250 DEVELOPMENT AND EMBRYOLOGY. [Chap. XIV. 



have been transmitted to the ofEspring at a correspond- 

 ing nearly mature age. Thus the young will not be 

 modified, or will be modified only in a slight degree, 

 through the effects of the increased 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 ma- 

 ture 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 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 under- 

 go any metamorphosis, whilst marine members of the 

 same groups pass through various transformations, Fritz 

 Miiller has suggested that the process of slowly modify- 

 ing and adapting an animal to live on the land or in 

 fresh water, instead (rf in the sea, would be greatly sim- 

 plified by its not passflig 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 



