6 THE GROWTH OF GROUPS 



There is, in my opinion, no such thing as an indivisible 

 group of vertebrate animals. 



HOW ARE SPECIES DEFINED? 



What is a species among vertebrate animals ? Species 

 are groups that have been defined as such, but these 

 groups are not equivalent things ; some are characterized 

 by many obvious attributes, others by a few less obvious 

 ones. Those groups, which are being described as species 

 at the present day, are for the most part of less weight 

 than those described in the past. But although the 

 conception has changed, species are merely groups that 

 have been defined and the problem is to describe how 

 they came to be as they are. In defining a group we are 

 at liberty to regard any characters we choose. Naturally, 

 we select the most obvious ones first. We select a number 

 of obvious characters and set up a convention in the 

 form of an assertion thus, " Every animal in the world 

 which possesses characters A, B, C, etc., is of the species X." 

 The systematist defines species in this manner, but in 

 doing so he does not claim to have described every 

 attribute which can be perceived by any means in every 

 individual of the species, he has merely selected a number 

 of obvious characters and defined the species by declaring 

 that every animal possessing them is of the species. In 

 this sense a species is a verbal description. In referring 

 to a species it is considered necessary to add the name of 

 its author ; by the species we mean the author's descrip- 

 tion of it. The author of any vertebrate species must 

 feel sure that he has overlooked a number of microscopic 

 or unobtrusive characters which are also present in every 



