108 GENEEAL EVOLUTION". 



and having thus been converted into hands, paddles, or wings." 

 He then inclines to assign this change to the necessity of external 

 circumstance. But such modification must be the same in kind 

 as others, which the same hypothesis must explain, and of which 

 the same author remarks (page 383) : " We can not, for instance, 

 suppose that in the embryos of the Vertebrata the peculiar loop- 

 like course of the arteries near the branchial slits are related 

 to similar conditions in the young mammal, which is nourished 

 in the womb of its mother, in the egg of the bird which 

 is hatched in a nest, and in the spawn of a frog under water. We 

 have no more reason to believe in such a relation than we have to 

 believe that the same bones in the hand of a man, wing of a bat, 

 and fin of a porpoise, are related to similar conditions of life. 

 No one will suppose that the stripes on the whelp of the lion, or 

 the spots on the young blackbird, are of any use to these animals, 

 or related to the conditions to which they are exposed." 



The law of natural selection, however, has no doubt been a 

 very important agency in the production of organic types in dif- 

 ferent periods of the world's history ; but the part it has played in 

 the determination of generic features would appear to have been 

 very small. 



In its first effect — that of producing a structure adapted for 

 a particular purpose — it would seem to have acted differently to 

 produce the same results, and hence not to have produced any of 

 the more extended groups, as families, where hundreds of species 

 are identical in a single feature. Witness the differences in di- 

 verse types of the tree-frogs, each type adapting its possessor to 

 an aboreal life : 



I. Claw-like, with globular base Hylid^. 



Lepiopelis. 

 II. Simple, obtuse-depressed at tip Ranid^. I an and III a. 



III. With a terminal transverse limb Ranid^, Hylarana et aff. 



Callula. 



Brach^merus. 



Hylodcs. 



IV. Bifurcate Balrachyla. 



Dendrobates. 



Polypedates. 



Rhacophoms. 



The short foot of the Testudinidse, where one row of pha- 

 langes is omitted, has been already alluded to. The gradual re- 

 duction of this set of bones, accompanying general modification 

 of form in the increased convexity of dorsal region, as we leave the 

 more aquatic and progress toward the terrestrial tortoises, would 



