CHAPTER V 



FUNCTIONAL VARIATION 



By functional variation is meant a deviation, not in form or in 

 the number of parts but in the functions that they perform. The 

 living animal (or plant) not only is something, but it does some- 

 thing, and plants and animals differ among themselves not only 

 in what they are but in what they do. 



Each portion of a highly differentiated organism has its own 

 peculiar activity, which is essentially different from that of any 

 other part of the same organism. These activities are not con- 

 stant but variable ; and inasmuch as many animals and not a 

 few plants are kept not for their appearance but for what they 

 can do, any deviations in their performance ability are of prime 

 importance to the breeder, who is bent upon their increased 

 efficiency and their permanent improvement for the service 

 of man. 



Now plants and animals are considered as high or low in their 

 development according to the degree of differentiation or division 

 of labor between their different parts. In the protozoon the func- 

 tions of life are few, and its relations to the environment are 

 simple. Accordingly its activities are exerted and its obligations 

 to life discharged by the common mass of undifferentiated pro- 

 toplasm, perhaps without so much as a stomach, reproduction 

 being effected by a direct division of the whole mass. 



In higher organisms (metazoans), however, life is more complex 

 and the responsibilities of existence are heavier. These are met 

 by specialized structures, such as the mouth to take food, the 

 stomach and intestines to dissolve and prepare it for use, the 

 liver to convert certain portions into specially usable form, 

 the lungs to absorb air, the blood vessels to carry it and the 

 digested food to all parts of the body, where each extracts what 

 it needs and can use. 



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