Page:Popular Science Monthly Volume 76.djvu/326

322 We can not, however, consider this more than a slight alteration from the normal condition for two reasons. First, because the silkworm is closely allied to forms of moths in which parthenogenesis is normal; and second, because, as Loeb states, "Siebold had already mentioned, and Nussbaum confirmed his observations, that a small number of such eggs develop without these means." This is about the last we shall hear of artificial parthenogenesis, as our review takes us higher and higher in the organic scale of evolution. The results of such attempts as have been made to induce this form of modification among the vertebrates have been very unsatisfactory. Nothing can be made to develop beyond the segmentation stage.

Some experiments like those of Dorfmeister, Weismann, Eimer, Merrifield and Poulton show the direct results of changes in temperature, food and even the color of the surroundings; but these affect the character of the pigmentation in these normally highly pigmented forms, and we shall see, as we go higher in phylogeny, that pigmentation is one of the easiest characters to alter through environmental differences.

Eegeneration experiments are naturally not numerous on the bodies of insects, for practical reasons, but their power to regenerate lost legs and wings falls in line with our generalization. The legs of the lower insects, like the walking-stick and cockroaches, will grow again if amputated, the legs of the higher insects, butterflies, ants, bees and wasps, not so well. Larval insects are placed by Przibram about on a par with adult mollusks, leeches and fishes, and a little behind Amphioxus as far as general power of regeneration is concerned. Insects in the final or adult stage are placed in the highest class along with mammals and birds, but since adult mammals and birds can not regenerate lost limbs it seems as if further subdivision might have been made. Experiments which consist of grafting parts of different species on to one another are possible among insects at least during the pupal stage. But the "integumentary organs" alone show successful union.

There is one way in which insects appear to show less modification than the vertebrates. Removal of the sexual glands from birds and mammals produces, as is well known, certain marked anatomical and physiological changes. These are supposed to be due to the cessation of an internal secretion which the gland normally produces. Removal of the sexual glands from insects does not cause changes in the secondary sexual characters. It seems fair to assume that this is because the sexual glands of insects do not produce any internal secretion. If