CONVERGENT EVOLUTION 453 



allied to the Coots and Water-hens, which are also lobe-footed. 

 There is, however, little of real value in this contention. 



That homoplasy should play a very important part in the 

 evolution of the Class Aves is only what might be expected from 

 the uniformity of structure which these animals present. 



The factors which are concerned in the production of these 

 homoplasts are by no means so easily detected as some would 

 have us believe. Doubtless they are the same as obtained in 

 other cases of adaptation to environment, but this assurance 

 leaves the key to the problem still undiscovered. 



"When," says Sir Ray Lankester, "identical or nearly 

 similar forces, or environments, act on two or more parts of an 

 organism which are nearly or exactly alike, the resulting modi- 

 fications of the various parts will be exactly or nearly alike. 

 Further, if instead of similar parts in the same organism, we 

 suppose the same forces to act on parts in two organisms, 

 which parts are exactly or nearly alike, and sometimes homo- 

 genetic, the resulting correspondences called forth in the several 

 parts of the two organisms will be nearly or exactly alike. . . ." 

 As generally interpreted these forces, or environments, act by 

 selecting variations favourable to the maintenance of life in the 

 particular environment in question. 



Thus, variations tending to increase the length of the body 

 and shift the legs backwards have been selected in birds which 

 swim and dive much ; the selection of variations which tended 

 to lengthen the legs and strengthen the toes and claws has 

 transformed the limbs of the Owls from normal perching limbs 

 to organs fitted for grasping and holding living prey, and so on. 



According to some, these variations are more or less fortuit- 

 ous : that is to say they are not stimulated in any way by the 

 environment. Others, however, like Professor Osborn, hold 

 that in every animal there are fundamental predispositions to 

 vary in certain directions, predispositions which are aroused 

 under certain exciting causes. But this hypothesis, judging 

 by the evidence now before us, seems to prove too much. 

 Perchance a satisfactory solution may be found in a compromise. 

 This assumes a potentiality to respond directly to any long-sus- 

 tained stimulus. Many will see in this last explanation a virtual 

 concession to the theoretical doctrine of the transmission of 

 acquired characters. Nevertheless, it does not necessarily im- 

 ply this. 



