14 THE MODERN STUDY OF ZOOLOGY 



feature of the older system, are here swept 

 away ; the resemblances of animals are made 

 of more weight than their differences ; and no 

 attempt is made to define the limits of the several 

 groups. 



This doctrine of animal types was first brought 

 forward prominently by Cuvier and Von Baer at 

 the commencement of the present century. Cuvier, 

 in his latest system of classification, distinguished 

 four leading types or plans of structure in the animal 

 kingdom, to one or other of which all animals could, 

 according to him, be referred. Mainly owing to 

 the weight of Cuvier's authority this doctrine of 

 types made considerable progress during the first 

 half of the present century ; it never, however, 

 wholly replaced classification by definition, and 

 probably never would have done so ; for, in the 

 first place, the essence of classification is conveni- 

 ence, and classification by definition is far more 

 convenient for the ordinary purpose of a zoologist 

 than classification by type ; and, secondly, although 

 the idea of types expressed a great and important 

 truth ; yet it was but the partial expression of a 

 still greater one, which, when fully developed, 

 was destined to completely overthrow all former 

 attempts, and to reveal the only true and un- 

 assailable basis of classification. The gradual 

 rise of this new doctrine we have now to notice 

 briefly. 



About the commencement of the present century 

 two new influences began to make themselves felt 

 in zoology, two new branches that were afterwards 



