THE PHENOMENA OF ORGANIC NATURE 109 



it arises from the simple fact of their being shut up and 

 deprived of their liberty, or not, we do not know, but 

 they certainly will not breed. What an astounding thing 

 this is, to find one of the most important of all functions 

 annihilated by mere imprisonment 1 



So, again, there are cases known of animals which have 

 been thought by naturalists to be undoubted species, which 

 have yielded perfectly fertile hybrids ; while there are 

 other species which present what everybody believes to 

 be varieties * which are more or less infertile with one 

 another. There are other cases which are truly extra- 

 ordinary ; 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, fertilizes 

 the female element of the other, B ; while the male element 

 of B will not fertilize the female element of A ; so that, 

 while the former 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 cannot 

 explain and the thing that upsets you altogether. 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 hypothesis ; in this case all you can say is, 

 that your hypothesis is in the same position as a good 

 many others. 



Now, as to the third test, that there are no other causes 

 competent to explain the phenomena, I explained to you 

 that one should be able to say of an hypothesis, that no 

 other known causes than those supposed by it are competent 



* And as I conceive with very good reason ; but if any objector 

 urges that we cannot prove that they have been produced by arti- 

 ficial or natural selection, the objection must be admitted ultra- 

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



