( cii ) 



We now turn to the consideration of interspecific sterility, 

 which many have supposed to be an infallible criterion. 

 Huxley himself felt this so strongly that he was, in consequence, 

 never able to give his full assent to natui-al selection. 

 The grounds of his objection were the subject of prolonged 

 correspondence with Darwin. In order to prove that natural 

 selection has produced natural species separated rigidly, as he 

 believed, by the barrier of sterility, Huxley maintained that we 

 ouglit to be able to produce the same stei ility between our artifi- 

 cially selected breeds ; and until this had been done he could 

 not thoroughly accept the theory of natural selection. This 

 objection he expressed, or implied, in many speeches and 

 writings up to within a few months of his death. One of 

 the simplest statements is contained in a letter to the 

 late Charles Kingsley. Huxley wrote, April 30, 1863, 

 " Their produce [viz. that of Horse and Asa] is usually 

 a sterile hybrid. 



"So if Carrier and Tumbler, e.g., were physiological species 

 equivalent to Horse and Ass, their progeny ought to be 

 sterile or semi-sterile. So far as experience has gone, on the 

 contrary, it is perfectly fertile — as fertile as the progeny of 

 Carrier and Carrier or Tumbler and Tumbler. 



" From the first time that I wrote about Dai'win's book 

 in the Times, and in the Westminster, until now, it has been 

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

 doctrine. He has shown that selective breeding is a vera 

 cmisa 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 prodvice physiological species by 

 selection — only the feat has not been performed yet."* 



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

 "Lectures to Working Men" in 1863, that Darwin argued 

 with convincing force in many 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 : — 



* " Life and Letters of Thomas Henry Huxle}'," vol. i, p. 239. 



