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 nearly all cases the body juices of one animal, when 

 introduced into the blood or tissues of a different kind of 

 animal, is capable of acting as an antigen, i.e., of exciting 

 a disturbance in the form of a chemico-physiological "re- 

 action." 



The reactions thus induced vary according to the nature 

 of the experiment, as will be shown in the chapter upon 

 Infection and Immunity. When the antigens are adminis- 

 tered in small doses, frequently repeated, so that no harm is 

 effected, they usually induce the formation of self-dissolving, 

 self-neutralizing, self-agglutinating, or self-precipitating 

 antibodies. Zooprecipitins, phytopredpitins, hemolysins, 

 cytotoxins, and antitoxins are examples later to be discussed. 



The term "blood-relationship" has been introduced 

 by Nuttall to express certain physiologico-chemical 

 resemblances found to obtain among different kinds of 

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