SUMMARY. 283 



solved. Practically, all scientists accept evolution 

 as expressing a fact of nature. Having thus by 

 their theory eliminated special creation of species, 

 it becomes a logical necessity to show how the 

 working of natural laws could have produced an 

 evolution. To admit that the present species are 

 descended from older ones is no advantage, unless it 

 can be shown that new species can arise from old 

 ones by the working of acknowledged laws of or- 

 ganic being. Two series of facts, it is plain, must 

 furnish the data for all explanation : heredity and 

 variation. By heredity, species reproduce their own 

 kind ; by variation, they produce offspring some- 

 what different from themselves. These two series 

 of data are not theoretical, but actual. It is a uni- 

 versally recognized fact that animals and plants in- 

 herit from their parents, and it is also universally 

 acknowledged that they vary very mnch. Out of 

 these two series of facts, then, must the explanation 

 arise. 



Darwin turned his attention to the facts and selec- 

 tion of variation. This subject he patiently studied 

 for many years, collecting a vast amount of material. 

 With all his work he was unable to discover any im- 

 portant laws which regulated the appearance of vari- 

 ation. So irregular are they in their appearance, so 

 indefinite in direction, and so completely beyond the 

 realm of prediction, that he expresses his conclusion 

 by saying that organisms have an innate tendency 

 to vary, and that the variations are chance varia- 

 tions. By this he simply means, first, that though 

 they all have an efficient cause, the causes are com- 



