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 réle, 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 
