GENERA OF THE ARCIFEROUS ANURA. 101 



This view has not been overlooiced by Darwin, who, however, treats of it very 

 briefly, and appears to attach it to the theory of adaptations, or modification for a 

 physiological purpose. He says. Origin of Species, 388, (American Edition, 1860) : 

 " We may extend this view to whole families or even classes. The fore limbs, 

 which served as legs in the parent species, may become, by a long course of modifica- 

 tion, adapted in one descendant to act as hands, in another as paddles, in another as 

 wings; and on the above two principles, — namely, of each successive modification 

 supervening at a rather later age, and being inherited at a correspondingly late age, 

 — the fore limbs in the embryos of the several descendants of the parent species will 

 still resemble each other closely, for they will not have been modified. But in each 

 individual new species, the embryonic fore limbs will differ greatly from the fore 

 limbs in the mature animal ; the limbs in the latter having undergone much modifi- 

 cation at a rather late period of life, and having thus been converted into hands, 

 paddles or wings." He then inclines to assign this change to the necessity of exter- 

 nal 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 (p. 382) : 

 " We cannot, 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 con- 

 ditions 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." 



Among the higher groups of animals can be detected series " homologous " on the 

 same principle as the alcohols (? compound radicals) and their derivatives ; and the 

 component types of each can be, and have been in many instances, shown to be 

 " heterologous," as are the ethers, — mercaptans, aldehydes, acids, etc. Among 

 the Mammalia two (homologous series) have been pointed out : Implacentialia and 

 Placentialia;* possibly such are the types Altrices and Prgecoces among Aves. Of a 

 lesser grade in this class are the parallel series of Pullastrse and Gallinse, of Clama- 

 tores and Oscines. Among Tortoises I have alluded to the Pleurodiraf as compared 

 with the remainder of the order, already parallelized by Wagler ; and of lesser grade, 

 the series among Lacertilia of Acrodonta and Iguania, parallelized by Dumeril and 

 Bibron, and of Teidae and Lacertidge, compared by Wiegraann : I have discovered a 



* Perhaps more exist, as Flower and Huxley cast doubts upon some of the supposed distinctions of these 

 two. Professor Dana's Megasthenes and Microsthenes may also be such, but they lack as yet the necessary 

 anatomical demonstration. 



t Proc. Acad. Philada., 1864. 



26 



