apparently exclusive self-fertilization in Rhabditidæ (round worms),
it seems fair to say that they have not been as yet sufficiently
investigated to admit of a secure appreciation of their value in their
theoretical bearings. Cf. _Arch. Zool. Exper._, 3rd ser., vol. viii,
1900.
We must now briefly consider the higher plants with reference to the
maintenance of diversity in the germ-plasm through crossing.
We saw in an earlier lecture that most flowers are hermaphrodite,
but that they do not fertilize themselves, and are adapted for
crossing, since the pollen of one flower is carried by insects to
the pistil of another, which cannot be reached by its own pollen,
either because it ripens too early or too late, or because the
stigma, notwithstanding its proximity, is so placed as to be out of
reach of the pollen from the adjacent stamens. I showed, following
the fundamental investigations of Sprengel, Charles Darwin, Hermann
Müller, and other successors of Darwin, that the flowers may in a sense
be regarded as the resultants of the insect-visits, since all their
accessory adaptations--large coloured petals, fragrance, nectar, and
even little minutiæ of colour and markings (honey-guides)--as well
as their detailed shape, as seen in 'landing stages,' corolla tubes,
and so on, are only intelligible when we refer their existence to
natural selection. We assume that each of these adaptations secured
some advantage for the species concerned, and that therefore their
first beginnings as slight germinal varieties were accepted, and were
brought gradually to their full expression by the united operation
of germinal and personal selection. This at least is how we should
express ourselves now that we have become acquainted with the factor
of germinal selection. The advantage secured by every such improvement
in a flower's means of attracting insects is obvious, as soon as it
is established that cross-fertilization is more advantageous for the
species than self-fertilization.
We have discussed this already; we saw that experiments instituted
by Darwin proved that seedlings which had arisen through
cross-fertilization were superior to those arising through
self-fertilization, and that in many cases the mother-plant itself
produced fewer seeds when self-fertilized than when cross-fertilized.
This discovery afforded an explanation of the cross-fertilization
of flowers by insects which Sprengel had previously observed. We
understand how the flowers must have become so adapted through
processes of selection that they were unable to fertilize themselves,
but attracted insects, and, so to speak, compelled these to dust them
with pollen from another plant of the same species. We also understand
how self-fertilization remained possible for many flowers in the
event of cross-fertilization through insects not being effected,
since after a certain period of waiting, a curvature of the stamens
or the pistil may take place and lead to the stigma being dusted with
the pollen of the same flower. Obviously the development of _fewer_
seeds is preferable to complete sterility. It is a well-known fact
that peculiar inconspicuous and closed flowers, designed solely for
self-fertilization, may occur along with the open flowers, as in the
case of the so-called cleistogamous flowers of the violet (_Viola_)
and the little dead-nettle (_Lamium amplexicaule_), and the phyletic
origin of these becomes intelligible as soon as it is established that
cross-fertilization is more advantageous than self-fertilization.
Now, however, it seems as if the fundamental proposition of this theory
of flowers will have to be rejected. Not only do the cleistogamous
flowers just mentioned exhibit a great fertility, not at all less
than that of the open flowers of the same species which are adapted
for cross-fertilization, but there is a small number of plants which
produce seeds by _self-fertilization alone_. Thus in _Myrmecodia_
cross-fertilization is absolutely prevented by the fact that the
flowers never open, and according to Charles Darwin _Ophrys apifera_
also reproduces by self-fertilization alone, and is nevertheless a
thoroughly vigorous plant. There are several other cases of this sort,
and particularly among the orchids, though the whole of the structure
of their flowers is specially adapted for pollination by insects.
Many of them are only rarely visited by insects, some not at all, we
know not why, but it is readily intelligible that in such cases they
should have adapted themselves to self-fertilization wherever that was
possible. For this _no great_ variation was necessary; it was enough
that the pollinia, which formerly only became detached from their
attachment at a touch or a push from an insect, should free themselves
spontaneously. And this, according to Darwin, is what happens,
for instance, in _Ophrys scolopax_, which at Cannes is frequently
self-fertilizing. For the development of seed, however, it is not
enough that the pollen should reach the stigma; the pollen-grain has
to send out its tube and penetrate into the ovary, and in many orchids
this does not happen; they are infertile with their own pollen. Various
other plants are also non-fertile with their own pollen, for instance
the common corydalis, _Corydalis cava_, or the meadow cuckoo-flower,
_Cardamine pratensis_ (Hildebrand).
