LECTURE VII 



THE ORIGIN OF VERTEBRATED ANIMALS 



In order to illustrate the conclusions arrived at in 

 previous lectures, and to test the validity of the 

 Darwinian theory, I propose to apply it, in a more 

 detailed manner than we have yet considered, to one 

 group of animals. The group selected is that of 

 Vertebrates, as being a well-defined group, consist- 

 ing of forms which are familiar and usually of com- 

 paratively large size ; and also for the reasons that 

 the fos sil ^Jorms in this group are numerous and 

 characteristic, and the embryology of the group has 

 been worked out with more detail than in the Inver- 

 tebrates. Moreover, Vertebrates have a special 

 interest from the fact that it is to this group that 

 man himself belongs. 



The problems we have before us are, first, to 

 determine the mutual affinities of different groups 

 now living ; secondly, to determine the relations of 

 Vertebrates to other animals ; this being the less 

 important of the two problems at the present time. 

 The evidence available is of three chief kinds : 

 (i) Comparison of Structure. — For example, a cat 

 and a dog are clearly more closely allied to one 

 another than is either of them to a fish or a bird. 



