THE PAST EVOLUTION OF MAN 77 



species. Even in the case of quite fresh stains this 

 method is, however, somewhat uncertain ; and it is 

 inapplicable to those stains — constituting the large 

 majority — in which the corpuscles are no longer 

 recognisable. 



Recently, however, students of the blood have 

 been enabled to elaborate a method of identification 

 which is based upon a quite different set of facts. 

 If a few drops of blood obtained from any species 

 of animal be injected, with due antiseptic or aseptic 

 precautions, into the veins of another animal of the 

 same species — as from man to man, or from cat to 

 cat — no results of any kind are to be observed. 

 The injected drops find themselves at home, and 

 then corpuscles mingle with those of the second 

 animal's own blood, without any ill results to either. 

 It is found, however, that very striking results follow 

 the injection of blood from an animal of one species 

 into the veins of an animal belonging to another 

 species : as from a cat to a dog. If the species are 

 distinct, the result is obtained, even though they 

 be so closely allied as are cat and dog. The red 

 blood-corpuscles of the second animal's blood are 

 found to undergo a rapid disintegration and dissolu- 

 tion. On analysis it is discovered that this result is 

 due to the presence in the fluid of the blood of any 

 animal, of certain substances which have received the 

 appropriate name of cytolysins : a term accurately 

 to be translated as meaning cell-dissolvers. These 

 cytolysins exert no deleterious action upon the cells 

 of the blood in which they naturally occur, nor 

 upon the cells of the blood of any animal that 

 belongs to the same species. It does not matter 



