260 THE ORIGIS OF SPECIES 



members of the great class of lusccts. With respect to 

 the tinal 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 modi- 

 lied 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 transformations, Fritz Miiller has suggested that 

 the process of slowly modifying and adapting an animal 

 to live on the laud or in fresh water, instead of in the 

 sea, would be greatly simplified by its not passing through 

 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 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 favored 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 difEerent from their 



