402 DEVELOPMENT CiiAr. XIII. ' 



and greatly-changed habits of life, could be found unoccupied 

 or ili-occupit'd by other organisms. In this case the gradual 

 acquirement at an earlier and earlier age of the adult structure 

 would be favored by natural selection, and all traces of formei 

 metamorphoses would finally be lost. 



If, on the oilier liand, it })rofitetl the young of an animal to 

 follow hal)its of life slightly dillerent from those of the parent- 

 form, and consequently to be constructed in a slightl^'-dif- 

 ferent manner, or if it profited alarva already widely different 

 from its parent to change still further, then, on the principle 

 of inheritance at corresponding ages, the young or the larvas 

 might be rendered by natural selection more and more differ- 

 ent from their parents to any conceivable extent. Differences 

 in the larvie might, also, become correlated with successive 

 stages of development ; so that the larvee, in the first stage, 

 might come to differ gTcatly from the larva? in the second 

 stage, as is the case Avith many animals. The adult might 

 also become fitted for sites or habits, in which organs of loco- 

 motion or of the senses, etc., 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 Avith changed habits 

 of lif(% together with inheritance at corresponding ages, animals 

 in certain cases might come to pass through stages of develop- 

 ment, perfectly distinct from their primordial, adult condition. 

 Fritz Muller, Avho has recently discussed this subject with much 

 ability, believes that the progenitor of all insects resembled an 

 adult insect, and that the caterpillar or maggot stages, as 

 Avell as the cocoon or pupal stages, have subsequently been ac- 

 (|uired; but from this view many naturalists, for instance, Sir 

 J. Lubbock, who has likewise recently discussed this subject, 

 would, it is probable, dissent. That certain unusual stages in 

 the metamorphoses of insects have been acquired through adap- 

 tation to peculiar habits of life, there can hardly be a doubt ; 

 thus the hrst larval form of a certain beetle, the Sitaris, as de- 

 scribed by M. Fabre, is an active, minute insect, furnished with 

 six legs, two long antenna^, and four eyes. These larvro are 

 hatched in the nests of bees ; and Avhen the male-bees emerge 

 fiom their burrows in the spring, which they do before the 

 females, the larvte spring on them, and afterward crawl on the 

 female while paired with the males. As soon as the females 

 lay their eggs on tlie surface of the honey stored in their cells, 

 the larvas of the Sitaris leap on the eggs and devour them. 



