the pattern of development. Among human beings, for example, we differ 

 as to the time when the first teeth appear, or the last teeth. We differ as to 

 the age at which we begin to walk or to talk, as to the time at which we ma- 

 ture, and as to how long we remain at each stage. And especially do we differ 

 as to how long we postpone the end of individual existence. In addition to 

 having such inherited differences, the individual's pattern of development 

 is frequently altered and blocked. Yet it has been difficult to find out what 

 brings about "natural death". One reason is that very few human beings die 

 a "natural death". There have been many theories regarding the chemical 

 and physical changes which lead to death, where no injuries have taken place. 



August Weismann, already mentioned as the author of the "germ plasm" 

 idea (see page 507), pointed out that the protozoa (and one-celled plants too) 

 are potejjtially immortal. In this way he emphasized the idea that under suit- 

 able conditions a line of such simple protoplasm can remain alive indefinitely 

 through successive cell-divisions. There is no natural death in these species, 

 as we have seen. 



In the more complex many-celled species the germ plasm may continue 

 indefinitely, so long as reproduction takes place. The individual body, or 

 soma, however, which we conceive to be an offshoot of the germ plasm, may 

 have a limited duration, except where there is vegetative propagation (see 

 illustration, p. 508). 



According to this view, life became "mortal" when it acquired a many- 

 celled body, in which germ cells are differentiated from soma, or body, cells. 

 But we must not confuse the idea that "protoplasm continues to live" with 

 the idea that a particular "individual" or person continues to live. Even in 

 the case of the ameba or paramecium the life of the individual has a definite 

 limit. 



What Are the Advantages of Specialization? 



Division of Labor We can see the advantages of "physiological" divi- 

 sion of labor from our experience with social or economic division of labor. In 

 fact, we are so familiar with specialized organs carrying on specialized functions 

 that we find it in some ways more difficult to understand a "simple" system, 

 like a bacterium, than a complex one, like a human body. 



Imagine the life of, say, a dozen scattered human beings roaming over 

 several square miles, each one living by himself. Compare these with a group 

 or family of the same number living together. In the simplest of human 

 societies, where there is only a family group, division of labor is already pres- 

 ent. The men hunt while the women look after the children and prepare food 

 and shelter. Members too old to take part in the strains and dangers of hunt- 

 ing keep weapons in repair or make new ones. Children too young to do more 

 difficult work can fetch and carry for the older members, saving the time of 



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