Evolution 335 



exact spot of origin is not fully known, although some maintain it must 

 have been North America. They base this conclusion chiefly on the 

 fact that the greater part of the evolution of the horse did occur on this 

 continent. Wherever it did arise, Equus soon had migrated throughout 

 Europe, Asia, Africa and all parts of North America. One major mys- 

 tery — probably never to be solved — is what happened to the horses in 

 North America ? At the time of the arrival of the first white men there 

 was none. Thus the horse had to be reintroduced to the land of its origin. 



SOME SPECIAL ASPECTS OF EVOLUTION 



It has been emphasized repeatedly in this discussion that varia- 

 tion within a population occurs in all possible directions. Many of 

 these variations naturally are doomed to extinction while others have a 

 high survival value. Often the examination of some animal reveals 

 characters that are extreme in their adaptation to an unusual situation. 

 At times such adaptations are very similar to those found in the ani- 

 mals of a totally unrelated phylum. These characters can be either 

 of structure or of habit. 



Convergence. — The random variations of characters are of value 

 to the individual only as they allow for successful survival in any one 

 environment ; thus the environment is of great importance in determin- 

 ing the direction of evolution. This is clearly demonstrated by the fact 

 that animals living in a single environment often tend to have like 

 characteristics even though they may belong to completely unrelated 

 phyla. Such development is termed convergence. 



Innumerable examples of convergence may be cited. Animals 

 dwelling within caves usually are without or have only reduced eyes 

 and are white in color. These characters can be seen in fish, insects, 

 spiders, and crustaceans. Obviously such characters did not have com- 

 mon origins, but were independently achieved. Such characters un- 

 doubtedly appear as mutations in populations outside the cave, but 

 inasmuch as the animals would not survive competition, they would 

 not become part of the breeding population. Such a mutation would 

 thus be lost. In the cave, however, color is of no importance and 

 eyes are useless. 



Remarkable convergence also occurs among aquatic vertebrates. 

 Streamlining resulting in a torpedo-shaped body is found in several 

 different classes. These include most fish, the now-extinct reptilian 



