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 that
they do exist; so far, therefore, the hypothesis does not break down.

But in the next place comes a much more difficult inquiry:--Are the
causes indicated competent to give rise to the phenomena of organic
nature? I suspect that this is indubitable to a certain extent. It is
demonstrable, I think, as I have endeavoured to show you, that they are
perfectly competent to give rise to all the phenomena which are
exhibited by RACES in nature. Furthermore, I believe that they are