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. 



"It has been objected to this theory 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." 1 



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). 



