Chap. XIV.] DEVELOPMENT AND EMBRYOLOGY. 251 



greatly changed habits of life, would commonly be found 

 unoccupied or ill-occupied by other organisms. In this 

 case the gradual acquirement at an 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 con- 

 structed 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 corre- 

 sponding ages, the young or the larvae might be rendered 

 by natural selection more and more different from their 

 parents to any conceivable extent. Differences in the 

 larva might, also, become correlated with successive 

 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 corres- 

 ponding 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 



