598 THE HISTORY OF BIOLOGY 



the antitoxin, their blood-serum afterwards being used as a counter-poison 

 against the disease. The actual elements — both toxins and antitoxins — 

 that operate in this process always exist in very small quantities, and for 

 that reason and owing to their complicated composition it has not been 

 possible to analyse them; their quantitative effect upon one another has 

 nevertheless been investigated by Svante Arrhenius and others. 



In connexion with this research work the nature of the blood in gen- 

 eral has been the object of very detailed investigations, often with quite 

 remarkable success. Among the best-known of these is the precipitin re- 

 action, which was discovered by Paul Uhlenhuth (born 1870, Behring's 

 successor at Marburg). If we take serum from an animal — for instance, a 

 dog — and inject it into a rabbit, we obtain from the rabbit's blood within 

 some days a serum that produces precipitation in the serum of the dog's 

 blood. This reaction is specific and can therefore be utilized in medico-legal 

 investigations in order to distinguish, for instance, human blood from ani- 

 mal blood. In this, however, similar forms act in an identical way; for ex- 

 ample, human blood and the blood of anthropoid apes produce the same 

 reaction. When this chemical resemblance between allied organisms was dis- 

 covered, it excited great enthusiasm as a phylogenetical argument; closer 

 consideration, however, at once makes it clear that this agreement of chemi- 

 cal composition demonstrates just as much or just as little as the morphologi- 

 cal resemblance that can be demonstrated by the old comparative method; 

 the fact that resemblance in bodily structure, food, and habit is accompanied 

 by corresponding chemical agreement is essentially so obvious that the con- 

 trary would be more surprising. 



We may here cite one more example of discoveries in this sphere. We 

 have previously mentioned how the Russian naturalist Ilja METscHNiKorF 

 (1845-1916), who after studying at German universities was a professor at 

 the Pasteur Institute in Paris, produced the so-called phagocyte theory; he 

 found, as indeed Haeckel had already observed, that the white blood-cor- 

 puscles absorb foreign substances into the body; in particular, he discovered 

 that the leucocytes in this way free the body from bacteria that enter it, pro- 

 vided the latter are not too strong and do not get the upper hand. In more 

 recent times it has been found that special substances are produced, which 

 have been called "opsonins," which possess the ability to increase the 

 leucocytes' power of killing the bacteria. These substances, however, are 

 at present little known, but they are, of course, of the very greatest interest. 

 They have been mentioned here as examples of the wide possibilities with 

 which modern biochemistry has to reckon. The immense practical benefits 

 that this research work has brought humanity can only be hinted at here; 

 some of the theoretical speculations to which it has given rise will be dealt 

 with in the following. 



