Cii.vr. XIII. AND EMBRYOLOGY. 401 



and having been inherited at a corresponding age, the young 

 will be left l)iit little modifuMl and will resemble each other 

 much more closely than do the adults — just as we have seen 

 with the breetls of the pigeon. We may extend this view to 

 widely-distinct structures and to whole classes. The fore- 

 liiiihs, for instance, which once served as legs to a remote ])ro- 

 genitor, may have become, through a long course of modiJica- 

 tioii, 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 the embryonic forc- 

 liinb will differ greatly from that in the adult. Whatever in- 

 fluence, moreover, long-continued use or disuse may have had 

 in modifying the limbs or other parts of animals, this will 

 chiefly or solely have affected them when mature, and when 

 they hatl to use their full powers to gain their own living; 

 and the elTect thus jiroduced will be transmitted to the oil- 

 spring at a ccjrrcsponding mtitvn'e age. Thus the young will 

 not be modified or will be modiiied in a less degree. 



In other cases successive variations may have supervened 

 at a very early period of life, or the steps may have been in- 

 herited at an earlier age than that at which they first occurred. 

 In either case, as we have seen Avith the short-faced tumbh^r, 

 the young or embryo would closely resemble tlie mature par- 

 ent-form. And this is the rule of development in certain whole 

 groups or sub-grouj^s, as Avith cuttle-fish, land-shells, fresh-wa- 

 ter crustaceans, spiders, and some members of the great class 

 of insects. AVith respect to the fmal cause of the 3'^oung in 

 these 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 Iheir own Avants, and from their following the same habits 

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

 ]iensable for their existence that they should be modiiied in 

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

 singular fact of so many terrestrial and fresh-water animals not 

 undergoing any metamorphosis, while marine members of the 

 same groups pass through various 1 ransformal ions, Fritz MuUer 

 has suggested that the process of slowlv modifying and adapt- 

 mg an animal to live on the land or in fresh water, insttnvd 

 of in the sea, would be greatly simjjliHed 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 



