320 



ANIMAL BIOLOGY 



Pressure. The metabolism of organisms, in common with 

 chemical processes in general, is influenced by the surrounding 

 mechanical pressure. Therefore it is evident that the pressure 

 of either the water or air plays an important part in the operation 

 of the life functions. We find organisms adapted to the greatest 

 depths of the ocean where the pressure is several tons to the 

 square inch — so great that some forms burst when rapidly 

 brought to the surface; while others are adapted to live at high 

 altitudes where the air pressure is relatively low. And again, the 

 higher Vertebrates present an adaptive mechanism which ren- 

 ders them less dependent on a constant atmospheric pressure. 



These few examples must suffice to emphasize the general en- 

 vironmental conditions which are necessary for life, as we know 

 it, to exist, and to suggest that within these broad limits organisms 

 are adapted to special environmental conditions so that there is 

 scarcely a niche in nature untenanted. 



2. Adaptations Essentially Structural 



We may now broaden our view of the plasticity of organisms 

 by a brief consideration of adaptations which are essentially struc- 



Fig. 201. — Gymnura. (From Horsfield and Vigors.) 



tural. But here as elsewhere it is absolutely impossible to distin- 

 guish sharply between structure and function which, obviously, 

 are only reciprocal aspects of the fitness of living creatures. 



Adaptive Radiation of Mammals. In the group of Mammals, 

 forms are to be found which are extraordinarily modified in adapta- 

 tion to the most diverse environmental conditions. From a more 

 or less primitive type, or focus, there radiate, as it were, types 

 which are specialized for very different habitats and modes of 



