THE EPIC OF EVOLUTION 523 



before Mendel and his followers had laid bare the essential mechanism 

 of heredity. In a later paragraph, reference will be made to how 

 Darwin's great German disciple, Weismann, came to the rescue and 

 made the hypothesis of pangenesis unnecessary. 



Isolation 



"The nearest relative of any species is not to be found in the same 

 area, nor in a far distant area, but in a nearby area, separated from 

 it by barriers." This is the Law of Isolation as formulated by 

 David Starr Jordan. Unless some sort of isolation prevents or 

 minimizes the swamping effect of promiscuous interbreeding, a 

 newly "selected" species has difficulty in maintaining its indepen- 

 dence. 



There are first of all geographic barriers that lead to isolation, as, 

 for example, the water barrier when continental islands are cut off 

 from the mainland. In such cases, since the island types can no 

 longer breed with the continental forms from which they arose, there 

 is furnished an opportunity, due to isolation, for them to maintain 

 the modifications which make them different species. Oceanic islands 

 also illustrate the part isolation plays in establishing new species. 

 For instance, the volcanic island of Oahu, on which Honolulu is 

 situated, is fluted with valleys as the result of erosion, and each 

 valley, as Gulick has shown, has its own peculiar species of land snail 

 of the genus Achatinella. These snails live in trees and are isolated, 

 each species in its own valley, because the mountainous ridges be- 

 tween the valleys furnish a barrier to their intermingling with each 

 other. 



In addition to geographic barriers there are biological harriers that 

 provide isolation for newly formed species, protecting them from 

 the leveling effects of mixture with the parental stocks. Plants, 

 for example, may maintain their individuality, even while remaining 

 in the same habitat with contaminating relatives, by practicing 

 self-fertilization, or by establishing a different period of sexual 

 maturity. 



There are a dozen different species of albatross in the Southern 

 Hemisphere which mingle freely throughout the whole range of their 

 wanderings except during the breeding season, when the members 

 of each species are segregated in their own quarters to reproduce. 

 This behavior is true of migrating birds in general so that, so far as 

 breeding goes, there is virtual isolation among them. 



