are often two or more of these animals together in the intestine
of a host at the same time. But even where there is only one,
self-fertilization on the part of the joints, that is, the sexual
individuals, is prevented, and by the same device, metaphorically
speaking, as in the case of the oyster, for in each joint the male
elements mature first and the female elements afterwards. In certain
parasitic Isopods of the genus _Anilocra_ and related forms close
inbreeding is prevented in the same way--by a difference in the period
at which the two sets of gonads in the hermaphrodite individual become
mature (dichogamy).
This is secured in a different way in Crustaceans which have grown to
maturity in a sedentary state, like the Cirrhipeds. These animals,
known as 'acorn-shells' and 'barnacles,' are sedentary, sometimes
on rocks and stones, sometimes on a movable object, the keel of a
ship, floating pieces of wood, cork, or cane, or sometimes attached
to turtles or whales, and although they generally occur in great
numbers together, they are probably only able to fertilize each
other occasionally, and are therefore essentially dependent upon
self-fertilization. But Charles Darwin discovered long ago that many
of them, notwithstanding their hermaphroditism, have males which are
small, dwarf-like, and very mobile organisms, destined only for a
very brief life. These seemed quite superfluous in association with
hermaphrodite animals, and they have therefore long been regarded as
vestigial males, as the last remnant, so to speak, of a past stage of
the modern Cirrhipeds, in which the sexes were separate. It is obvious,
however, that we must now attribute to them a deeper significance,
for these so-called 'primordial males,' although extremely transitory
creatures without mouth or intestine, represent a means of securing
the cross-fertilization of the species. What importance nature
attaches to their preservation is shown especially by the parasitic
Cirrhipeds which have been so carefully studied by Fritz Müller
and Yves Delage--those sac-like Rhizocephalidæ or root-crustaceans
which are altogether disfigured by parasitism. The fully developed
animals are hermaphrodite and live partly in, partly upon crabs and
hermit-crabs (Fig. 112, _C_, _Sacc_). These hermaphrodites indeed
fertilize themselves, but in their youth they are of distinct sexes,
and the females are so constituted that they lay eggs for the first
time just when the males of the current year are appearing. Thus the
first batch of eggs liberated by the females are fertilized by the
minute free-swimming 'primordial males,' but after that the females
themselves develop testes, and then fertilize themselves; the males
die very soon after copulation, and only appear the following year
in a new generation. They are therefore far from being mere historic
reminiscences, vestiges of the early history of the modern species,
for they are the instruments of a regular cross-fertilization of
the species, and therefore of a constant mingling of new ids in
the germ-plasm. This is not the place to discuss the marvellous
life-history of these parasites in detail; I can only say that when
we inquire into the whole story, and appreciate the difficulties
associated with the persistence of these 'primordial males,' we can no
longer doubt that crossing is an indispensable feature of amphimixis--a
feature which must at least occasionally occur if amphimixis is to
retain its significance. This is shown, it seems to me, especially by
these numerous instances of what we may call compulsory retention of
ephemeral males in hermaphrodite, self-fertilizing animals; it follows
also from the theory, for with continued self-fertilization all the ids
in the germ-plasm of an individual would tend to become identical, and
the mingling of two germ-plasms which contained _identical_ ids would,
at least according to the germ-plasm theory, have no meaning at all.
[Illustration: FIG. 112 (repeated). Development of the parasitic
Crustacean _Sacculina carcini_, after Delage. _A_, Nauplius stage.
_Au_, eye. _I_, _II_, _III_, the three pairs of appendages. _B_,
Cypris stage. _VI-XI_, the swimming appendages. _C_, mature animal
(_Sacc_), attached to its host, the shore-crab (_Carcinus mænas_),
with a feltwork of fine root-processes enveloping the crab's viscera.
_S_, stalk. _Sacc_, body of the parasite. _oe_, aperture of the
brood-cavity. _Abd_, abdomen of the crab with the anus (_a_).]
