INVASION 231 
In I, a species A, becomes scattered over a large area in a series of places, 
m. . . mm", 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, in the course of migration find themselves 
respectively in a more mesophytic habitat, #i, and a more xerophytic one, +, 
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 polygerctic 
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 monogenetic, 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 water-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. 
lt 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- 
chronic and monochronic are proposed for similar origin in time. 
