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. Tlie spots in which these nascent species, such as Galium 

 boreale hylocolum, Aster lezns 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 dift'erence 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 

 polyphyletic 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. 



