466 THE CAUSES OF THE xi 



which are more or less infertile with one another. 

 There are other cases which are truly extraordi- 

 nary ; there is one, for example, which has been 

 carefully examined, of two kinds of sea- weed, of 

 which the male element of the one, which we may 

 call A, fertilises the female element of the other, 

 B ; while the male element of B will not fertilise 

 the female element of A ; so that, while the for- 

 mer experiment seems to show us that they are 

 varieties, the latter leads to the conviction that 

 they are species. 



When we see how capricious and uncertain this 

 sterility is, how unknown the conditions on which 

 it depends, I say that we have no right to affirm 

 that those conditions will not be better understood 

 by and by, and we have no ground for supposing 

 that we may not be able to experiment so as to 

 obtain that crucial result which I mentioned just 

 now. So that though Mr. Darwin's hypothesis 

 does not completely extricate us from this difficulty 

 at present, we have not the least right to say it 

 will not do so. 



There is a wide gulf between the thing you can- 

 not explain and the thing that upsets you alto- 

 gether. There is hardly any hypothesis in this 

 world which has not some fact in connection with 

 it which has not been explained, but that is a very 

 different affair to a fact that entirely opposes your 



artificial or natural selection, the objection must be admitted 

 ultra-sceptical as it is. But in science, scepticism is a duty. 



