ITS ZOOLOGICAL IMPORTANCE. 123 



ble each other more strongly in proportion as we examine 

 them at an earlier period. We have already stated that, 

 during almost the whole period of embryonic life, the young 

 fish and the young frog scarcely differ at all : so it is also 

 with the young snake compared with the embryo bird. The 

 embryo of the crab, again, is scarcely to be distinguished 

 from that of the insect ; and if we go still farther back in 

 the history of development, we come to a period when no 

 appreciable difference whatever is to be discovered between 

 the embryos of the various departments. The embryo of the 

 snail, when the germ begins to show itself, is nearly the 

 same as that of a fish or a crab. All that can be predicted 

 at this period is, that the germ which is unfolding itself 

 will become an animal ; the class and the group are not yet 

 indicated. 



321. After this account of the history of the develop- 

 ment of the egg, the importance of Embryology to the 

 study of Zoology cannot be questioned. For evidently, if 

 the formation of the organs in the embryo takes place in an 

 order corresponding to their importance, this succession 

 must of itself furnish a criterion of their relative value 

 in classification. Thus, those peculiarities that first ap- 

 pear should be considered of higher value than those 

 that appear later. In this respect, the division of the Ani- 

 mal Kingdom into four types, the Vertebrates, the Articu- 

 lates x the Mollusks, and the Radiates, corresponds perfectly 

 with the gradations displayed by Embryology. 



322. This classification, as has been already shown (61), 

 is founded essentially on the organs of animal life, the 

 nervous system and the parts belonging thereto, as found in 

 the perfect animal. Now, it results from the above account, 

 that in most animals the organs of animal life are precisely 

 those that are earliest formed in the embryo ; whereas those 

 of vegetative life, on which is founded the division into 



