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