142 CHARLES DARWIN. 



in his lectures to working men (" Darwiniana," pages 

 367, 368)— 



"... that the more extensive verifications are, — that the 

 more frequently experiments have been made, and results of 

 the same kind arrived at, — that the more varied the conditions 

 under which the same results are attained, the more certain is 

 the ultimate conclusion . . . ." 



And again — 



" In scientific enquiry it becomes a matter of duty to expose 

 a supposed law to every possible kind of verification, and to 

 take care, moreover, that this is done intentionally, and not 

 left to a mere accident . . . ." 



It may well be that the length of time required 

 before an artificially-selected race will exhibit, when 

 interbred with the parent species, phenomena of 

 hybridism similar to those which are witnessed when 

 distinct natural species are interbred — will be fatal 

 to the production of this important line of evidence. 

 But there is nothing to hinder us from holding the 

 reasonable belief that such evidence might be ob- 

 tained if we had command of the necessary conditions; 

 and in the meantime other evidence of the most 

 satisfactory kind is accumulating, and on a vast scale. 

 Whenever a naturalist approaches a problem in the 

 light of the theory of natural selection, and is able, 

 by its aid, to predict a conclusion which subsequent 

 investigation proves to be correct, he is helping in 

 the production of evidence in favour of the theory. 

 When a naturalist has found the formula "if natural 

 selection be true so-and-so ought to happen " the 



(I 



