806 



Comparative Morphology of Chordates 



Fig. 608. {Left) Paleolithic drawing of elephant showing heart; from the cavern 

 of Pindal, Asturias, northwestern Spain. {Right) Magdalenian drawings of bison 

 with arrows embedded in the heart; from the cavern of Niaux on the Ariege, 

 southern France. (After Cartailhac and Breuil. Courtesy, Singer: "The Evolution 

 of Anatomy," New York, Alfred A. Knopf, Inc.) 



definite lines of specialization, each resulting in a type of mammal 

 anatomically and physiologically highly adapted to a particular en- 

 vironment and mode of living. The group attains a high degree of 

 success, but within the limitations imposed by its specialization. Such 

 relatively unspecialized animals as the insectivores live where they 

 best can. Their continued survival is due partly to retreat from com- 

 petition and partly to great fecundity. 



Primates have hit upon an entirely unique line of specialization. 

 Retaining, with only minor modifications, the primitive "generalized" 

 structure of their insectivore ancestors, they act upon their environ- 

 ment in such a way that it not only more readily yields them the 

 primary necessities of living, but so that, in ever increasing degree, 

 they become able to extend their habitats and utilize energies external 

 to their bodies in the performance of a great diversity of activities of a 

 sort never before achieved by any animal. 



THE PRIMATE LINE OF SPECIALIZATION 



The primates have reversed the old order of evolution. From the 

 most ancient invertebrates onward, it has always been a tedious process 

 of structural change requiring thousands or millions of years for ani- 

 mals to emerge from aquatic to effective terrestrial living or to acquire 

 capacity for flight. Modification of environment has been limited to 



