486 ORIGIN OF SPECIES 



a long course of modification, adapted in one descendant to 

 act as hands, in another as paddles, in another as wings; 

 but on the above two principles the fore-limbs will not have 

 been much modified in the embryos of these several forms; 

 although in each form the fore-limb will differ greatly in the 

 adult state. Whatever influence long-continued use or disuse 

 may have had in modifying the limbs or other parts of any 

 species, this will chiefly or solely have affected it when nearly 

 mature, when it was compelled to use its full powers to gain 

 its own living; and the effects thus produced will have been 

 transmitted to the offspring at a corresponding nearly mature 

 age. Thus the young will not be modified, or will be modi- 

 fied only in a slight degree, through the effects of the in- 

 creased use or disuse of parts. 



With some animals the successive variations may have 

 supervened at a very early period of life, or the steps may 

 have been inherited at an earlier age than that at which 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 contin- 

 gencies ; 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 indispensable 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, whilst 

 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 

 land 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 



