338 Cell as Basis oj Organic Activity 



from more vigorous forms or the fact that their particular position in 

 the habitat is already occupied. The lack of some one food or the 

 presence of a parasite may also prevent further spread of any form. 



Isolation. — Once any group of animals has spread into a new 

 area, it may become isolated by some barrier such as those mentioned 

 above. In such a case, it will proceed to evolve in a manner which 

 may be quite unlike that of its near relatives in other areas. 



TO ALL PARTS 

 OF OLD WORLD 



/ 



THE WANDERINGS OF HORSES 



Fig. 107. — The migration of horses. The origin of the horse family is unknown, 

 but after their origin, North America became their home. From here they spread 

 periodically to other parts of the world over two land bridges: the Bering Bridge 

 and the Panama Bridge. (From Horses by George Gaylord Simpson. Copyright, 

 1951, by Oxford University Press, Inc.) 



(Continued from opposite page.) 



is an isolated species of Cocos Island finch. The ground finches are mainly seed- 

 eaters; 9, 10, and 11 are large, medium, and small in size; 12 is sharp-beaked; 

 13 and 14 are cactus eaters. These finches, isolated on the Galapagos Islands 

 (600 miles west of Ecuador), appear to be derived from a single colonizing form. 

 They still bear close resemblance to one another, but their beaks are adapted to 

 different diets. This indicates a method by which new species may arise when a 

 single form is isolated, in a new environment. (From the Scientific American, 

 drawing by Eric Mose, in article, Darwin's Finches by David Lack.) 



