CHAPTER XIV 

 INTRODUCTORY STATEMENT 



Any investigation of the causes of evolution must be preceded by a 

 survey of the facts to be explained. Some of the principal facts 

 which must be taken into account have already been placed before the 

 reader in the preceding section dealing with evidences of evolution. 

 If there were no other good reason for dealing with those materials 

 before beginning a discussion of causal theories of evolution, the peda- 

 gogical reason would be sufficient, because, until there is something 

 to explain, the necessity for an explanation does not arise. We are of 

 course aware that some writers prefer to deal with the facts of palaeon- 

 tology, geographic distribution, classification, comparative anatomy, 

 embryology, etc., after a discussion of the causes of evolution. Their 

 avowed reason for this order of treatment is that the net results of a 

 discussion of the causes underlying evolution may be used as a means 

 of more fully analyzing the facts. This is indeed true, but it is also 

 true that facts should come first and explanations afterward. As a 

 final step, the facts profitably may be re-examined in the light of causal 

 hypotheses. 



One of the outstanding facts of animate nature is the phenomenon 

 of adaptation. No naturalist has failed to note and marvel at the 

 adaptiveness or fitness of organisms to their environment and that of 

 parts of organisms for particular functions or activities. One of the 

 most difficult problems in evolution is the problem of the origin and the 

 perfection of adaptations, and most causal theories of evolution have 

 been aimed largely at an explanation of adaptation. Consequently, 

 before we enter upon a formal discussion of the causal theories we shall 

 introduce an outline of some of the main facts about adaptations. 



By way of introduction it should also be pointed out that the 

 causes of evolution are not all of equal value. Some of the causes are 

 to be conceived of as primary, others as secondary, or even tertiary. 

 Variation, for example, is absolutely primary in importance. Without 

 variation, change, which is the very essence of evolution, would of 

 course be impossible. Not less important is heredity; for unless there 

 be some factor which fixes variation so that it becomes a racial asset, 



IQI 



