Specializations among animals have developed in the direction of greater 

 mobility and of greater sensitivity to what happens in the environment. 

 This involves a greater consumption of materials in the release of energy, as 

 against the mere accumulation of materials in which sun energy is latent. 

 It involves also more rapid exchange of materials between the interior and 

 the exterior. And, in the larger animals, it involves a remarkable combina- 

 tion of (1) rapid transportation of materials through an "inner ocean"; 

 (2) rapid interchange of materials between the several millions of living 

 cells and this ocean; and (3) a high degree of stability, or homeostasis, in 

 the internal fluids. 



Specializations in animals are thus related to more complex mechanisms 

 of (1) attacking and taking in outside materials, including oxygen; 

 (2) transforming and distributing these materials to the ultimate consum- 

 ers in the diverse kinds of cells; (3) collecting wastes and by-products of 

 the cells and tissues and discharging or excreting them. Incidental to these 

 processes are means of locomotion and of defense, as well as of attack; 

 specialized sense organs related to getting food and escaping enemies; and, 

 again, means of resisting or surviving periods during which the ordinary 

 life activities cannot be carried on. 



In the highest animals, birds and mammals, the organism supplies its liv- 

 ing cells a well-protected and stabilized inner environment, a fluid medium 

 at constant temperature and constant acid-alkaline balance. Materials are con- 

 tinually diffusing into and out of the blood at varying rates. Yet the concen- 

 trations of sugar, proteins, fats, mineral salts, oxygen, carbon dioxide, and 

 nitrogenous wastes fluctuate within very narrow limits, regulated by nerves, 

 muscles, and special chemical "messengers", the hormones (see page 304). 



The circulating blood distributes whatever heat there is throughout the 

 body and so helps the organism to react to its environment as a whole. It 

 is impossible to have a sick foot or liver and not have the whole body 

 affected. Another remarkable specialization of blood is the rapid mobiliza- 

 tion of white corpuscles at points of injury or infection. Other activities 

 are slowed up until damage is repaired. 



We may describe the specialized structures and processes of plants and 

 animals in terms of the common activities — food-getting, oxidation, excre- 

 tion, and so on. It is necessary to keep constantly in mind, however, that 

 the unity of the organism is not a private possession, so to speak. Each 

 species has, indeed, its characteristic details; it has its own way of dealing 

 with the outside world. But these characteristics are related to other living 

 things, and not merely to the salt water of the ocean or some abstract 

 supply of food. To keep whole and to keep going, a plant or animal must 

 carry on certain processes inside itself; but these involve intimate adjust- 

 ments in dealing with other organisms, both friends and enemies. 



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