XI PHENOMENA OF ORGANIC NATURE 463 
this enormous lapse of time while almost every- 
thing else was changed and modified. 
Thus I have no doubt that Mr. Darwin’s hypo- 
thesis will be found competent to explain the ma- 
jority of the phenomena exhibited by species in 
nature ; but in an earlier lecture I spoke cautiously 
with respect to. its power of explaining all the 
physiological peculiarities of species. 
There is, in fact, one set of these peculiarities 
which the theory of selective modification, as it 
stands at present, 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 obli- 
gations of an hypothesis, Mr. Darwin, in order to 
place his views beyond the reach of all possible 
