OTHER THEORIES OF SPECIES-FORMING 269 
first stages before they attain selective value; secondly, it would make 
correlated adaptations feasible by supplying ontogenetic (individually 
acquired) modifications, until the material for the appropriate germi- 
nal adaptations arose. 
“Tt has been objected to this ee that, since the individually 
acquired modifications possess the main selective value in these 
instances, there is no reason why the corresponding germinal variations 
should be fostered at all. The individuals with the right, but slight, 
congenital variations would have no advantage over their fellows who 
show no such coincident variations. Nor is there any ground to 
assume that individuals with the greatest amount of plastic modifica- 
tion in a given direction will tend to exhibit similar innate variations 
to a greater degree than those individuals not possessing this plas- 
ticity.” 
ISOLATION THEORIES 
One of the objections to natural selection was that a favorable 
variation appearing in one or a few individuals would be lost because 
the individuals possessing it would interbreed with those not possessing 
it, which presumably would be much more numerous. If there were 
any kind of agency whose effect would be a partial or complete inhibi- 
tion of intercrossing, the favorable character would have a chance to 
survive. 
Several related theories have arisen that deal with possible isola- 
tive or segregative agencies that might serve to prevent promiscuous 
intercrossing, and these have received the names geographic isolation, 
climatic isolation, reproductive isolation, and physiologic isolation. 
Geographic isolation.—Moritz Wagner was the founder of this 
theory. He was a very extensive traveler and had a vast knowledge 
of the details of the geographic distribution of animals. He believed 
that isolation was absolutely essential in the differentiation of species. 
He at first thought of his theory as auxiliary to natural selection, but 
later, strongly impressed by the facts he had collected, he concluded 
‘that isolation was an independent and alternative explanation of 
species-forming. The underlying idea is one that has already received 
attention in chapter vii, under “Evidences from Geographic Distri- 
bution.” Any successful species tends to spread in all directions until 
checked by barriers. Some few members of a species under favorable 
conditions may surmount the barrier and become isolated. The result 
1 From S. Herbert, First Principles of Evolution (1913). 
