SOURCES OF THE VEGETATION 65 



distinctions it suggests the possible origin of new species from 

 the invading stock. 



As to whether a species is immigrant or indigenous is often 

 difficult or impossible to determine. Tn some cases it may be 

 apparent from geographical evidence that a species has entered 

 from a contiguous or distant area, but in others it may appear 

 to have originated within the area, and if so the question of 

 interest is the identity of the parent race or species, a fact which 

 may be suggested by a comparison with associated or neighboring 

 species, but which, if at all susceptible of proof, can be deter- 

 mined only by detailed structural comparisons or by experiment- 

 al means. In this paper such conclusions as may reasonably be 

 drawn from geographic evidence or other external facts will be 

 presented, but with appreciation of the uncertainties involved. 



With reference to the geographic evidence it would seem 

 that at least three considerations may be recognized. (1) The 

 edge or limits of the range of any species may fall within the 

 region under examination and if so its source may logically be 

 referred to the center from which it seems originally to have 

 emerged. Southward extensions along the Rocky Mountains of 

 boreal transcontinental species, or western margins of the ranges 

 of conspicuous eastern or middle states species lying along the 

 eastern base of the Rocky Mountains or eastern margins of 

 ranges in the case of typically Pacific Coast forms would be 

 cases in point. (2) The degree of development of a species as 

 representing the supposed center of its distribution. The Doug- 

 las spruce falls under this category. This species occurs in 

 extraordinary vigor and abundance along the Pacific Coast west 

 of the Cascades, whence it seems to have emerged eastward until 

 it has found its effective barrier in the plains between the Rocky 

 Mountains and the Mississippi River. While this is not usually 

 a safe argument in view of the well known facts of the luxuri- 

 ance of many introduced forms in various parts of the world, 

 it seems more logical in the case of naturally developed species 

 in their proper ranges to view them as indigenous to the regions 

 of their greatest and most varied development. (3) The geo- 

 graphic center in which the genus or family are conspicuous or 

 characteristic may be regarded as the source from which the 

 outlying species have emerged. The cacti and yuccas of the 