Thus we see that in the animal kingdom hermaphroditism is always
associated with cross-fertilization in some way or other, even though
the latter may occur rarely, being usually periodically interpolated,
and thus bringing new ids into the germ-plasm which is rapidly becoming
monotonous or uniform. Adaptations quite analogous to these are found
in relation to parthenogenesis, and it will repay us to give a brief
summary of these.
Parthenogenesis effects a very considerable increase in the fertility
of a species, and in this increase the reason for its introduction
among natural phenomena obviously lies. By the occurrence of
parthenogenesis, the number of ova produced by a particular colony of
animals may be doubled, because each individual is a female, and as the
multiplication increases in geometrical ratio a few parthenogenetic
generations result in a number of descendants enormously in excess of
those produced by bi-sexual reproduction. We can therefore understand
why parthenogenesis should obtain among animals whose conditions of
life are favourable only for a short time, and are then uncertain and
dangerous for a long period. This is the case with the water-fleas,
the Daphnids (see Figs. 57 and 58), whose habitats--pools, ponds, and
marshes--often dry up altogether in summer, or freeze in winter, so
that it becomes almost if not quite impossible for the colonies to go
on living, and the preservation of the species can only be secured
by the production of hard-shelled 'lasting' eggs, which sink to the
bottom, dry up in the mud, or become frozen, or at least remain latent
in a sort of slumber. As soon as the favourable conditions reappear,
young animals which emerge from the eggs are all females and reproduce
parthenogenetically, so that after a few days there is a numerous
progeny swimming freely about, which in their turn are all females,
and reproduce after the same manner. In many Daphnids this goes on
for a series of generations, and there thus arises an enormous number
of animals, which may fill a marsh so densely that, by drawing a
fine net a few times through the water, one can draw out a veritable
animal soup. In our ponds and lakes these little Crustaceans form the
fundamental food of numerous fishes. But notwithstanding the enormous
havoc wrought among them by enemies, large numbers remain at the end
of a favourable season, and these produce the lasting eggs, _after
fertilization_. For shortly before the end of the season males appear
among the progeny of the hitherto purely parthenogenetic females.
Although each female will only produce a few of these 'lasting' eggs,
which require fertilization and are richly supplied with yolk, the
whole number in each colony is a very large one, because the number of
individuals is very large; and it must be so, since the eggs, though
secure against cold and desiccation, are very imperfectly protected
against the numerous enemies which may do them injury.
Of course the number of individuals which form a colony may vary
greatly in the different species, and the same is true of the number
of parthenogenetic generations which precede the bi-sexual generation.
I have already shown in detail that this depends precisely on the
average duration of the favourable conditions, so that, for instance,
a species which lives in large lake-basins will produce many purely
parthenogenetic generations before the bi-sexual one, which only
appears towards autumn, while species which live in quickly-drying
pools have only a few parthenogenetic generations, and the true
puddle-dwellers give rise to males and sexual females along with the
parthenogenetic females as early as the second generation.
We thus find in the Daphnids an alternation, regulated and made
normal by natural selection, of purely parthenogenetic with bi-sexual
generations, and the result is that the uniformity of the germ-plasm,
which is the necessary consequence of pure parthenogenesis, is
interrupted after a longer or shorter series of generations by
the occurrence of amphimixis. That the number of parthenogenetic
generations may be so varied, though with a definite norm for each
species, indicates again that amphimixis is not an absolute condition
of the maintenance of life, not an indispensable rejuvenation, designed
to counteract the exhaustion of vital force--whether this be meant
in a transcendental sense or otherwise--but that it is an important
advantage calculated to keep the species at its highest level, and that
its influence appears whether it occurs in the species regularly, or
frequently, or only rarely.
