GENERAL ZOOLOGY 



tions such as digestion, respiration, circulation, excretion, and coordination, 

 to say nothing of reproduction and development, are alike in all the animals 

 of a certain kind? What determines the pattern; what perpetuates it? The 

 importance of enzymes as directives in the chemical reactions of living cells 

 has been pointed out in many discussions. Were we to delve more exhaus- 

 tively into the "how" of life, almost innumerable instances could be cited of 

 the ubiquity of these uniquely sculptured protein molecules which somehow 

 provide the special niches in which the vital reactions occur. We can go 

 a little farther in our search for a "constancy factor," a "directing agent," 

 a "set of instructions" which is not only effective among individuals of the 

 same generation but is handed on to successive generations. This leads to a 

 consideration of the phenomena of heredity and variation which we shall 

 next undertake. 



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