CHAPTER ONE 



Evolution by Natural Selection: 

 Darivin's Conception 



Evolution may be defined as "descent with modification" (Darwin), 

 closely related species resembling one another because of their common 

 inheritance, and differing from one another because of the hereditary dif- 

 ferences accumulated since the separation of their ancestors. Stated in 

 another way, evolution is the derivation of new species ( or higher groups ) 

 of plant or animal from previously existing ones. 



These definitions, as well as others commonly given, are general in their 

 terms. They do not imply a particular line of descent for any specified 

 organism. The descent of man from "monkeys" is not a point of definition 

 for evolution. Nor does any competent student of evolution regard man 

 as derived from any other organism now living. But both man and the 

 great apes are regarded as coming from a common ancestor, an unknown 

 primate. Man does not even play a very useful role in the study of evolu- 

 tion, because he is not available for laboratory experimentation to nearly 

 the extent that other animals are, and because primitive man was rarely 

 fossilized, though apparently more frequently fossilized than other primi- 

 tive Primates. However, the student of evolution belongs to an egocentric 

 species, and so a chapter on the evolution of man will be included in this 

 book. 



DARWINIAN PREMISES 



Evolution, as conceived by Darwin, can be briefly summarized. All plants 

 and animals reproduce in excess of the numbers which can actually sur- 

 vive, yet adult populations remain relatively constant. Hence, there must 

 be a struggle for survival. Now the members of any species vary one from 

 another. Some of the variations may be neutral, but others help or hinder 

 the organism in its struggle for survival. As a consequence, the "survival 

 of the fittest" ( Spencer ) variants will be expected, with the less fit being 

 destroyed either by their physical or biotic environments (natural selec- 

 tion ) . Thus species will be gradually modified in the direction of the most 

 advantageous variants. 



