THE EVIDENCE FOR EVOLUTION 729 



tion no longer occurs. When serum from an animal other than man is 

 mixed with rabbit serum containing antibodies for human serum pro- 

 teins, there is either no precipitation at all, or else a precipitation occurs 

 only with concentrated antigen solutions. By testing in turn the sera of a 

 variety of animals with rabbit serum containing antibodies for human 

 serum proteins, the degree of similarity between the proteins can be 

 determined. If the serum of another animal contains proteins which are 

 similar to those of man, a precipitation will occur. In this way, man's 

 closest "blood relations" have been found to be the great apes, and then, 

 in order, the Old World monkeys, the New World monkeys, and finally 

 the tarsioids. The serum of the lemur gives the smallest amount of pre- 

 cipitation when mixed with antibodies specific for human serum. 



The biochemical relationships of a variety of forms, tested in this 

 way, correlate with and complement the relationships determined by 

 other means. Cats, dogs and bears are closely related, as determined by 

 this test; cows, sheep, goats, deer and antelopes constitute another closely 

 related group. This test reveals that there is a closer relationship among 

 the modern birds than among the mammals, for all of the several 

 hundred species of birds tested give strong and immediate reactions with 

 serum containing antibodies for chicken serum. From other tests it was 

 concluded that birds are more closely related to the crocodile line of 

 reptiles than to the snake-lizard line, which corroborates the paleonto- 

 logic evidence. Similar tests of the sera of crustaceans, insects and mol- 

 luscs have shown that those forms regarded as being closely related from 

 morphologic or paleontologic evidence also show similarities in their 

 serum proteins. 



It might seem unlikely that an analysis of the urinary wastes of 

 different species would provide evidence of evolutionary relationship, 

 yet this is true. The kind of waste excreted depends upon the particular 

 kinds of enzymes present, and the enzymes are determined by genes 

 which have been selected in the course of evolution. The waste products 

 of the metabolism of purines (one of the constituents of nucleic acids) 

 are excreted by man and other primates as uric acid, by other mammals 

 as allantoin, by amphibians and most fishes as urea, and by most inverte- 

 brates as ammonia. Vertebrate evolution has been marked by the suc- 

 cessive loss of enzymes required for the stepwise degradation of uric 

 acid. Joseph Needham made the interesting observation that the chick 

 embryo in the early stages of development excretes ammonia, later it 

 excretes urea, and finally it excretes uric acid. The enzyme uricase, which 

 catalyzes the first step in the degradation of uric acid, is present in the 

 early chick embryo but disappears in the later stages of development. 

 The adult frog excretes urea but the larval form excretes ammonia. 

 These are biochemical examples of the principle of recapitulation. 



313. Evidence from Embryology 



The importance of the embryologic evidence for evolution was em- 

 phasized by Darwin and brought into even greater prominence by Ernst 

 Haeckel in 1866 when he developed his Biogenetic Law, that embryos. 



