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. 
