CHAPTER XIII. 

 BLOOD RELATIONSHIP. 



It may justly be supposed that, as living things di- 

 verged morphologically, they also diverged physiologi- 

 cally, and indeed this fact has been dwelt upon in 

 various relations. Thus, with reference to nutrition, we 

 find plants agreeing in the function of constructing their 

 protoplasm out of inorganic compounds, and animals 

 diverging from them in the loss of this function. Aquatic 

 and terrestrial forms of life differ in their method of 

 absorbing oxygen and in the quantity essential to their 

 metabolism. Salt- and fresh-water organisms differ in 

 their tolerance to sodium chloride and other salts. Differ- 

 ences in diet, in function, and in metabolism continue to 

 increase, until we find it not infrequently happening that 

 the flesh of one animal is poisonous for another, and 

 occasionally happening that the body juice of one 

 animal is poisonous when introduced into another. 



In this way it comes about that the body juice of al- 

 most any living being when introduced into the body 

 of some other warm-blooded animal is capable of acting 

 as an antigen. The reactions induced by these antigens 

 vary according to the method of treatment, as will be 

 shown in the chapter upon Infection and Immunity; 

 but when it is administered in small doses, frequently 

 repeated, so that no harm is effected, induces the for- 

 mation of an antibody capable of precipitating it. 

 Thus come about Zooprecipitation and Phytoprecipita- 

 tion the latter, when the juices of plants are used as 

 antigens. 



The term "blood-relationship" has been introduced 

 by Nuttall to express certain physiologico-chemical 

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