
Darwin's view. Self-fertilisation is, for example, of greater importance
for annuals than for perennials as without it seeds might fail to be
produced. Even in the case of annual plants with small inconspicuous
flowers in which self-fertilisation usually occurs, such as Senecio
vulgaris, Capsella bursa-pastoris and Stellaria media, A. Bateson (Anna
Bateson, "The effects of cross-fertilisation on inconspicuous
flowers", "Annals of Botany", Vol. I. 1888, page 255.) found that
cross-fertilisation gave a beneficial result, although only in a slight
degree. If the favourable effects of sexual reproduction, according to
Darwin's view, are correlated with change of environment, it is quite
possible that this is of less importance in plants which die after
ripening their seeds ("hapaxanthic") and which in any case constantly
change their situation. Objections which are based on the proof of the
prevalence of self-fertilisation are not, therefore, pertinent. At first
sight another point of view, which has been more recently urged, appears
to have more weight.
W. Burck (Burck, "Darwin's Kreuzeungsgesetz... ", "Biol. Centralbl".
XXVIII. 1908, page 177.) has expressed the opinion that the beneficial
results of cross-fertilisation demonstrated by Darwin concern only
hybrid plants. These alone become weaker by self-pollination; while
pure species derive no advantage from crossing and no disadvantage from
self-fertilisation. It is certain that some of the plants used by Darwin
were of hybrid origin. (It is questionable if this was always the case.)
This is evident from his statements, which are models of clearness and
precision; he says that his Ipomoea plants "were probably the offspring
of a cross." ("Cross and Self fertilisation" (1st edition), page 55.)
The fixed forms of this plant, such as Hero, which was produced by
self-fertilisation, and a form of Mimulus with white flowers spotted
with red probably resulted from splitting of the hybrids. It is true
that the phenomena observed in self-pollination, e.g. in Ipomoea, agree
with those which are often noticed in hybrids; Darwin himself drew
attention to this.
Let us next call to mind some of the peculiarities connected with
hybridisation. We know that hybrids are often characterized by their
large size, rapidity of growth, earlier production of flowers, wealth of
flower-production and a longer life; hybrids, if crossed with one of the
two parent forms, are usually more fertile than when they are crossed
together or with another hybrid. But the characters which hybrids
exhibit on self-fertilisation are rather variable. The following
instance may be quoted from Gartner: "There are many hybrids which
retain the self-fertility of the first generation during the second
and later generations, but very often in a less degree; a considerable
number, however, become sterile." But the hybrid varieties may be more
fertile in the second generation than in the first, and in some hybrids
the fertility with their own pollen increases in the second, third,
and following generations. (K.F. Gartner, "Versuche uber die
Bastarderzeugung", Stuttgart, 1849, page 149.) As yet it is impossible
to lay down rules of general application for the self-fertility of
hybrids. That the beneficial influence of crossing with a fresh stock
rests on the same ground--a union of sexual cells possessing somewhat
different characters--as the fact that many hybrids are distinguished
by greater luxuriance, wealth of flowers, etc. corresponds entirely
with Darwin's conclusions. It seems to me to follow clearly from
his investigations that there is no essential difference between
cross-fertilisation and hybridisation. The heterostyled plants are
normally dependent on a process corresponding to hybridisation. The view
that specifically distinct species could at best produce sterile hybrids
was always opposed by Darwin. But if the good results of crossing were
EXCLUSIVELY dependent on the fact that we are concerned with hybrids,
there must then be a demonstration of two distinct things. First, that
crossing with a fresh stock belonging to the same systematic entity
or to the same hybrid, but cultivated for a considerable time under
different conditions, shows no superiority over self-fertilisation,
and that in pure species crossing gives no better results than
self-pollination. If this were the case, we should be better able to
understand why in one plant crossing is advantageous while in others,
such as Darwin's Hero and the forms of Mimulus and Nicotiana no
advantage is gained; these would then be pure species. But such a
proof has not been supplied; the inference drawn from cleistogamous and
cleistopetalous plants is not supported by evidence, and the experiments
on geitonogamy and on the advantage of cross-fertilisation in species
which are usually self-fertilised are opposed to this view. There are
still but few researches on this point; Darwin found that in Ononis
minutissima, which produces cleistogamous as well as self-fertile
chasmogamous flowers, the crossed and self-fertilised capsules
produced seed in the proportion of 100:65 and that the average bore the
proportion 100:86. Facts previously mentioned are also applicable to
this case. Further, it is certain that the self-sterility exhibited by
many plants has nothing to do with hybridisation. Between self-sterility
and reduced fertility as the result of self-fertilisation there is
probably no fundamental difference.
It is certain that so difficult a problem as that of the significance
of sexual reproduction requires much more investigation. Darwin was
anything but dogmatic and always ready to alter an opinion when it was
not based on definite proof: he wrote, "But the veil of secrecy is
as yet far from lifted; nor will it be, until we can say why it is
beneficial that the sexual elements should be differentiated to a
certain extent, and why, if the differentiation be carried still
further, injury follows." He has also shown us the way along which
to follow up this problem; it is that of carefully planned and exact
experimental research. It may be that eventually many things will be
viewed in a different light, but Darwin's investigations will always
form the foundation of Floral Biology on which the future may continue
to build.