150 DARWINISM TO-DAY. 



of many species of mammals, birds, insects, spiders, etc., 

 differ from the females. And if sexual selection does not 

 explain them then some other explanation is necessary. But 

 the lack of this explanation does not invalidate the general 

 theory of natural selection as one of the factors in organic 

 evolution and indeed one of the most important and far- 

 reaching ones. 



The difficulty of a satisfactory discussion of the objection 



that natural selection rests too largely on an assumed likeness 



to artificial selection, while the differences in 



Consideration ^he two processes, especially in their results, are 



of the objection * 



that natural se- too radical to allow us to rest any confidence 



on this a PP arent homology, is, that despite the 

 assumed analogy several thousand years through which artificial 

 selection Cla selection has been followed and studied we 



still know too little of the real character of it, 

 especially of its results. Most selectionists now admit that 

 the argument for natural selection on the basis of its sim- 

 ilarity to artificial selection has been given too great promi- 

 nence and relied on too strongly, but that the observed 

 processes of the one do teach us much of truth about the 

 unobservable processes of the other the Darwinians firmly 

 maintain. As Plate says, "The great value of artificial 

 selection consists in this, that it shows first, that a gradual 

 cumulation of characteristics in definite directions is actually 

 possible through successive selections, and second, that it 

 has afforded us a rich mass of data concerning variation, 

 inheritance, and the influence of changing intrinsic condi- 

 tions or influences. When Darwin showed what a high 

 plasticity the domestic animals possess he built for his 

 theoretical explanation of descent an indubitable necessary 

 foundation, for the changes which a domestic animal passes 

 through in the hands of man must of necessity be able to 

 be called forth in similar manner in the feral animals by 

 the creative force of nature, for the domestic animals cer- 



