76 GENERAL EVOLUTION. 



certain features whicli are only ordinal or generic in meaning, yery 

 erroneous conclusions may be reached by the inexact student as 

 to the want of parallelism of classes to each other. Thus Rathke 

 says of the development of the eye of the snake Tropidonotus, at 

 a certain period, that it is far in advance of that of the mammal 

 at the same stage. Here, says the objector, is a case where their 

 parallelisms do not coincide ; the mammal is really similar to a 

 younger stage of the reptile. 



But, in fact, the size of the eye is but a generic or family char- 

 acter ; if the development of the lemur had been compared with 

 the snake, the mammal would have been found to be in advance ; 

 of the mole, much farther behind. If the snake selected were the 

 purblind Atractaspis, almost any mammal would have been in 

 advance ; if, on the other hand, the great-eyed Dii3sas, but few 

 Mammalia would have been parallel to it. 



In a word, to find exact parallelism it is necessary to examine 

 the closest allies. 



It is also of first importance to distinguish between the exist- 

 ence of generic or higher characters, and their condition under 

 various circumstances of individual life. If a foetal or larval 

 character be conserved through the adult life of a type, it will be 

 of course adapted to the functions of mature age. Thus the un- 

 developed character of the horns of the genus of deer, Rusa, are 

 not accompanied with the marks of individual youth of the cor- 

 responding stage of Cervus ; its individuals are fully grown and 

 functionally perfect. The species of Hyla are not small and in- 

 capable of self-preservation and reproduction, as is the corre- 

 sponding stage of Trachycephalus ; they are functionally devel- 

 oped. The student need not be surprised, then, if, when identity 

 or exact parallelism is asserted, he finds some differences depend- 

 ent on age and adaptation, for if he be an anatomist he need not 

 be informed that a morphological relation constitutes types what 

 they are, not a physiological. 



II. OF KETARDATIOK AI^D ACCELEEATIOK 11^ GENERIC CHAE- 



ACTEES. 



First. Of adult metamorphosis : 



The question has necessarily arisen, Have these remarkable 

 relations between genera resulted from an arrangement of distinct 

 generations according to a permanent scale of harmony, or have 

 the same genetic series of individuals been made to assume the 



