INVASION 231 



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

 w . . . 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 mesophytic tendency, B,^n the course of migration find themselves 

 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. 



230. Proofs of polygenesis. In affirming the probability of a polyge^rtic 

 origin of species, there is no intention of asserting that all species origi- ate 

 in this way. It seems evident that a very large number of species of re- 

 stricted range are certainly mcnogenetic, 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 ex- 

 tensive area, in which recur instances of the same or similar habitat. The 

 relative frequence and importance of the two methods can hardly be con- 

 jectured as yet, but origin by monogenesis would seem to be the rule. 



The arguments adduced by Engler in support of polygenesis are in 

 themselves conclusive, but the investigations of the past decade have brought 

 to light additional proofs, especially from the experimental side. In de- 

 termining the physical factors of prairie and mountain formations, and 

 especially by methods of experimental ecology, the author has found that 

 habitats are much less complex than they are ordinarily thought to be, 

 since w-ater-content and humidity, and to a less degree light, constitute the 

 only factors which produce direct modification. In addition, it has been 

 ascertained that the minimum difference of water-content, humidity, or light, 

 necessary to produce a distinguishable morphological adjustment is much 

 greater than the unit differences recorded by the instruments. In short, 

 the differences of habitats, as ascertained by thermograph, psychrometer 

 and photometer, are much greater than their efficient differences, and, with 

 respect to their ability to produce modification, habitats fall into relatively 

 few categories. A striking illustration of this is seen in the superficially 

 very different habitats, desert, strand, alkali plain, alpine moor, and arctic 

 tundra, all of which are capable of producing the same type of xerophyte. 

 It follows from this that many more or less plastic species of extensive 



1901). Since polygenesis expresses the idea of origin, and applies to multiple origin 

 in time as well as in space, it is retained as the name of this concept. Polytopic and- 

 monotopic .are adopted for multiple and single origin in space respectively, and poly- 

 chrome and monochrome are proposed for similar origin in time. 



