Page - 487 - in The Origin of Species

DEVELOPMENT AND EMBRYOLOGY 487 habits of life, would commonly be found unoccupied or ill- occupied by other organisms. In this case the gradual ac- quirement at ati earlier and earlier age of the adult structure would be favoured by natural selection ; and all traces of former metamorphoses would finally be lost. If, on the other hand, it profited the young of an animal to follow habits of life slightly different from those of the parent-form, and consequently to be constructed on a slightly different plan, or if it profited a larva already different from its parent to change still further, then, on the principle of inheritance at corresponding ages, the young or the larvae might be rendered by natural selection more and more dif- ferent from their parents to any conceivable extent. Differ- ences in the larva might, also, become correlated with suc- cessive stages of its development ; so that the larva, in the first stage, might come to differ greatly from the larva in the second stage, as is the case with many animals. The adult might also become fitted for sites or habits, in which organs of locomotion or of the senses, &c., would be useless; and in this case the metamorphosis would be retrograde. From the remarks just made we can see how by changes of structure in the young, in conformity with changed habits of life, together with inheritance at corresponding ages, animals might come to pass through stages of development, perfectly distinct from the primordial condition of their adult progenitors. Most of our best authorities are now convinced that the various larval and pupal stages of insects have thus been acquired through adaptation, and not through inheritance from some ancient form. The curious case of Sitaris—a beetle which passes through certain unusual stages of development—will illustrate how this might occur. The first larval form is described by M. Fabre, as an active, minute insect, furnished with six legs, two long antennae, and four eyes. These larvae are hatched in the nests of bees; and when the male-bees emerge from their burrows, in the spring, which they do before the females, the larvae spring on them, and afterwards crawl on to the females whilst paired with the males. As soon as the female bee deposits her eggs on the surface of the honey stored in the cells, the larvai of the Sitaris leap on the eggs and devour them.