This kind of alternation of generations, that is, the alternation
between unisexual (female) and bi-sexual generations, has been called
heterogony. In the Daphnids, certainly, a difference in form between
the parthenogenetic and the bi-sexual generation does not exist,
for the same females which produce eggs requiring fertilization can
also produce parthenogenetic ova, although these are very different
from each other, as we have already seen. The difference between
generations, therefore, does not lie in their structure, but in their
tendency to parthenogenetic or to amphigonous reproduction, and in the
absence or presence of male individuals. There are, however, other
cases of alternation of generations in which the different generations
diverge from each other in structure. One of the most remarkable of
these is that of the gall-wasps (Cynipidæ). In many of these little
Hymenoptera, which form galls on leaves, blossoms, buds, and roots,
especially of the oak, two generations occur annually, one in summer,
the other in early spring, or even in the middle of winter. The latter
consists of females only and reproduces parthenogenetically. We can
readily understand this from the point of view of adaptation to
particular conditions, since the young wasps which emerge from their
galls in winter, or in the middle of a raw spring, are exposed to many
dangers and are terribly decimated before they can succeed in laying
their eggs in the proper place on the plant. Moreover, much precious
time would be lost by the mutual search of the sexes for each other,--a
search which would often be entirely without result. Thus the wingless
female of _Biorhiza renum_ (Fig. 124, _A_), which is not unlike a plump
ant, attempts, without taking food, and often interrupted by a spell
of cold or a snowstorm, to reach a neighbouring oak-shrub, creeps up
on it, and lays its eggs in the heart of a winter bud, whose hard
protecting scales it laboriously perforates by means of its short,
thick, sharp ovipositor.
[Illustration: FIG. 124. Alternation of generations in a Gall-wasp.
_A_, winter generation (_Biorhiza renum_). _B_ and _C_, summer
generations (_Trigonaspis crustalis_). _B_, male. _C_, female. After
Adler.]
After it has succeeded in sinking its ovipositor into the heart of the
bud, it goes on working for hours, piercing the delicate tissue with a
multitude of fine canals, one close beside the other, and then deposits
an egg in each of these. The whole detailed piece of work requires,
according to Adler, uninterrupted active exertion for about three days,
even though in the end only two buds may be filled with eggs. If at
every egg-laying the arrival of a male had to be waited for, an even
larger number of females would fall victims to the unfavourable weather
and other dangers, while at the same time the number of emerging
females could be only half as large as it is. It is obvious that in
this case parthenogenesis is of very great advantage.
In summer the climatic conditions are incomparably more favourable for
the gall-wasps, and accordingly we find that the summer generation
is bi-sexual, but, strangely enough, is so different from the winter
generation that the relationship of the two forms was for a long time
overlooked. The antennæ, the legs, and particularly the ovipositor,
the whole shape of the animal, its size, the length of the abdomen,
the structure of the thorax, and many other points are so different
that as long as the structural features afforded the only criterion
of relationship, the systematists quite naturally placed the winter
and summer forms in different genera. It was only when Dr. H. Adler
succeeded in breeding the one form from the other that people were
convinced that such marked differences in structure could be found
within the same life-cycle.
[Illustration: FIG. 125. The two kinds of Galls formed by the species.
_A_, the many-chambered galls produced by the parthenogenetic
winter form, _Biorhiza renum_. _B_, those produced on oak-leaves by
_Trigonaspis crustalis_, the bi-sexual form. After Adler.]
But we see here quite clearly why the two generations had to become so
different; simply because the winter generation had to adapt itself
to different conditions from the summer generation, above all as to
the laying of its eggs within the tissues of a plant of a different
constitution. In our example, the winter form _Biorhiza renum_ pierces
the terminal buds of the oak, and lays in each of them a large number
of eggs, sometimes as many as 300, so that a very large gall is
formed, in which a great many larvæ can find food, and grow on to the
pupa-stage. From this spongy gall, something like an inverted onion in
shape, and about the size of a walnut (Fig. 125, _A_), there emerge in
July the slender, delicately formed male and female gall-wasps which
were long known as _Trigonaspis crustalis_. Both males and females
are winged, and fly rapidly about in the air (Fig. 124, _B_ and _C_).
