670 LECTURE XXIX. 



produced which are so constituted that they exactly correspond to the 

 structure of the foreign product. In this way the active groups which 

 would be injurious to the body-cells are rendered harmless. The "bio- 

 logical reaction" of the animal organism is, from this point of view, to 

 be regarded merely as a means of protection. 



Let us see what subsequent investigation has done for us in this direc- 

 tion towards establishing our conception of species. In the first place 

 it could be shown that the formation of precipitins was not confined to 

 one species, but that the specific result of the reaction was limited to 

 related animals within such narrow limitations that the relationship 

 between animals which from morphological and other similarities are 

 usually grouped together could be confirmed by means of the "biological 

 reaction. " Nuttal l found, for example, that the serum of a rabbit into 

 which the serum from dog's blood had been injected would give a pre- 

 cipitate with the blood of eight different kinds of Canidce, but not with 

 the blood of any other species. Friedenthal 2 showed, furthermore, that 

 only the anthropoid apes showed a marked blood-relation to man, 

 whereas the lower apes showed but slight indication of a common 

 origin. We may add that the different kinds of birds have also been 

 compared in this way, and that recently Uhlenhut 3 has succeeded in so 

 perfecting the method of carrying out the biological reaction that it has 

 become possible to differentiate and distinguish between closely related 

 kinds of blood. The reaction has become of considerable importance in 

 forensic blood determinations. 



It would be, of course, desirable to ascertain what compounds the cells 

 and body-fluids make use of for carrying out this specific reaction. We 

 would naturally think of the proteins in this connection, for, on account of 

 their extremely complicated composition, they are most suited to serve as 

 carriers of specific groups of atoms. As a matter of fact, it has been found 

 possible to prove that after the injection of protein, e.g., serum-albumin, 

 perfectly specific precipitins are formed; 4 and, indeed, it was even found 

 possible to distinguish by this means between the casein of different kinds 

 of milk. We do not yet know whether in such cases the individual protein 

 substances come into consideration, or whether the effect is produced by 

 impurities which adhere to them. At all events, it is of great interest to 



1 Cf. Nuttal: Proc. Roy. Soc. 69, 150 (1901). Blood Immunity and Blood Rela- 

 tionship. Clay & Sons, London, 1901. 



2 Arch. Anat. Physiol. 1900, 494. Sitzsber. Berliner Akad. 1902. Verhandl. Ber- 

 liner physiol. Gesell. 1904. Uhlenhut: Arch. Rassen- und Gesellschaftsbiologie, 1, 682 

 (1904). 



3 Uhlenhut: Deut. med. Wochschr. 1905, 42. 



4 Cf. among others, L. Michaelis: Deut. med. Wochschr. 1902, 41. L. Michaelis and 

 Carl Oppenheimer: Arch. Anat. Physiol. Sup. 1902, 336. F. Obermayer and E. P. Pick: 

 Wien. klin. Wochschr. 1904, 10. Andrew Hunter: J. Physiol. 32, 327 (1905). 



