148 mr. darwin's work and 



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

 functions annihilated by mere imprisonment ! 



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 every- 

 body believes to be varieties * which are more or less 

 infertile with one another. There are other cases which 

 are truly extraordinary ; 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 fer- 

 tilize 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 



* And as I conceive with very good reason ; bnt 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. 



