Chap. XIV.] DEVELOPMENT AND EMBRYOLOGY. 251 



changed habits of life, would commonly be found unoc- 

 cupied or ill-occupied by other organisms. In this case 

 the gradual acquirement at an earlier and earlier age 

 01 the adult structure would be favoured by natural selec- 

 tion; 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 ren- 

 dered 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 cor- 

 responding ages, animals might come to pass through 

 stages of development, perfectly distinct from the pri- 

 mordial 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 



