(oa) 
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 natural 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 
ought to be able to produce the same sterility 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 Ass] 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 Darwin’s book 
in the 7imes, 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 
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 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 Huxley,” vol. i, p, 239. 
