464 THE CAUSES OF THE XI 
assault, ought to be able to demonstrate the possi- 
bility of developing from a particular stock by se- 
lective breeding, two forms, which should either 
be unable to cross one with another, or whose 
cross-bred offspring should be infertile with one 
another. 
For, you see, if you have not done that you have 
not strictly fulfilled all the conditions of the prob- 
lem; 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 experiments have 
gone, it has not been found possible to produce 
this complete physiological divergence by selective 
breeding. I stated this very clearly before, and I 
now refer to the point, because, if it could be 
proved, not only that this Has not been done, but 
that it cannot be done ; if it could be demonstrated 
that it is impossible to breed selectively, from any — 
stock, a form which shall not breed with another, 
produced from the same stock; and if we were 
shown that this must be the necessary and inevit- 
able results of all experiments, I hold that Mr. 
Darwin’s hypothesis would be utterly shattered. 
But has this been done? or what is really the 
state of the case? It is simply that, so far as we 
have gone yet with our breeding, we have not pro- 