How are we to reconcile these apparently absolutely contradictory
facts? On the one hand, the innumerable devices for securing crossing
lead us to conclude that it is necessary, or at least advantageous,
and on the other we find a small number of plants which reproduce
continually by self-fertilization and yet remain strong and vigorous.
And again there are many plants which yield seed when fertilized with
their own pollen, and others which remain absolutely sterile in the
same circumstances, yielding no seed or very little, and there is
indeed one on which its own pollen has the effect of a poison, for if
it reaches the stigma the flower dies. If there is anything injurious
in self-fertilization (Darwin), we can understand that it will be
avoided, but how can it be continued so long in many cases, and even
become in others the exclusive method of fertilization without visible
evil results?
It seems to me that in these facts, established by observation, the
results of two quite different processes have been confused, and that
we can only gain clearness by studying them apart from one another; I
mean the processes involved in the mechanism of fertilization and those
involved in the mingling of the germ-plasms.
In many cases self-fertilization is said to yield less seed and weaker
seedlings. Let us for the present take this statement as the basis of
our consideration; it does not seem to me conceivable, though here I
am not in agreement with views that have been expressed by others,
that both effects should depend upon the same causes, for the smaller
number of seeds cannot possibly depend upon the mingling of the two
parental germ-plasms, and thus not upon the process of amphimixis
itself, since the effect of the mingling does not make itself felt
until the organism of the offspring is being built up. Of course the
plant seed is the embryo of the young plant, but it will hardly be
thought probable that its development could be absolutely prevented
by the too close relationship of the two germ-cells, and thus the
number of the developing seeds cannot depend on the quality of the ids
co-operating in the segmentation-nucleus, but presumably on the number
of ova awaiting fertilization in the ovary, which are reached by a
pollen-tube and then by a paternal sex-nucleus. This again will depend
upon the impelling and attracting forces of the pollen-grain on the one
hand, and of the stigma and 'embryo-sac' of the flower on the other. In
other words, the fertility of a flower with its own pollen will depend
upon whether the two products of the flower are adapted for mutual
co-operation, and in what degree they are so. We are here dealing not
with the _primary_ reactions of the germ-plasms, which are as they are
and cannot be varied, but with secondary relations, which may be thus
or thus--in short, with _adaptations_.
By what adaptations the pollen of a flower can be made ineffective
for that flower is a question which we must leave the botanists to
answer; in any case it must have been possible, and we see clearly
that it depends upon adaptation when we consider the numerous stages
which occur--from the rare case of the actually poisonous influence
of self-pollination already noticed, to complete sterility, and from
lessened fertility to greater or even perfect fertility. It is possible
that chemical products, secretions of the stigma or the pollen-grain,
or the so-called synergid-cells, have to do with this, or that the
size and therewith the penetrating power of the pollen-cell in
self-fertilization stand in inverse ratio to the length of the pistil,
as has been proved in regard to heterostylism by Strasburger; but in
any case it was possible for Nature, by means of slight variations in
the characters of the male and female parts of the flower, to diminish
the certainty of the meeting of the two germ-cells, even to the total
exclusion of the possibility of any union of these.
If, then, self-fertilization had to be guarded against or at least
rendered difficult because its consequences were injurious, all
variations pointing in the direction of safeguarding would necessarily
be preserved and increased. In many cases variations in the structure
of the flower were sufficient; but when, as in _Corydalis cava_, the
pollen could not readily be prevented from falling upon the stigma, the
pollen might be made sterile as far as its own flower was concerned
by a process of selection, in which on an average those plants would
remain successful which produced the largest number of cross-fertilized
seeds, and in this case those which did so were those whose pollen
reacted most feebly to the stimulus of their own stigma.
[Illustration: FIG. 128. Heterostylism (_Primula sinensis_), after
Noll. Two heterostylic flowers from different plants. _L_, the
long-styled form. _K_, the short-styled form. _G_, style. _S_, anthers.
_P_, _p_, pollen-grains. _N_, _n_, stigmatic papillæ of the long-styled
and short-styled forms respectively. _P_, _p_, _N_, _n_, magnified 110
times.]
