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COLLEGE ZOOLOGY 



Figure 439. The Ancon (short-legged) mutation in sheep (ewe in center, ram at right) com- 

 pared with a normal ewe at left. This is the earliest recorded mutant among domestic animals. 

 (Courtesy of Life magazine.) 



sible for changes in the structure, physiology, 

 or behavior of the organism. Such changes 

 are now known to be numerous but small, 

 and not rare or always conspicuous as was 

 formerly supposed. How they arise is not 

 known. They are inherited, however, and 

 may be preserved or eliminated by natural 

 selection. Thus, gene mutations are one 

 source of the variability among animals 

 which make possible the appearance of bet- 

 ter-adapted individuals, and hence the evo- 

 lution of the group. 



Isolation as an 

 evolutionary factor 



Isolation of a group of animals from 

 others of their kind seems to favor the prog- 

 ress of evolution in a species. This idea first 

 originated as a result of the study of the 



faunas of oceanic islands. Such islands are 

 usually inhabited by species that differ from 

 those on the nearest mainland. Birds, bats, 

 and insects are common inhabitants, no 

 doubt, because wings give them the neces- 

 sary powers of locomotion to reach islands, 

 but amphibians and mammals, other than 

 bats, are generally absent. Often the species 

 of birds and insects on islands have become 

 wingless. 



Another example of isolation that has 

 been carefully studied is that of snails living 

 in the valleys along the sides of the moun- 

 tains on Oahu in the Hawaiian Islands. 

 They live in trees and ordinarily do not cross 

 over the ridges from one valley to another. 

 Practically every valley is inhabited by its 

 own particular species or subspecies of snails, 

 which differ in size, shape, or color of shell, 

 but which are closely related. Chance colo- 



