THE GROWTH OF GROUPS 33 



hills, the range having been formed from a continuous 

 ridge. Species, as estimated by naturalists of the old 

 school or by sportsmen, are things separated by wide 

 gaps which are for the most part the results of the 

 elimination of intermediate forms which occupied them, 

 but the gaps between our modern species are very much 

 narrower. We all see an immense difference between a 

 horse and a fish, a great difference between a horse and 

 a dog, a smaller but obvious difference between a horse 

 and an ass, there is a difference between a chestnut and 

 a black horse, and an appreciable difference even between 

 two black horses. We speak of these degrees of difference 

 as gaps of varying width, they are obvious to every one, 

 although we cannot devise a unit to measure their width. 

 In the wide gaps are placed many intermediate forms, 

 some living, others extinct, but in the narrowest gaps 

 there is no room for intermediates. We may be sure 

 that there were extinct forms between the horse and ass, 

 but we may well doubt that black and chestnut horses 

 stand out as groups because of the elimination of inter- 

 mediates. 



The selection theory in its various aspects includes a 

 number of propositions, some of which may be called 

 broad statements, others narrow or precise. The fol- 

 lowing statements are examples of the two kinds : 



1 (broad). The gaps between the various forms of 

 organism are the result of the elimination or extinction 

 of intermediate forms. 



2 (precise). Groups of like organisms cannot come 

 into being as groups in any other manner than by the 

 elimination of intermediate forms. 



The first of these propositions is not in dispute, it is 



D 



