THE EVOLUTION OF ANIMAL LIFE 



like other characteristics, be the result of artificial selection in these domes- 

 ticated stocks. 



Until comparatively recently, the breeding of animals has developed as a 

 skill, rather than as a science. Folk traditions and superstitions have guided 

 the rule-of-thumb operations of breeders, and we still encounter references 

 to ''blood lines" which reflect the intuitive reasoning of the ancients that 

 the blood of the individual was responsible for its characteristics. With the 

 rise of genetics as an exact science, however, as discussed at length in 

 Chapter 6, the application of its principles to selective breeding has removed 

 much of the guesswork from planning and carrying out the production of new 

 and superior breeds of domestic animals and plants. Heredity has without 

 doubt played the same kind of part in natural evolution as it plays in selective 

 breeding under domestication. 



Summarizing the argument, we may state that the production of new 

 types of animals and plants under domestication illustrates the kind and 

 magnitude of changes which can occur in the characteristics of organisms with 

 the passage of time. If this much evolution has occurred in a relatively 

 short time under the selective guidance of man, it is only to be expected that 

 much more impressive changes should have occurred in all living things 

 during the extremely long period of time since life first appeared on earth. 



Summary of Evidence for Organic Evolution 



In summary, it may be repeated that evidence from many fields has led 

 biologists to accept the evolutionary concept as the most logical explanation 

 of the development of new kinds of animals. The geologic succession of ani- 

 mal types, as revealed in the fossil record, and their present geographical 

 distribution, can be most reasonably interpreted according to the theory of 

 organic evolution. Animals resemble each other in structure, in chemical 

 composition, and in patterns of development in such a way as to indicate 

 different degrees of interrelationship. The assumption that these facts reflect 

 different lines of descent from common ancestry best explains the available 

 evidence. Further evidence may be adduced from the production of new 

 breeds of animals and plants under domestication. Very often the evidence is 

 indirect or circumstantial, but it is so extensive, and so many lines of evi- 

 dence support the same conclusion, that one cannot reasonably remain un- 

 convinced of the reality of animal evolution. 



Human Evolution 



Man as a Primate. If the evolution of all other forms of life be accepted 

 as a reasonable probability, it is illogical to exclude man from the universal 

 process of organic evolution. Man's structure and embryonic development; 



637 



