560 EVOLUTION 



variations disappear with the individuals in which they occur. 

 To become a characterizing feature of a group of plants or animals 

 a variation must be more or less constant. It must repeat itself 

 in successive generations. This means that it must become a part 

 of the heritage of the organisms which it characterizes. Thus 

 through variations which become established one type of organ- 

 isms cab arise from another. It is evident that if we thoroughly 

 understood variations apd heredity, we would know much 

 about evolution, and it is in connection with these two factors 

 that moBt of the controversies concerning evolution have arisen. 

 No one disputes the fact that organisms vary and that there is 

 heredity, but concerning the causes of variations, their per- 

 manence, and the ways variations and heredity work in con- 

 junction with environment in estabUshing new forms there is still 

 much to be learned. 



Organic Evolution and origin of species. — The theory of or- 

 ganic evolution centers about the origin of species. The term 

 species is apphed to a group of plants or animals in which the 

 individuals are alike in their essential features and remain so 

 throughout generations under similar conditions. On the basis 

 of similarities species are grouped into genera, genera into fam- 

 ihes, families into orders, and so on. The unit of classification is 

 the species. Before we can have new genera, new families, etc., 

 there must be new species. Therefore, if we accept the theory 

 of evolution, the explanation of how species come into existence 

 is the explanation of organic evolution. For this reason the 

 term "origin of species" may be used interchangeable with the 

 term "organic evolution." 



History of the Theory of Evolution. — Rarely are theories ac- 

 cepted by the majority of people when first proposed. Commonly 

 a theory is accepted only by a few and is either accepted or rejected 

 by the majority after much debating which often extends over 

 many years or even centuries. Such is the history of the theory 

 of evolution. 



As far back as historical records go, man has wondered about 

 his origin and how all living beings and all nature came into 

 existence. He has not only wondered, but has offered explana- 

 tions, the eariiest of which were purely mythical. The question 

 of origins is the first to receive attention in the Bible. According 

 to the literal interpretation of the Book of Genesis all things were 