That self-sterility in all these different degrees is not a primary
character of the species, but an adaptation to the advantages of
cross-fertilization, is apparent--if indeed it seems doubtful to any
one--especially from cases of heterostylism. I refer to the dimorphism
and trimorphism which Darwin discovered in many flowers, and which
shows itself in the fact that flowers otherwise almost exactly
alike, as, for instance, primroses, may exhibit a long style in some
individuals, and in others a short one (Fig. 128). At the same time,
there is a difference in the position of the stamens, which are placed
higher up in flowers with short styles, and much lower down in those
with long styles. Experiments have proved that the dusting of the
stigma has the best results if pollen from the short-styled reaches
the stigma of the long-styled form, or if pollen from the long-styled
form reaches the stigma of the short-styled. Thus we have again to deal
with an arrangement for crossing, an adaptation to the advantages of
cross-fertilization, and we can in this case see the reason why the
pollen has a different effect upon the two stigmas; the pollen-grains
of the flowers with short style are larger than those of the flowers
with long style, and as the length of the pollen-tube that can be sent
out must depend upon the mass of protoplasm within the pollen-grain,
it follows that the smaller pollen-grains will send out too short a
tube to reach through the long style to the embryo-sac. In addition to
this there is a difference in the papillæ of the stigmas, and it is
possible that these may form an obstacle to the penetrating of pollen
from a similar type. The process of selection which gives rise to such
arrangements as we find in Primulas may easily be imagined, as soon as
we are able to assume that cross-fertilization is more advantageous
than self-fertilization as regards progeny, that is, as regards the
continuance of the species.
We have already seen that uninterrupted self-fertilization is unknown
among animals, but that it is not even very rare among plants, and
this emphatically corroborates our previous conclusion, that the
reason for which amphimixis was introduced as a normal event in nature
is not to be sought for in the necessity for a renewing of life, or
'rejuvenation.' It cannot be a necessity, but only an advantage, which
can in certain circumstances be dispensed with.
Although it is obvious enough that continued inbreeding in its
most extreme form, self-fertilization, does not imply an absolute
abandonment of amphimixis, the adherents of the rejuvenescence theory
have regarded the unfavourable consequences of pure inbreeding as a
confirmation of their assumption, according to which amphimixis is
indispensable to the continuance of the life of the species, and it
is therefore an important fact, if it can be proved, that continued
self-fertilization can occur persistently, among plants at least, and
yet not cause any injurious results to the species.
But how can this fact be understood from our point of view? How does it
happen that crossing is striven after in so many different ways and yet
so often given up again, and continued self-fertilization resorted to?
To this it may be answered, in the first place, that it is not,
as far as we can see, for _internal_ reasons that persistent
self-fertilization becomes the rule; there is no peculiar condition
of the germ-plasm which makes it disadvantageous or superfluous
that the diversity of the id-combinations should be maintained;
self-fertilization is due to _external_ influences which bring it about
that the plant has only the alternative of producing no seeds at all
or of producing them by self-fertilization. In this connexion Darwin's
experiments with orchids are particularly noteworthy.
In this very diversified order of plants there are numerous species
whose flowers are infertile with their own pollen, although it
does not reach the stigma in natural conditions, and therefore
there was no necessity--as far as we can see--for guarding against
self-fertilization by 'self-sterility.' These flowers are thus doubly
adapted, so to speak, for crossing by means of insects. But as regards
many of these, as well as many other modern orchids, insect-visits are
very rare, and in some cases do not occur at all, and therefore these
species cannot produce seed or can do so only exceptionally.
This is true of most of the Epidendra of South America, and of
_Coryanthus triloba_ of New Zealand, two hundred blossoms of which only
yielded five seed-capsules, and also of our _Ophrys muscifera_ and _O.
aranifera_, the latter of which yielded only a single seed-capsule
from 3,000 flowers gathered in Liguria. We might expect that the
species in question must have become very rare, but this is not always
the case, since each of these capsules contains an enormous number
of seeds, sometimes many thousands. As soon as the visits of insects
cease altogether, the species must die out in the particular locality
concerned, unless it can revert to self-pollination and self-fertility.
There is a whole series of species in which the stigma of the flower
is sensitive to its own pollen, and in many of these an adaptation
to self-fertilization has actually been effected, for the pollinia
detach themselves from their anthers at maturity and fall upon the
stigma. I have already mentioned _Ophrys apifera_, which, according to
Charles Darwin, is no longer visited by insects, although its flowers
still possess the structure required for insect-fertilization. This
species has saved itself from extinction by the normal occurrence of
self-fertilization.
This seems to me noteworthy in two respects. In the first place, it
shows that pure self-fertilization need not necessarily result in a
weakening of the species, and secondly, it affords a clear instance
of a species being transformed in one minute character only, all the
other characters remaining unaltered. In this case it was only the
pollinia that required to vary a little in their mode of attachment and
maturation, in order to effect the transformation of the flower for
self-fertilization, and in point of fact that is all that has varied.
