1 16 mr .darwin's work and 



is not wholly competent to explain, and that is the group 

 of phenomena which I mentioned to you under the name 

 of Hybridism, and which I explained to consist in the 

 sterility of the offspring of certain species when crossed 

 one with another. It matters not one whit whether 

 this sterility is universal, or whether it exists only in a 

 single case. Every hypothesis is bound to explain, or, 

 at any rate, not be inconsistent with, the whole of the 

 facts which it professes to account for ; and if there is 

 a single one of these facts which can be shown to be 

 inconsistent with (I do not merely mean inexplicable 

 by, but contrary to,) the hypothesis, the hypothesis 

 falls to the ground, — it is worth nothing. One fact 

 with which it is positively inconsistent is worth as 

 much, and as powerful in negativing the hypothesis, 

 as five hundred. If I am right in thus defining the 

 obligations of an hypothesis, Mr. Darwin, in order to 

 place his views beyond the reach of all possible assault, 

 ought to be able to demonstrate the possibility of deve- 

 loping from a particular stock by selective breeding, 

 two forms, which should either be unable to cross one 

 with another, or whose cross-bred offspring should be 

 infertile with one another. 



Tor, you see, if you have not done that you have not 

 strictly fulfilled all the conditions of the problem ; you 

 have not shown that you can produce, by the cause 

 assumed, all the phenomena which you have in nature. 

 Here are the phenomena of Hybridism staring you in 

 the face, and you cannot say, "I can, by selective 

 modification, produce these same results." Now, it is 

 admitted on all hands that, at present, so far as experi- 

 ments have gone, it has not been found possible to pro- 



