Page:Popular Science Monthly Volume 16.djvu/187

 to an ovary, but not a true ovary, these larvæ end another generation of larvæ like themselves, which in their turn, by internal budding, form a third generation, and so on until autumn, when the last generation develop into perfect winged insects, male and female. These last coöperate to produce eggs which hatch next spring, to commence another cycle of changes.

Here, then, we observe as before the lower form of reproduction in the larva, and the higher in the perfect insect. Here, again, we have non-sexual mode preceding the sexual mode in ontogenesis, suggesting a similar succession in phylogenesis. But in addition we observe here that the form of non-sexual reproduction very closely simulates sexual reproduction; for the budding is from an internal organ set apart for the purpose and very closely resembling a true ovary.

The next step in the chain of approximation is found in parthenogenesis or virgin generation. This consists in the formation, in a perfect female capable of sexual generation, of ovules which develop into embryos without the coöperation of the male element. In bees and wasps the ovules are sometimes fertilized and sometimes unfertilized. The fertilized eggs always produce females, the unfertilized always males. In this case the analogy to non-sexual reproduction is not close; because the female is, of course, the sex absolutely necessary to carry on the succession of generations, and it is this sex which it requires fertilization to produce. But in other cases, for example, in certain moths and in some phyllopod crustaceans, according to Siebold, the unfertilized eggs produce females and the fertilized males. In such cases, it is evident, a succession of females may be formed without the cooperation of the male; and thus we have continuous generation which is completely intermediate between sexual and non-sexual. It is sexual in that an embryo is developed from an ovule formed in a perfect ovary, it is non-sexual in that the coöperation of the male element is unnecessary even for an indefinite succession of generations.

On the other hand, the case of moths and phyllopod crustaceans approaches equally the case of aphids already mentioned—so much so, indeed, that the larval reproduction of these latter have often been classed under parthenogenesis. The difference is this: true parthenogenesis takes place in perfect females, capable of sexual union and of fertilization, possessing perfect ovaries and producing true ovules which develop into embryos without fertilization. The larval aphid, on the contrary, is not a perfect female, is not capable of sexual union nor of fertilization; its ovary-like organ is not a true ovary, does not produce true ovules which develop into embryos, but forms an embryo at once within, which then is born in an active state. Still the resemblance to parthenogenesis is undoubted, and together they almost wholly fill up the gap between the sexual and non-sexual modes of reproduction.

There is still another fact which must be brought forward to fill this gap. True sexual reproduction, as we have seen, consists