CAIN : CENTER OF ORIGIN 139 



It is reasonable that, with migration from the center of origin, a species popula- 

 tion may encounter more favorable conditions than those that prevailed where 

 it arose. Hulten (1937) says concerning the "mass center" hypothesis, "Christ 

 and other authors considered that a plant is likely to have originated in a dis- 

 trict where its most numerous individuals are now found. Heer already opposed 

 this view. It is natural that if a plant at the border of its perhaps wide ori- 

 ginal area should find favourable conditions and multiply freely, so that 

 numerous individuals are developed, such a phenomenon will afford no indica- 

 tion of the earlier history of the species." Such cases are apparently found in 

 certain weedy species of Tradescantia that have obtained wide areas and rela- 

 tively high abundance in the eastern grassland and agricultural areas (Ander- 

 son and Woodson, 1935). Also, as with criterion 1, we can conceive of climatic 

 deterioriation causing a reduction in numbers of individuals at the center of 

 origin. 



Shreve (1937) has pointed out that shrubs of the Sonoran desert with 

 hard wood, sparse branching, and determinate growth (Cassia, Mimosa, 

 Acacia, Croton, Karzmnskia, Caesalpinia, Lysiloma, Bauhinia, Acalypha, etc.) 

 belong to genera which are well represented in the thorn-forest, both with 

 respect to numbers of species and abundance of individuals. Furthermore, dis- 

 tributional data indicate that this type has spread from the thorn-forest into the 

 desert. However, Shreve (1934) has clearly shown for Larrea tridentata and 

 Franseria dumosa what is probably a widespread relationship— that variations 

 in plant size and abundance, and degree of dominance are correlated with en- 

 vironmental conditions, and not with the center of origin. 



It is of interest to inquire further into certain characteristics of the distribu- 

 tion of individual plants. Gleason (1925) has studied this matter statistically 

 and concluded that environmental differences are not of sufficient magnitude 

 to affect the distribution of the species within an association, and that the num- 

 ber of individuals of a species, other things being equal, is an index to its adap- 

 tation to the environment. But what, we may ask, is the behavior of the species 

 outside its native association, or at the margin of its range ? When the area of 

 a population of a new species or subspecies is expanding from its center of 

 origin, and when natural barriers have not yet established a boundary, there 

 will naturally be a centrifugal decrease of density. This would seem to be an 

 inevitable result of numbers and random dispersal, and to provide a case in 

 which the criterion is true. Let us assume, however, that a species population 

 has extended its area to its maximum, having met barriers of one sort or 

 another on all sides. Under these conditions it would seem that there would be 

 a tendency for a greater density of individuals to exist toward the center of 

 area because of a central harmony between ecological requirements and ecolog- 

 ical conditions. Everywhere outside of this central "typical" climatic region 



