POLYGENESIS 



73 



following diagrams will serve to make the above distinctions 

 more evident : 



I. Polygenesis II. Polyphylesis III. Monogenesis 



(Monophylesis) 



7h^ 



8 a a 



c a. 



In I, a species A, becomes scattered over a large area in a 

 series of places, m . , . m"^ , with the same physical factors, 

 in any or all of which may arise the new species a. In II, 

 a species with xerophytic tendency, A, and one with meso- 

 phytic tendency. B, in the course of migration find them- 

 selves respectively in a more mesophytic habitat, m, and a 

 more xerophytic one, x, in which either may give rise to the 

 new form, c, which is more or less intermediate between A 

 and B. In III, the method of origin is of the simplest type, 

 in which a species is modified directly into another one, or 

 is split up into several. 



In afflrming the probability of a polygenetic origin of 

 species, there is no intention of asserting that all species 

 originate in this way. It seems evident that a very large 

 number of species of restricted range are certainly mono- 

 genetic, at least as far as origin in space is concerned : it is 

 possible that any species may arise at two or more distinct 

 times. Polygenesis can occur readily only in species of 

 more or less extensive area, in which recur instances of the 

 same or similar habitat. The relative frequence and 



