100 TJic Unit// of the Organism 



As is" now widely known, tliis hjpotliesis is supported by 

 anotlRT liToat mass of evidence from (iiiite a different source, 

 which tli()u<(h not as directly chemical as that just adduced, 

 is still so clearly so in its implications that reference to it 

 in tlii> chapter is undoubtedly justifiable. What is in mind 

 are tlie discoveries of recent years touchinir the compatibility 

 and non compatibility of the blood of one kind of animal 

 for that of another kind; discoveries in other words, con- 

 cerning- the so-called "})recipitin reaction" as between or- 

 tranisms of different kinds. Although this subject has at- 

 tracted a good measure of attention, only a portion of its 

 more fundamental significations has been much regarded. 

 Its bearings on ])roblems of affinity and racial descent, for 

 example, have elicited their due of interest. But its con- 

 tribution to light upon the opposite aspect of animal nature, 

 namely, that of difference as well as of likeness between 

 kinds, has not been appreciated in proportion to its merits. 

 Once grasp the conception of each organic species, to say 

 nothing of each indi\ddual, as something genuinely unique 

 in the world in certain of its more obvious attributes, as a 

 scheme of organization, shape, etc., and then extend this 

 down into basal composition and process, so that the organ- 

 ism is seen in its role not merely of transformer and creator, 

 but to some extent of exclusive transformer and creator of 

 the elements of which it is constructed, and these and kin- 

 dred discoveries fall into their right perspective of meaning 

 and interest. 



The Tuiderlying general principle of the precipitin reac- 

 tion is that of the production within the organism of aiiti- 

 hodies as a result of injecting into it certain foreign sub- 

 stances which, when the reaction occurs, are known as anti- 

 gens, the anti-gens and anti-bodies usually reacting definitely 

 and s])ecifically upon each other. In one form of this re- 

 action the antibody acting upon certain proteins, forms a 

 precipitate, this precipitate carrying down both the antigen 



