232 THE FORMATION 
geographical area will find themselves in similar or identical situations, 
measured in terms of efficient differences, and will be modified in the 
same way in two or more of these. In mountain regions, where interrup- 
tion of the surface and consequent alternation are great, the mutual invasion 
of contiguous formations is of frequent occurrence, often resulting in 
habitat forms. The spots in which these nascent species, such as Galium 
boreale hylocolum, Aster levis lochmocolus, etc., are found, are often so 
related to the area of the parent species as to demonstrate conclusively 
that these forms are the result of polygenesis and not of migration. Na- 
turally, what is true of a small area will hold equally well of a large region, 
and the recurrence of the same habitat form may be accepted as conclusive 
proof of polygenesis. The most convincing evidences of multiple origin, 
however, are to be found in what DeVries has called “mutations.” It 
makes little difference whether we accept mutations in the exact sense of this 
author, or regard them as forms characterized by latent variability. The 
evidence is conclusive that the same form may arise in nature or in cultiva- 
tion, in Holland or in America, not merely once, but several or many times. 
In the presence of such confirmation, it is unnecessary to accumulate proofs. 
Polygenesis throws a new light upon many difficult problems of invasion 
and distribution, and, as a working principle, admits of repeated tests in 
the field. It obviates, moreover, the almost insuperable difficulties in the 
way of explaining the distribution of many polygenetic species on the basis 
of migration alone. 
281. Origin by polyphylesis. In 1898, the author first advanced a ten- 
tative hypothesis to the effect that a species homogeneous morphologically 
may arise from two distinct though related species. During subsequent 
years of formational study, the convinction has grown in regard to the prob- 
ability of such a method of origin. Since the appearance of Engler’s work, 
a polyphyletic origin for certain genera has been very generally accepted 
by botanists, but all have ignored the fact that the polyphylesis of genera 
carries with it the admission of such origin for species, since the former 
are merely groups of the latter. I can not, however, agree with Engler, that 
polyphvletic genera, and hence species also, are necessarily unnatural. If 
the convergence of the lines of polyphylesis has been great, resulting in 
essential morphological harmony, the genus is a natural one, even though 
the ancestral phyla may be recognizable. If, on the other hand, the con- 
vergence is more or less imperfect, resulting in subgroups of species more 
nearly related within the groups than between them, the genus can hardly 
be termed natural. This condition may, however, prevail in a monophyletic 
genus with manifest divergence and still not be an indication that it is 
artificial, 
