ADAPTATION OF ORGANISMS 313 



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 carrying on of the life functions. We find organ- 

 isms adapted to the greatest depths of the ocean where the 

 water pressure is several hundred atmospheres 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 renders them less 

 dependent on a constant barometric pressure. 



These few examples must suffice to emphasize the general 

 environmental 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 condi- 

 tions so that there is scarcely a niche in nature untenanted. 



2. Adaptations Essentially Structural 



We may now broaden our view of the plasticity of organ- 

 isms by a brief consideration of adaptations which are 

 essentially structural. But here as elsewhere it is absolutely 

 impossible to divorce structure and function which, ob- 

 viously, are only reciprocal aspects of the fitness of living 

 creatures. 



ADAPTIVE RADIATION OF MAMMALS. In the group of 

 Eutherian Mammals, forms are to be found which are ex- 

 traordinarily modified in adaptation 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 different habitats and modes of life. (Fig. 

 161.) We may select a small Malayan insectivorous animal 

 known as Gymnura, which is allied to the Hedgehogs, as 



