THE EVOLUTION THEORY 137 



We have already become acquainted with the valuable faculty 

 of the living organisms to generate for each toxin introduced an 

 antitoxin which combats the dangerous effects of the former. 

 It is owing to this faculty that the organism is able, by a gradual 

 increase in the doses, to become immune against the most 

 dangerous poisons. When we inject the blood of a distant species 

 certain elements are formed in the body, called ' precipitins,' 

 which under a continued systematic treatment with a certain 

 kind of blood are capable of accumulating in the blood of the 

 animal under treatment. These ' precipitins ' possess the 

 faculty of causing in the blood- serum of the species from which 

 the blood for the injection was taken remarkable precipitates, 

 whilst leaving unaffected the blood of all other animals. 



If, for instance, we inject at regular intervals of two to 

 six days ten ccm. of blood from a fowl the dose may be 

 increased after the rabbit has become accustomed to the foreign 

 blood we shall obtain at last a rabbit serum (rabbit-fowl- 

 antiserurn) which mixed with fowl's blood causes in it strong 

 turbid precipitates. But the fact chiefly of interest to us is that 

 the rabbit serum has this action not only upon fowl's blood, but 

 also, though in a lesser degree, upon the blood of related species. 



This faculty has been made by Nuttall and others the starting- 

 point of a series of comprehensive biochemical experiments, in 

 order to ascertain the degree of relationship of various different 

 species. These tests generally corroborate the results which 

 have been obtained by a study of comparative anatomy and 

 phylogeny. It must, however, be understood that it is in no case 

 possible to say how closely the examined animals are related ; 

 it is only possible to ascertain from the strength of the precipitate 

 that an animal A is more nearly related to an animal B than to 

 an animal C. 



By an ingenious method Nuttall was able to compare the 

 degree of relationship simply by measuring the volume of the 

 precipitates. Thus an anti-sheep serum, obtained from a rabbit 

 by systematic injections of sheep's blood, when mixed with 

 sheep's blood yielded a strong precipitate the volume of which 

 was assumed to equal 100 per cent. Mixed with bullock's blood, 

 the precipitates now equalled only 80 per cent., with that of the 

 antelope 50 per cent., reindeer 30 per cent., pig 20 per cent., 



