that the assumed causes of the phenomena are
competent to produce such phenomena as those which we wish to explain by
them; and in the last place, we ought to be able to show that no other
known causes are competent to produce those phenomena. If we can succeed
in satisfying these three conditions we shall have demonstrated our
hypothesis; or rather I ought to say we shall have proved it as far as
certainty is possible 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 knowledge. It was because it satisfied these
conditions 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 apprehend it--for I have put
it into a shape more convenient 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 VARIABILITY, 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
matter is surrounded--that these put together are the causes of the
Present and of the Past conditions of ORGANIC NATURE.

Such is the hypothesis as I understand it. Now let us see how it will
stand the various tests which I laid down just now. In the first place,
do these supposed causes of the phenomena exist in nature? Is it the
fact that in nature these properties of organic matter--atavism and
variability--and those phenomena which we have called the conditions of
existence,--is it true that they exist? Well, of course, if they do not
exist, all that I have told you in the last three or four lectures
must be incorrect, because I have been attempting to prove that they do
exist, and I take it that there is abundant evidence