34 THE GROWTH OF GROUPS 



accepted as true by almost all naturalists, but opinion 

 concerning the second proposition is sharply divided. 

 To return to our former illustration, the horse and ass 

 are, broadly speaking, animal forms, the great gap between 

 them was no doubt occupied by intermediate forms 

 which have become extinct ; this is not in dispute, but 

 within the group indicated by the word horse, colour is 

 a group distinction. There are chestnut and black horses, 

 and each kind constitutes a group of like animals. It may 

 well be doubted whether such groups are the result of 

 the elimination of intermediate coloured forms. In fact 

 the second or precise proposition appears to be untrue. 



This particular example is used here merely to illus- 

 trate the grounds of dispute. The distinction between 

 black and chestnut horses is scarcely typical of those 

 distinctions which are commonly used to separate our 

 modern species, by many it would be called a varietal 

 distinction. However, when searching among the multi- 

 tude of animal forms it is impossible at present to say 

 with confidence that one is a variety, another a species. 

 Whether there is an essential difference between them 

 or not, each species or variety is a group of like organisms, 

 and we wish to know how they become groups. 



It is indisputable that the conception of a species 

 has changed considerably of late years. It often happens 

 that a species defined fifty years ago is redefined as a 

 genus containing a large number of species. The horse 

 and ass are typical of the older idea of species, but not 

 of our modern species, the gaps between the latter are 

 much smaller. Frequently they are inappreciable by 

 the casual observer, often they are so small that it is 

 difficult to imagine that they were ever occupied by 



