514 THE CHANGING WORLD 



Evidence from Serology 



When serum from human blood is injected at repeated intervals 

 into a rabbit, the rabbit eventually develops antibodies and becomes 

 sensitized to human blood. When the blood thus prepared is added 

 to human blood, it produces a precipitate, a chemical change that 

 does not occur if it is added to the blood of other animals. Unsensi- 

 tized rabbit blood does not react to human blood. Rabbit blood 

 can also be sensitized to horse serum, or to that of other animals 

 such as the pig or fowl. 



Such blood tests constitute a chemical method which enables the 

 experimenter to determine whether or not a specimen of unknown 

 blood is human, a technique that has proved very useful to the 

 criminologist. In Germany blood tests have even been employed 

 to determine the composition of suspected sausages. 



By this method also the degrees of relationship between different 

 animals can be determined. In the case of man, for example, when 

 human-sensitized rabbit blood is added to that of anthropoid apes, 

 the precipitation is almost as complete as with human blood. The re- 

 action occurs in diminishing degree with Old World catarrhine mon- 

 keys, and still less with New World, long-tailed platyrrhine monkeys. 

 It does not react in any appreciable degree with the blood of lemurs. 

 This is in confirmation of anatomical and embryological evidence 

 that the order of relationship among primates is man, apes, Old 

 World monkeys. New World monkeys, and lemurs. 



Rabbit blood sensitized to horse serum will react against the blood 

 of a zebra, but less positively than to horse blood. By similar blood 

 tests it is shown that whales are more akin to ungulates than to car- 

 nivores, that birds are closer to turtles than to lizards, and that the 

 horseshoe crab Limulus, as already mentioned, is more of a scorpion 

 or spider than it is a crab, which it externally resembles. 



Evidence from Human Interference 



The part man has played in directing the course of evolution is 

 apparent in domesticated animals and plants, which often differ to a 

 remarkable degree from wild ancestral forms, as a visit to a flower, 

 dog, or poultry show demonstrates. 



What man has done has not been creative but selective. He has 

 employed neither laws nor methods which were not already in opera- 

 tion. Nature has furnished the plastic variable organisms, and man 



