244 EVOLUTION IN NATURE AND UNDER DOMESTICATION. 



The isolation required to make the proportion of intra-spe- 

 cif ic matings to inter-specific ones sufficiently high to make a 

 species of a number of individuals is brought about by a good 

 many factors, which sustain and counteract each others action. 

 Actual geographic barriers are not necessary to keep species 

 apart. Organisms with a quick rate of reproduction and with a 

 quick rate of dispersal will probably require actual barriers to 

 keep species apart. But it is evident that a slow moving 

 group of organisms may be differentiated into several species 

 without the existence of geopgraphic barriers of any kind. 



When we speak of isolation as of the necessary requirement 

 for species-formation we do not quite follow Wagner in assum- 

 ing the need of an actual isolation in space. Isolation simply, 

 is the most common cause for the required proportion between 

 the number of intra and inter-specific matings. But it is evi- 

 dent, that any other cause bringing about this proportion 

 acts in this way. 



In a slowly dispersing animal the chances for individuals 

 wandering out of the territory of their species into that of a 

 neighbouring one is small. Snails are for this reason especially 

 apt to form local species, very much more so than weasels. 



New species can only originate in the territory occupied by 

 a parent-species, or by a closely related one, or they can only 

 reenter this territory if they are for some reason protected 

 from crossing freely with it. By far the most common case 

 will be the one, in which a new species fits into a somewhat 

 different ecological niche, but by no means the only one. A 

 difference in size may preclude crossing to an extent suffic- 

 ient specific distinctness. Or a difference in mating season, or 

 one in the structure of the sexual organs may have this influ- 

 ence. The case of the two house-rats differing in size in Java 

 and British India, is a good instance of two distinct species, 

 very closely related, fitting the same ecological niche and yet 

 remaining distinct as species. 



If we should state with Jordan that two very closely related 

 species can only occur separated by a barrier, or if we enlarge 



