43^ EVOLUTION, GENETICS, AND EUGENICS 



A good example of geographic races or sub-species arising in an 

 extensive area unbroken by effective barriers is seen in the case of the 

 wrens of South America. Wrens are found practically all over that 

 continent; but those in one region differ from those in others in color 

 patterns, size, proportions, and habits. In the absence of barriers all 

 these local races grade into each other. In certain regions, however, 

 that happen to be more than usually isolated, as when a high mountain 

 range intervenes, transitional forms are missing. The point is, how- 

 ever, that whether barriers are present or not, sheer distance apart 

 tends to produce local races. 



c) Climatic isolation. — There often occur distinct and more or less 

 abrupt climatic differences within the range of a species which may act 

 as barriers to migration or involve different adaptive changes on the 

 part of individuals living under these different climatic conditions. 

 Thus, without the presence of any geographic barriers, a species may 

 split up into a northern and a southern race or sub-species that are 

 genetically distinct, neither thriving in the territory occupied by the 

 other. Similarly, in the oceans there exist rather abrupt differences 

 in temperatures that act as effective barriers. The waters north and 

 south of Cape Cod, for example, have almost entirely different faunas. 

 The reason for this is that the shore-line north of Cape Cod is made 

 cold by the Labrador Current and that south of the Cape is more 

 influenced by the warm Gulf Stream. Species north of the Cape and 

 south of the Cape are almost as distinct as if they belonged to different 

 oceans. 



d) Biotic isolation. — The territory occupied by any species of ani- 

 mal or plant is always occupied by many other species. Some of 

 these associated species may occupy one part, some another part, of 

 the range of the species in question. Now each species affects all the 

 other species with which it is spatially associated; each species is a 

 part of the biotic environment of all the others. If, then, the biotic 

 environment varies within the range of the species, the pressure of 

 selection will vary also, certain characters or combinations of char- 

 acters being more favorable for one biotic environment than another. 

 This being the case, groups in different biotic environments will tend 

 to be isolated from one another and will diverge in both adaptive and 

 non-adaptive directions. 



e) Reproductive isolation. — The simplest type of reproductive iso- 

 lation is one that involves changes due to mutations in the copulatory 

 organs. In some insects, for example, the copulatory organs are con- 



