INTRODUCTION. XXXV 



sents the characters that are common to all, in a more 

 remarkable and complete manner than the rest ; and this 

 is called the type of the group. Thus each genus has its 

 typical species ; each family its typical genus, and so on ; 

 the type being, in each instance, that form to which our 

 minds naturally revert as best exhibiting the characters 

 that belong to the entire group. Some are very close 

 to the type, others differ from it to such a degree that we 

 might have failed to recognise the connection, were it not 

 from the presence of intermediate links. These are called 

 aberrant forms. 



It may be asked, are these divisions, which we have here 

 indicated, natural, i. e. marked out by nature, or, in other 

 words, designed ? That some of them are so, we may, I 

 think, safely infer from the example already quoted of the 

 Vertebrata. Here we have at least four, some say five, 

 great divisions marked out by nature so broadly that the 

 distinctions are in most cases recognizable and patent to 

 all; and, in each of these classes so clearly marked, 

 there are certain divisions apparent even to the un- 

 instructed ; such for example, among birds, are the Birds 

 of Prey, Owls, Finches, Game birds. Ducks, &c., &c. 

 Many genera, too, are undoubtedly exceedingly natural 

 and clearly defined ; and, on the whole, I think, we may 

 conclude that nature herself (could we but correctly read 

 her lessons) has pointed out most of the divisions ; or, in 

 other words, has varied each group, small as well as great, 

 in a certain and definite method.* Many natural divisions 

 however appear to grade into each other, and have no 

 definite limits ; yet, for purpose of study, we must assign 

 limits and characters ; and the afiinities, by which they 



* This subject will be mooted more at largo in the Introduction to Mammalia. 



