rapidly in some regions or among some species than in others. None of the 

 physical or chemical features in the conditions of living is known to give rise 

 to mutations. Some of the mutations certainly are incapable of perpetuating 

 themselves. From the facts that we do know, however, it seems reasonable 

 to assume that (1) mutations have taken place among living things throughout 

 the centuries; (2) some of the existing species arose, through mutation, from 

 ancestors having somewhat different characteristics. 



Does the Idea of Evolution Apply to Human Beings? 



Kinds of Resemblances On the basis of structure and form, human 

 beings are most like the apes and monkeys. For the zoologist Homo sapiens 

 represents one family of the order Anthropoidea. The other famiUes of this 

 order are represented by the marmosets, the New World monkeys, the Old 

 World monkeys, and the simians, or apes (see p. 53 and Appendix). We have 

 seen that in hundreds of details the homologies of structure show remarkable 

 similarities between man and the other mammals, but more specifically the 

 other anthropoids. The teeth, for example, vary among the primate families, 

 but the numbers and kinds of teeth are the same in men and the apes. 



In the course of its development the human embryo passes through stages 

 which are impressively like those of other vertebrates, of other mammals, and 

 especially, again, of the other primates (see illustration, p. 459). During this 

 development the embryo puts on details of structure that recall details in 

 other species, but that have no relation to the human mode of life (see pages 

 174 and 460). We might conceive all these resemblances to be merely coin- 

 cidences, and without any bearing upon man's history or ancestry. 



Chemical Resemblances Some of the similarities between man and 

 the other primates, however, appear more significant. The human race, as a 

 whole, is immune to certain species of microbes that cause disease in other 

 species, but the apes are susceptible to about the same diseases as men are. 

 That is, there is a chemical similarity between man and the other primates, 

 as well as a physical, or structural, similarity. The parasitic protozoon that 

 causes the disease syphilis affects other primates, but with a virulence that is 

 almost in direct proportion to their structural resemblance to man: the resem- 

 blance is strongest in apes, weaker in monkeys. 



We have seen that bringing foreign substances into the blood of an animal 

 leads to the formation of specific antibodies (see page 233). White-of-egg, for 

 example, would result in one kind of antibody, and the protein of a fish would 

 result in a different kind. This general fact was at first put to practical use in 

 deciding whether blood-stains had been made by human blood or by the 

 blood of some other animal. 



If small quantities of human blood are repeatedly injected into a rabbit 



514 



