78 'WHAT IS A SPECIES?" 



'Their produce [vi/. that of Horse and Ass] is usually 

 a sterile hybrid. 



* So if Carrier and Tumbler, e.^:^ were physiological 

 species ec^uivalent to Horse and Ass, their progeny ought 

 to be sterile or semi-sterile. So far as experience has 

 gone, on the contrary, it is perfectl)- fertile — as fertile 

 as the progeny of Carrier and Carrier or Tumbler and 

 Tumbler. 



* From the first time that I wrote about Darwin's book 

 in the Tivics and in the \Vcst7uijister until now, it has 

 been obvious to me that this is the weak point of Darwin's 

 doctrine. He Jias shown that selective breeding is a vera 

 causa for morphological species ; he has not yet shown it 

 a vera causa for physiological species. 



' But I entertain little doubt that a carefully devised 

 system of experimentation would produce physiological 

 species b}- selection — only the feat has not been per- 

 formed vet.' ' 



It was against this same view, as expressed in Huxley's 

 Lectures to Working Meji in 1863, that Darwin argued 

 with convincing force in man)- letters. The main facts 

 with which he confronted Huxley again and again were 

 the artificially selected races of certain plants which are 

 sterile inter se. The position is clearly expressed in the 

 following amusing, vehement passages from two letters : — 



'Dec. 18, [1862.J 



' Do you mean to sa}' that Gartner lied, after experi- 

 ments by the hundred (and he a hostile witness), when he 

 showed that this was the case with I'erbascuni and with 

 maize (and here you have selected races) : does Kolreuter 

 lie when he speaks about the varieties of tobacco ? I\Iy 

 God, is not the case difficult enough, without its being, as 

 I must think, falsely made more difficult ?* 1 believe it is 



m\- own fault — my d d candour: I ought to have 



made ten times more fuss about these most careful 

 experiments.' ^ 



' Life and Letters of Thomas Henry Huxley, vol. i, p. 239. 

 * More Letters, vol. i, p. 230, Letter 156. 



