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.
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 (E1 and E2); from these the sexual generation arises, the female (F1) and the male (F2); 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 (E1 and E2), and from these there emerge, late in autumn, the members of the single bi-sexual generation, males and females (F1 and F2), 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.