464 DBVELOPMENT AND EMBRYOLOGY. 



they first occurred. In either of these cases, the young or 

 embryo will closely resemble the mature parent-form, as 

 we have seen with the short-faced tumbler. And this is 

 the rule of development in certain whole groups, or in 

 certain sub-groups alone, as with cuttle-fish, land-shells, 

 fresh-water crustaceans, spiders, and some members of 

 the great class of insects. With respect to the final 

 cause of the young in such groups not passing through 

 any metamorphosis, we can see that this would follow 

 from the following contingencies: namely, from the 

 young having to provide at a very early age for their 

 own wants, and from their following the same habits of 

 life with their parents; for in this case it would be in- 

 dispensable for their existence that they should be modified 

 in the same manner as their parents. Again, with respect 

 to the singular fact that many terrestrial and fresh-water 

 animals do not undergo any metamorphosis, while marine 

 members of the same groups pass through various trans- 

 formations, Fritz Miiller has suggested that the process of 

 slowly modifying and adapting an animal to live on the 

 land or in fresh water, instead of in the sea, would be 

 greatly simplified by its not passing tlirough any larval 

 stage; for it is not probable that places well adapted for 

 both the larval and mature stages, under such new and 

 greatly changed habits oriife, 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 favored 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 ani- 

 mal 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 dif- 

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

 IDrinciple of inheritance at corresponding ages, the young 

 or the larvaj might be rendered by natural selection more 

 and more different from their parents to any conceivable 

 extent. Differences in the larva might, also, become corre- 

 lated 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 ani- 



