142 CHARLES DARWIN. 



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

 367, 368) 



. . . that thg 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 



