130 THE THEORY OF MUTATION 



methods of evolution are now known to be available, 

 the burden of proof of this proposition seems to 

 lie with those who maintain the all-important influ- 

 ence of continuous variation and selection. At present 

 we are free to reply in the words of Malthus, who long 

 ago protested against the extravagant powers which 

 were ascribed to the selection of small differences. 



' I have been told,' Malthus writes, ' that it is a 

 maxim among some of the improvers of cattle that 

 you may breed to any degree of nicety you please, 

 and they found this maxim upon another, which is, 

 that some of the offspring will possess the desirable 

 qualities of the parents in a greater degree. In the 

 famous Leicestershire breed of sheep, the object is to 

 procure them with small heads and small legs. Pro- 

 ceeding upon these breeding maxims, it is evident that 

 we might go on until the heads and legs were evan- 

 escent quantities ; but this is so palpable an absurdity 

 that we may be quite sure the premises are not just, 

 and that there really is a limit, though we cannot see 

 it or say exactly where it is.' * 



The only recorded example I am aware of in the 

 case of animals, which shows the result of long-con- 

 tinued selection acting upon a quantitative character, 

 is afforded by the case of the American trotting-horse. 

 In this case it appears highly probable that we are 

 dealing with a character which varies in a strictly 

 continuous fashion. In his book upon ' The Trotting 

 and Pacing Horse in America,' Hamilton Busbey gives 

 a table from which the diagram on the opposite page 

 * 'Essay on Population,' 6th ed., vol. ii., p. n. 



