370 THE CHANGING GENERATIONS 



our base with which to compare the serums of other animals. First an antiserum 

 (in this case an anti-human serum) is produced in the blood of some laboratory 

 animal such as the rabbit, by means of repeated injections of small quantities 

 of human serum or the proteins of human serum. When the anti-human precipitin 

 has reached a high level of concentration in the rabbit's blood, the animal is 

 killed, the blood is drawn off and allowed to clot, and the serum is sterilized by 

 filtration and preserved for use as a chemical reagent. When a drop of this anti- 

 human serum is added to a dilute solution of human serum, a dense white pre- 

 cipitate is formed. This simple and rather crude test has become routine procedure 

 in crime investigation, for distinguishing human from other types of blood. 



When the precipitin reactions were discovered in 1897, they were 

 thought to be absolutely specific. A given antiserum was believed to react 

 only with the serum used in its formation. If this were true, the reactions 

 could be used for identifying serums but not for determining degrees 

 of likeness among them. It was soon found that the reactions are instead 

 quantitatively specific. An antiserum reacts most strongly with the antigen 

 mixture used to produce it and progressively less strongly with related 

 antigen mixtures in proportion to their degree of chemical similarity. 



In the early years of this century Nuttall made some 16,000 precipitin 

 tests of the blood serums of a large number of animal species. He used 

 anti-human, anti-pig, anti-pigeon, anti-turtle, anti-king crab, and other 

 antiserums as reagents. The general trend of his results was clear, and 

 many of his findings have been substantiated by later work. His methods 

 were not truly quantitative as he believed, however, and some of his 

 conclusions have been shown by more precise later work to have been 

 unfounded. 



Some of NuttalPs more important conclusions were as follows: (1) the 

 bloods of all mammals show chemical relationship ; (2) man is most nearly 

 related to the great apes and shows increasingly distant kinship to the 

 Old World monkeys, the New World monkeys, the marmosets, and the 

 lemurs, in that order; (3) all carnivores are more closely related to one 

 another than to other mammals; (4) relationships are close within the 

 families that include the pigs, the camels, the deer, and the cattle; (5) 

 whales and porpoises are distantly related to pigs and much more dis- 

 tantly to other mammals; (6) among reptiles the turtles, crocodiles, and 

 alligators form one assemblage and the snakes and lizards another; (7) 

 all bird serums are chemically very similar and are closer to those of 

 lizards and snakes than to the other groups of reptiles ; and (8) the horse- 

 shoe, or king, crab, Limulus, is closer kin to the arachnids (spiders, 

 scorpions, etc.) than to the crustaceans which it resembles. Except for 

 (5), these conclusions are in accord with the indications of relationship 

 given by morphology and taxonomy ; but the relations of the whales and 

 porpoises are probably with the carnivores and not with the pig?. 



