56 THE EVOLUTION OF MAN. 



passes lengthwise through the whole body of the embryo. 

 It is developed at a very early stage, and is the first form?,- 

 tion of the spine, the firm axis of Vertebrates. In the 

 Lancelet (Amphioxus), the lowest of all Vertebrates, the 

 entire inner skeleton is limited to this Chorda throughout 

 life. But in Man and all the higher Vertebrates, first the 

 spine, and later the skull, are developed round this cord. 



Important as these and many other discoveries of Baer's 

 were in the Ontogeny of Vertebrates, yet the great im- 

 portance of his researches rested especially on the fact that 

 he was the first to apply the comparative method to the 

 study of the evolution. It was, of course, the Ontogeny of 

 Vertebrates, and principally of Birds and Fishes, that Baer 

 first and especially investigated. Yet he by no means 

 limited himself to these ; for he included various Inverte- 

 brates in his investigations. The most general result of 

 these comparative embryological researches was that Baer 

 assumed four totally different courses of evolution for the 

 four principal groups of the animal kingdom. These four 

 chief groups, or types, which at that time had just begun 

 to be distinguished, in consequence of George Cuvier's 

 researches in Comparative Anatomy, are : (1) Vertebrates 

 (Vertebrata) ; (2) Articulated animals (Arthropoda) ; (3) 

 Soft-bodied animals (Mollusca) ; and (4) the lower animals, 

 which at that time were all erroneously grouped under the 

 term Radiata. Cuvier, in the year 1816, demonstrated for 

 the first time that these four groups of the animal kingdom 

 show very essential and typical distinctions in their whole 

 inner structure, and in the arrangement and position of the 

 organic systems; that, on the other hand, the internal 

 structure of all animals of one type, for example, of all Ver- 



