is 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
quite competent to account for all that we may call purely structural
phenomena which are exhibited by SPECIES in nature. On that point also
I have already enlarged somewhat. Again, I think that the causes assumed
are competent to account for most of the physiological characteristics
of species, and I not only think that they are competent to account
for them, but I think that they account for many things which
otherwise remain wholly unaccountable and inexplicable, and I may say
incomprehensible. For a full exposition of the grounds on which this
conviction is based, I must refer you to Mr. Darwin's work; all that I
can do now is to illustrate what I have said by two or three cases taken
almost at random.

I drew your attention, on a previous evening, to the facts which are
embodied in our systems of Classification, which are the results of
the examination and comparison of the different members of the animal
kingdom one with another. I mentioned that the whole of the animal
kingdom is divisible into five sub-kingdoms; that each of these
sub-kingdoms is again divisible into provinces; that each province may
be divided into classes, and the classes into the successively smaller
groups, orders, families, genera, and species.

Now, in each of these groups, the resemblance in structure among the
members of the group is closer in proportion as the group is smalle