DEVELOPMENT AND EMBRYOLOGY. 435 



the young will have been but little modified, and they will 

 still resemble each other much more closely than do the 

 adults, just as we have seen with the breeds of the pigeon. 

 We may extend this view to widely distinct structures and 

 to whole classes. The fore limbs, for instance, which once 

 served as legs to a remote progenitor, may have become, 

 through a long course of modification, adapted in one de- 

 scendant 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 sev- 

 eral forms ; although in each form the fore limb will differ 

 greatly in the adult state. Whatever influence long con- 

 tinued 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 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 dub^o 

 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 member 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 indis- 

 pensable 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 ani- 

 mals do not undergo any metamorphosis, while marine mem- 

 bers of the same groups pass through various transformations, 

 Fritz Miiller has suggested that the process of slowly modi- 

 fying 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 prob- 



