506 ESSENTIALS OP ZOOLOGY 



human blood serum are introduced at intervals into the blood stream 

 of a rabbit, in time there will be developed in the blood of the rabbit 

 a substance (antibody) which, when mixed with normal human 

 blood will cause precipitation of the proteins here. Serum from such 

 a rabbit is called antihuman serum. When this serum is mixed with 

 serum in a certain dilution taken from human, chimpanzee, gorilla, or 

 monkey, it will cause precipitation. If the dilution is increased, 

 there is no precipitation when mixed with monkey serum; at still 

 higher dilution there is no precipitation when mixed with gorilla 

 blood; and higher none for chimpanzee blood, until finally none for 

 human blood. On the basis of these sera precipitation tests the chim- 

 panzee is closest in its relationship to man, then the gorilla, and then 

 the monkeys. ' 



In making similar tests on other vertebrate groups, it is found that 

 crocodiles are more closely related to birds than are the other reptiles ; 

 also among reptiles, that snakes and lizards are more closely related 

 to each other than to turtles. Too, crocodiles show a closer relation- 

 ship to turtles than to the other groups of reptiles. Blood studies of 

 the various groups of vertebrates indicate that there is more similarity 

 in blood of closely related forms than of others. At the same time, it 

 is seen that a chemical relationship persists in the blood throughout 

 the chordate phylum. 



The breeding of plants and animals through long series of genera- 

 tions of domestication and laboratory experiments has yielded much 

 information concerning the ways of adaptation and phylogenetic 

 development. A significant result is the demonstration of changes 

 occurring in animals and plants. From such studies it seems quite 

 obvious that organisms now living have come to be what they are 

 by gradual change from generation to generation through a course 

 of descent from preexisting and varied ancestors, rather than by a 

 sudden and completely new development. Most of the various 

 breeds of cattle, chickens, dogs, horses, sheep, crops, etc., have been 

 developed in each case from a preexisting common ancestor. 



Darwin and Studies of Evolution. — Most discussions of organic 

 evolution usually begin with mention of Darwin's monumental work 

 on this subject, and difficult it is to get away from his fundamental 

 basic thinking on the subject. He was the first to survey thoroughly 

 the fields of morphology, embryology, and paleontology, and to re- 

 late logically the data found there to the theory of evolution. From 



