HISTORICAL ACCOUNT OF EVOLUTION THEORY 33 



that it is a major factor of as great importance in species forming, or 

 nearly so, as natural selection. But the prevailing opinion seems to be 

 that isolation is really a kind of selection, more like artificial selection 

 than anything else, which separates out certain pure lines and prevents 

 promiscuous interbreeding. Various agents are known to produce 

 isolation by erecting barriers to interbreeding between groups of 

 individuals within a species. These segregative factors may be 

 geographical, climatic, reproductive, physiological, or, in plants, the 

 result of soil diversity. Thus a mountain range, on the two sides of 

 which a species migrates, effectively separates the species into two 

 independent groups. Heat, cold, moisture, etc., separate others. 

 Reproductive incompatibility between new and older types is equally 

 effective, as is assortative mating of like with like. Like natural selec- 

 tion, isolation has nothing to do with the origin of new types, but 

 merely aids in the preservation of types when once formed. Were 

 there not spontaneous variations among animals and plants, there 

 would be nothing to isolate. Therefore isolation plays only an 

 auxiliary role, helping to preserve new races once they are formed. 



ORTHOGENESIS THEORIES 



"The orthogenetic evolution theories of various authors, based 

 upon the assumed occurrence of variations in determinate lines or 

 directions (a restricted and determinate variation as compared with 

 the nearly infinite, fortuitous, and indeterminate variation assumed 

 in the selection theories), are of several types. The mention of two 

 will reveal pretty well the more important characters of all. Not a 

 few biologists have always believed in the existence of a sort of mystic, 

 special vitalistic force or principle by virtue of which determination 

 and general progress in evolution is chiefly fixed. Such a capacity, 

 inherent in living matter, seems to include at once possibility of pro- 

 gressive or truly evolutionary change. Not all evolution is in a single 

 direct line, to be sure; ascent is not up a single ladder or along a single 

 geological branch, but these branches are few (as indeed we actually 

 know them to be, however the restriction may be brought about) 

 and the evolution is always progressive, that is, toward what we, 

 from an anthropocentric point of view, are constrained to call higher 

 and higher or more ideal life stages and conditions. 



"Other naturalists also seeming to see this source of determinate 

 or orthogenetic evolution, but not inclined to surrender their dis- 

 belief in vitalism, in forces over and beyond the familiar ones of the 



