366 THE BIOLOGY OF INSECTS 



Its publication was one of the most important events in the 

 history of science, because through its influence the principle 

 of evolution, regarded previously by most naturalists with 

 doubt and suspicion, came to be generally accepted. 

 Darwin's success in establishing this principle, in contrast 

 with de Lamarck's failure, was due largely to the reasonable 

 and convincing manner in which he marshalled the facts 

 of structure, classification, development, and palaeontology 

 in support of the *' doctrine of descent with modification." 

 His success was also due to the appeal made by his special 

 theory of " natural selection " which he invoked as an 

 explanation of the method by which evolutionary change 

 had been brought about. Natural selection is a secondary, 

 not a primar}^, factor in evolutionary change, as Darwin 

 himself clearly perceived, for he begins his arguments by 

 treating of variation '* under domestication " and " in a 

 state of nature," and demonstrating '* that a large amount 

 of hereditary modification is at least possible." The 

 primary factor therefore is heredity with variation. Darwin 

 next dwells on the importance of " the Struggle for Exist- 

 ence, amongst all organic beings throughout the world 

 which inevitably follows from the high geometrical ratio 

 of their increase." As this expression has often given rise 

 to misunderstanding, it is important to recall that Darwin 

 used " this term in a large and metaphorical sense, including 

 dependence of one being on another and including (which 

 is more important) not only the Hfe of the individual but 

 success in leaving progeny." From variation among living 

 creatures and the struggle for life, thus understood, there 

 follows the process which Darwin called Natural Selection. 

 He points out that " of the many individuals of any species 

 that are periodically born, but a small number can survive," 

 and argues that the survival of some and the elimination of 

 the rest depends on the fact that *' variations ... if they 

 be in any degree profitable to the individuals of a species 

 in their infinitely complex relations to other organic beings 

 and to their physical conditions of life will tend to the 

 preservation of such individuals and will generally be 



