SUMMARY OF EVIDENCE 77 



species of life have originated by natural evolution from 

 one or more primitive types has come to be accepted 

 by the scientific world as best agreeing with and ex- 

 plaining the known facts of biology and of related 

 spheres of investigation. The e^ddence by which the 

 evolutionary hypothesis is supported consists, there- 

 fore, of innumerable facts, which, when considered 

 together, suggest such a theory, and do not appear to 

 be susceptible of any other physical explanation that 

 is so satisfactory and so free from difficulties. These 

 facts are gathered from a wide variety of sources, but 

 especially from results of investigations in morphology, 

 embryology, the geological succession of organisms, 

 their geographical distribution, and the observed pro- 

 duction of new varieties and species by artificial selec- 

 tion and by natural mutation.^ 



I. The universal occurrence in the organic world 

 of variation, and of the perpetuation of its results by 

 inheritance, has led scientific observers to abandon 

 behef in the fixity of species, and to look for some other 

 explanation of their origin than the ancient behef in a 

 separate creation of each by divine fiat. Moreover, so 

 far as we can imagine, there are but two alternatives 

 to this view — their spontaneous but separate origin, 

 one by one, out of lifeless matter, and their natural 

 evolution by means of variation and inheritance. Of 



1 The evidences of natural evolution are given in most all the 

 works mentioned on p. 37, note 2, above. But note especially A. R. 

 Wallace, Darwinism; Chas. Darwin, Origin of Species, ch. xv; 

 A. Weismann, Evolution Theory, Lees, ii, iii; Thos. Huxley, Darwin- 

 iana, pp. 205-225; M. M. Metcalf, Organic Evolution, pp. 87-163. 



