148 THE FOUNDATIONS OF ZOOLOGY 



distinguish one species from another have been acquired since 

 the two diverged, and not even all of these slight differences. 



"We know that the duration of even the most persistent 

 species of the higher animals is only an infinitesimal part of 

 the whole history of their evolution, and it is clear that the com- 

 mon characteristics of two allied species must outnumber, thou- 

 sands of times, the differences between them. It follows that the 

 parents of any possible hybrid must be alike in thousands of 

 features for one in which they differ. Crossing simply results in 

 the formation of a germ by the union of a male and a female 

 element derived from two essentially similar parents, with, at 

 most, only a few secondary and comparatively slight differences, 

 all of which have been recently acquired." 



I trust that you will agree with me that due consideration of 

 the subject which is here presented might have saved much 

 unprofitable discussion of " the causes of variation " ; for it seems 

 clear that we must seek in the modern world, and not in the 

 remote past, for an explanation of that diversity among individ- 

 uals which passes under the name of "variation." 



I have called your attention to these facts because they serve 

 to introduce, and to throw light upon, the subject of the next 

 lecture, The Statistical Study of Inheritance ; although they seem 

 to me to throw light upon other zoological problems. 



If the extinction of a genetic line may be so slow that a fail- 

 ing stock may go on from bad to worse for many generations 

 before it is utterly destroyed, is it not clear that we can seldom 

 hope to discover what determines the ultimate survival or extinc- 

 tion of a genetic line? Is it not equally clear that artificial selec- 

 tion, by the sudden and utter destruction of the discarded, is no 

 measure of natural selection ? 



Unless individuals with the same useful quality breed together 

 it is hard to see how this useful quality can be intensified by 

 natural selection, and as it also seems hard to find in nature any 

 reason why these individuals should seek out and unite with each 

 other, this criticism of natural selection seemed to Darwin to be a 

 real difficulty ; but we must remember that while the sexual union 

 of those individual animals whose descendants would be the fit- 

 test to survive may be rare and exceptional, the survival of a 