The sexes pair, and the females lay their eggs in the cell-layers
on the under side of an oak-leaf, on which arise small, wart-like,
kidney-shaped galls (Fig. 125, _B_) which fall to the ground in autumn,
and from which there emerge, in the middle of winter, the plump,
wingless females, to which, as we have already seen, the name _Biorhiza
renum_ was given.
One generation, therefore, lays its eggs in the parenchyma of tender
leaves, and has only to pierce through a thin layer of plant-tissue,
while the other must penetrate deep down into the hard winter bud,
to be able to deposit its eggs in the proper place, and we therefore
find that in the two kinds of female the ovipositor differs in length,
thickness, and general structure, and so also does the whole complex
apparatus by which the ovipositor is moved. But these differences are
associated with the form of the abdomen, in which the ovipositor lies,
and with the strength and shape of the legs, which must be shorter
and stronger when the boring has to be performed through a hard
plant-tissue or to a considerable depth. We can readily understand how
numerous must be the secondary variations which a transformation of the
ovipositor brings in its train when we compare the ovipositor apparatus
in the two generations of one of these species (Fig. 126).
[Illustration: FIG. 126. Ovipositor and ovum of the two generations
of the same species of Gall-wasp. _A_, those of the winter form,
_Neuroterus læviusculus_. _B_, those of the summer-form, _Spathegaster
albipes_. _st_, ovipositor. _ei_, ovum. Similarly magnified. After
Adler.]
Figure 126 shows the ovipositor of another gall-wasp, of which the
winter form, _Neuroterus læviusculus_, also perforates the hard winter
buds of the oak, while the summer form, _Spathegaster albipes_, lays
its eggs in the tender young leaves of the same tree. The ovipositor of
the former is thin and long, that of the latter short and strong (Fig.
126, _A_ and _B_), and corresponding also to the depth at which the egg
must be sunk, or, so to speak, sown in the tissue of the plant, the egg
of the summer generation differs from that of the winter generation
by having a much shorter stalk (Fig. 126, _ei_). These little wasps
thus afford a beautiful example of the way in which even marked
changes in the conditions of life of a generation may be associated
with transformations in bodily structure, and we understand how it
was possible that by means of processes of selection the generations
which alternate periodically in the year should come to diverge very
considerably in structure. The example may also serve to illustrate how
diverse are the harmonious co-adaptations which such transformations
require, and how necessary, therefore, the continual re-combination of
the ids of the germ-plasm by means of amphimixis must be. We understand
why bi-sexual reproduction was only abandoned in one generation, and
that the one in which parthenogenesis was of considerable advantage.
But such transformations must have come about with extreme slowness,
since they were the result of climatic changes which only come about
very gradually. We thus come again to the same conclusion to which
we were led by our study of vestigial organs in Man, that numerous
species which appear to be at a standstill are continually working
towards their own improvement. But for this amphimixis is essential;
consequently the descendants which have arisen through amphimixis, and
whose ancestors have arisen in the same way, have an advantage over
those of parthenogenetic origin. On the whole, at least, this must be
so; in special cases it may be otherwise, namely, when the advantage
offered by parthenogenesis in respect to the maintenance of the species
preponderates over the advantage which amphimixis implies as regards
possibilities of transformation.
As far as we have seen from the case of the gall-wasps, the absence of
amphimixis in every second generation implies no disadvantage in regard
to the capability for transformation which the species exhibits. As to
whether any disadvantage would ensue if the number of parthenogenetic
generations in the life-cycle were greater we can only guess, since no
case is known which enables us to decide this point, _pro_ or _con_,
with any certainty. The heterogony of the plant-lice, the Aphides, and
their relatives might be cited as against the probability, for in this
case a long series of parthenogenetic generations often alternates
with a single bi-sexual one, but the difference in structure is not so
great in this case, although it does exist, and moreover we can quite
well assume that the adaptation to parthenogenesis was effected at the
beginning of heterogony, when it still consisted of a cycle of only
two generations, and that further virgin generations were interpolated
subsequently.
