INDIVIDUAL DIFFERENTIATION AND ADAPTATION. IO5 



II. Among animals of different species. 



1. Friendly and social. 



(a) Commensalism. 



(b) Symbiosis. 



2. Competitive. 



(c) The predaceous habit: adaptations for offense 

 and defense. 



(d) Parasitism. 



142. Adaptations to the Inorganic Environment. These 

 embrace such special structural devices as hair, feathers, the 

 blubber of whales, which enable the body to maintain its tem- 

 perature despite the condition of the medium. The habits of 

 burrowing and hibernation, the winter migrations of many 

 animals, especially birds, are examples of instinctive adapta- 

 tion to cold. The same end is obtained by man by artificial 

 clothing, by houses, and by the use of fire which has been one 

 of the most important instruments in his progress. Rotifers, 

 infusoria, and some other animals have become capable of 

 retaining life during thorough drying, and of resuming activ- 

 ity on the return of moisture. Adaptations to locomotion in 

 different media, earth, air, and water; to climbing; to station- 

 ary life, belong to this group. These are only a few of the many 

 instances of adaptations of organisms to the materials and 

 the forces about them. It is easy to see that some of the adap- 

 tations are of life and death importance, and without them the 

 species would become extinct. It is believed that these qualities 

 of the organism arise in a way something like this : owing to 

 the irritability of all protoplasm, the prevalent external factors 

 as heat, light, gravity, moisture, and chemically active sub- 

 stances must produce some change, that is, some response 

 on the part of the organism. Those organisms in which the 

 response is not in accordance with the best adjustment to the 

 special environment are less likely to survive in the struggle 

 for existence. Those which do survive and propagate their 

 kind because of their favorable responses to these stimuli are, 



