OTHER THEORIES OF SPECIES-FORMING. 239 



homogeneous in their character, the members showing indi- 

 vidual fluctuation and climatic effects, but remaining uni- 

 form in most regards, all representatives slowly changing 

 together in the process of adaptation by natural selection. 

 In regions broken by barriers which isolate groups of indi- 

 viduals we find a great number of related species, though in 

 most cases the same region contains a smaller number of 

 genera or families. In other words, the new species will be 

 formed conditioned on isolation, though these same barriers 

 may shut out altogether forms of life which would invade 

 the open district. 



"Given any species in any region, the nearest related 

 species is not likely to be found in the same region nor in 

 a remote region, but in a neighbouring district separated 

 from the first by a barrier of some sort. 



"Doubtless wide fluctuations or mutations in every species 

 are more common than we suppose. With free access to the 

 mass of the species, these are lost through interbreeding. 

 Isolate them as in a garden or an enclosure or on an island, 

 and these may be continued and intensified to form new 

 species or races. Any horticulturist will illustrate this. 



''In these and in all similar cases we may confidently 

 affirm : The adaptive characters a species may present are 

 due to natural selection or are developed in connection with 

 the demands of competition. The characters, non-adaptive, 

 which chiefly distinguish species do not result from natural 

 selection, but from some form of geographical isolation and 

 the segregation of individuals resulting from it. 



"In the animal kingdom, generally, we may say: When- 

 ever a barrier is to some extent traversable, the forms 

 separated by it are liable to cross from one side to the other, 

 thus producing intergradations, or forms more or less 

 intermediate between the one and the other. For every 

 subspecies, where the nature of the variation has been care- 

 fully studied, there is always a geographical basis. This 



