452 THE CAUSES OF THE ey 
we shall have proved it as far as certainty is pos- 
sible for us; for, after all, there is no one of our 
surest convictions which may not be upset, or at 
any rate modified by a further accession of know- 
ledge. It was because it satisfied these condi- 
tions that we accepted the hypothesis as to the 
disappearance of the tea-pot and spoons in the 
case I supposed in a previous lecture; we found 
that our hypothesis on that subject was tenable 
and valid, because the supposed cause existed in 
nature, because it was competent to account for 
the phenomena, and because no other known cause 
was competent to account for them ; and it is upon 
similar grounds that any hypothesis you choose to 
name is accepted in science as tenable and 
valid. 
What is Mr. Darwin’s hypothesis? As I appre- 
hend it—for I have put it into a shape more con- 
venient for common purposes than I could find 
verbatim in his book—as I apprehend it, I say, 
it is, that all the phenomena of organic nature, 
past and present, result from, or are caused by; 
the inter-action of those properties of organic 
matter, which we have called ATAVISM and VARIA- 
BILITY, with the CONDITIONS OF EXISTENCE, or, 
in other words,—given the existence of organic 
matter, its tendency to transmit its properties, and 
its tendency occasionally to vary ; and, lastly, given 
the conditions of existence by which organic mat- 
ter is surrounded—that these put together are the 
