100 HISTORY OF CRUSTACEA. Chap. X. 



formed in the sequence of their organic importance ; the 

 most essential always a])]pear first. Thus the organs of 

 vegetative life, the intestine, &c., appear later than 

 those of animal life, the nervous system, skeleton, &c., 

 and these in turn are preceded by the more general 

 phenomena belonging to the animal as such " (§ 318). 

 " Thus, in Fishes, the first changes consist in the seg- 

 mentation of the vitellus and the formation of a germ, 

 processes which are common to all classes of animals. 

 Then the dorsal furrow, characteristic of the Vertebrate, 

 appears — the brain, the organs of the senses ; at a later 

 period are formed the intestine, the limbs, and the per- 

 manent form of the respiratory organs, from which the 

 class is recognised with certainty. It is only after ex- 

 clusion that the peculiarities of the structure of the 

 teeth and fins indicate the genus and species" (§ 319). 

 '^ Hence the embryos of different animals resemble each 

 other the more, the younger they are'' (§ 320). " Conse- 

 quently the high importance of developmental history 

 is indubitable. For, if the formation of the organs takes 

 ^lace in the order corresponding to their imjjortance, this 

 sequence must of itself be a criterion of their compara- 

 tive value in classification. The peculiarities which 

 appear earlier should be considered of higher value 

 than those which appear subsequently" (§ 321). "Ä 

 system, in order to he true and natural, must agree with 

 the sequence of the organs in the development of the 

 embryo'' (§322). 



I do not know whether any one at the present day 

 will be inclined to subscribe to this proposition in its 