This assumption is supported by the fact that in some species of our
indigenous Ostracods, in _Cypris vidua_ and _Candona candens_, in
contrast to the Daphnids, several bi-sexual generations alternate
with one parthenogenetic generation. But in this case again there is
no difference whatever in the structure of the two generations, the
parthenogenetic generation being distinguished from the bi-sexual
generation simply by the absence of males.
The alternation of generations in the plant-lice is particularly
instructive, because it emphatically indicates how much Nature is
concerned with the retention of amphimixis, and how little mere
multiplication has to do with this. This is especially striking in the
case of the bark-lice; for instance, in their notorious representative,
the vine-pest, _Phylloxera vastatrix_.
[Illustration: FIG. 127. Life-cycle of the Vine-pest (_Phylloxera
vastatrix_), after Leuckart and Nitsche, and Ritter and Rübsamen. _A_,
the fertilized ovum. _B_, the resulting apterous and parthenogenetic
Phylloxera. _C_, its eggs, from which, as the uppermost arrow
indicates, there may arise similar apterous, parthenogenetic forms, or,
as the horizontal arrow indicates, winged forms (_D_), which produce
'female' and 'male' ova (_E^1_ and _E^2_); from these the sexual
generation arises, the female (_F^1_) and the male (_F^2_); the former
lays the fertilized ovum (_A_).]
As in all plant-lice, the advantage for the sake of which sexual
reproduction was given up depends upon the fact that a practically
unlimited food supply is at the disposal of these parasites of the
vine, which can be made full use of during the proper season, and
which, since every animal is female and produces eggs, results in an
enormous increase in the number of individuals, and thus secures the
continuance of the species. These insects emerge in spring from small
fertilized eggs, which have lain dormant throughout the winter (Fig.
127, _A_), and they develop rapidly into wingless females (_B_), which,
sucking the juice of the vine, multiply by producing large numbers
of little white eggs (_C_). These develop without fertilization into
similar wingless females. Several generations of females succeed each
other, but then, usually from August onwards, differently formed winged
females (_D_) make their appearance, and these, flying from plant to
plant, effect the distribution of the species. But these, too, lay
parthenogenetic eggs (_E^1_ and _E^2_), and from these there emerge,
late in autumn, the members of the single bi-sexual generation, males
and females (_F^1_ and _F^2_), both very minute and wingless, without
a piercing proboscis, and thus incapable of taking food. These pair,
and the female lays a single egg (_A_) under the bark of the vine,
from which the leaves are now falling; this egg survives the winter,
and from it in the following April or May there emerges once more a
parthenogenetic female.
It could hardly be more plainly shown than it is by this case that the
importance of amphimixis is something quite apart from reproduction
and multiplication, for here the number of individuals is not only not
increased by amphimixis, but is materially diminished, being indeed
lessened by a half. By the retention of amphimixis, the species gains
in this case no advantage _except_ the mingling of two germ-plasms.
Something similar occurs in plants which exhibit alternation of
generations, for instance the ferns, in which the sexual generation,
the so-called prothallium or prothallus, contributes nothing to
the _multiplication_ of the plant, since only a single egg-cell
is developed; and the same is true of the mosses. In both cases
_multiplication_ depends solely on the asexual generation, which, as
the so-called 'moss-fruit' or 'fern-plant proper,' produces an enormous
number of spores, in addition to multiplying by runners.
To sum up: we have seen that self-fertilization does occur in
hermaphrodite animals, where otherwise the species would be in
danger of extinction, but this is never the _sole_ and exclusive
mode of fertilization[25], for hermaphrodite species have always the
possibility of securing inter-crossing of individuals, and that in
various ways, whether by the intervention of 'primordial males' or
by an occasional or a periodic alternation of self-fertilization and
mutual fertilization. Pure parthenogenesis enduring through innumerable
generations does appear to occur, but in most cases unisexual
generations alternate with bi-sexual, so that a stereotyping of the
germ-plasm with complete uniformity of ids is obviated.