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. 



