together. We see mutilated animals, as well as plants, carry on instead of 

 being killed by the injuries they have received. Sometimes in ourselves 

 joints stiffen, vision dims, muscles are less prompt or less effective than we 

 should like. The more complex an organism is, the more likely is some 

 part to get out of step. But what is it that maintains the harmony when 

 nothing is out of order ? 



We have seen that homeostasis, or the constancy of the blood, is main- 

 tained by continuous and delicate adjustments to slight changes in the tem- 

 perature and chemical condition of the blood. The tropic movements of 

 plants also result from chemical responses to changes in temperature, illumi- 

 nation, pressure, and so on. Among simple animals too, many of the tropic 

 movements seem to come from chemical responses to stimuli, whether these 

 are originally electrical or mechanical, whether they are changes in light or 

 in temperature. In the most complex organisms, the warm-blooded birds 

 and mammals, the blood acts as a unifying medium, for it rapidly distributes 

 the chemical "messengers", or hormones, which the endocrine glands release 

 under various circumstances. These hormones stimulate various parts of 

 the body or retard their action in various ways. On the whole, however, the 

 net effect is to bring the behavior of the entire system into harmony. That 

 is, the endocrines harmonize the parts of the body in relation to one an- 

 other, while the body as a whole acts — in most cases — with relation to 

 existing conditions. 



In addition to the chemical processes which have the effect of unifying 

 the parts of the body, the nervous system does the same thing. In one-celled 

 animals it is possible to locate surface spots that are exceptionally sensitive 

 to stimuli, and also strands of protoplasm through which stimuli appear to 

 be transmitted. We may think of the sense organs and the nerves of many- 

 celled animals as elaborations of such areas. The sensitive spot comes to be 

 one of several special sense organs. The sensitive strand appears as a nerve 

 cell. There are simple nerve paths in animals connecting receptor directly 

 with effector. There are reflex arcs, and chains or groupings of reflex arcs. 

 In the backboned animals the central nervous system, with the brain and 

 the autonomic nerves, ties together the sensory and muscle systems with the 

 visceral and endocrine systems. We may say that as the one-celled animal 

 behaves as a unity because it is all one, a many-celled bird or mammal be- 

 haves as a unity because it is in all its parts firmly bound together by 

 chemical and nervous strands; it is hardly possible to touch a point without 

 affecting all parts, directly or indirectly. 



Instead of asking how the parts of a plant or animal do work together, 

 it might be more helpful to think of the organism as a distinct kind of 

 unity; or we might ask how this unity comes to have so many distinct kinds 

 of parts, or even why the parts sometimes fail to work together. 



325 