The case is not relevant to our investigation at this moment, but
cases of the kind can so rarely be clearly demonstrated that I cannot
lose the opportunity of calling attention to it. The germ-plasm of
this _Ophrys_ must have varied at an earlier stage, for otherwise the
detachment of the pollinia would not have become normal and hereditary,
but it can only have varied to the extent that the structure of this
one small part of the flower was affected by the variation; something
must have varied in the germ-plasm that had no influence upon the other
parts of the flower, that is, solely the _determinants_ of the pollinia.
Let us return after this digression to our previous train of thought;
we have to inquire how we can interpret the fact of continued
self-fertilization without any visible injurious results to the
species. If cross-fertilization be a material advantage as regards the
continuance of the species, how can it be transformed into its opposite
without evil effects? And there are no visible evil effects in _Ophrys
apifera_. It is indeed not so abundant as _Ophrys muscifera_, or other
allied species, but it certainly does not follow from that that it is
on the way to extinction; certainly no decrease either of vigour of
growth or of fertility can be observed.
If we inquire from the standpoint of our theory, how the composition
of the germ-plasm must have altered through continual inbreeding,
we have already found the answer--that through the reduction of the
number of ids at the maturation of every germ-cell the diversity of the
germ-plasm would gradually be lessened, that the number of different
ids would thereby be lessened possibly even to the identity of the
whole of the ids.
The consequences of such extreme uniformity of the germ-plasm would
not, according to our theory, necessarily be that the species would
be incapable of continued existence, but it would be that the species
would become incapable of adaptations in many directions. Adaptations
in one direction, such, for instance, as the variation in the mode of
attachment and detachment of the pollinia of an Orchid, would still be
possible. Thus a species which has long been perfectly adapted will be
able to make the transition to inbreeding without injury to its chances
of continued existence, if it be compelled by circumstances to do so.
Species, on the other hand, which are still undergoing considerable
transformations in many directions must be exposed by these to the
danger of degeneration, just as happens in the artificial experiments
with domesticated animals, whose secret weaknesses are greatly
exaggerated by inbreeding.
We might be inclined to regard the effects of inbreeding as similar to
those of parthenogenesis; they are certainly analogous, for both modes
of reproduction must lead to a certain degree of uniformity in the
germ-plasm. But there seems to me to be a difference and one which is
not without importance.
In parthenogenesis no amphimixis occurs, but neither does any reduction
of the number of the ids to one-half; all the ids present at the
beginning of parthenogenesis are retained; they are only no longer
mingled with strange ids. In inbreeding both amphimixis and reduction
take place, but the former soon ceases to convey any really strange
ids to the germ-plasm, but only the same as those which it already
contains, so that a rapidly increasing monotony of the germ-plasm
must result. To this must be added the possibility that among the
few ids which now--many times repeated--form the germ-plasm, some
must occur which exhibit unfavourable variational tendencies in
one or many determinants, and then the same thing will occur which
usually occurs in experimental inbreeding of domesticated animals,
namely, _degeneration of the progeny_. In parthenogenesis the case is
otherwise; unfavourable variational tendencies, as soon as they attain
selection-value, are, so to speak, eliminated root and branch, because
the individuals which exhibit them, and their whole lineage, are
exterminated, without their having any effect upon the other collateral
lines of descent. A purely parthenogenetic species will, therefore, not
degenerate as long as individuals of normal constitution are present,
for these reproduce with perfect purity. But if in later generations
unfavourable variational tendencies crop up in the germ-plasm through
germinal selection, the process of personal selection will be
reinforced on these or on their descendants, and it is conceivable, and
even probable, that in perfectly adapted species parthenogenesis may
last for a very long time without doing any injury to the constitution
of the species.
The same is true of purely asexual reproduction, to the investigation
of which we shall now turn.
Let us leave out of account the simplest animals (Monera) without
amphimixis, which we have already discussed. In simple animals
reproduction by budding or by fission is frequent, or it occurs in
alternation with sexual reproduction; in higher animals, Arthropods,
Mollusca, and Vertebrates, asexual reproduction is wholly absent.
In plants it plays an enormously greater part, and what is called
'vegetative reproduction,' which is purely asexual without any
amphimixis, is to be found in all groups of plants, especially in
the form of budding and spore-formation, besides which there is
multiplication by runners, rhizomes, tubers, bulbs, and bulbils. In
most cases there is, in addition to the purely asexual reproduction,
so-called sexual reproduction associated with amphimixis, and often
the sexual and asexual generations alternate with each other, so that
'alternation of generations' occurs, as is common in lower animals,
especially polyps, medusæ, and worms.
But it sometimes happens among plants that the sexual reproduction is
absent, and that a species reproduces by the asexual mode only, and
this is the case which we must now consider more closely.