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 smaller.
Thus, a man and a worm are members of the animal kingdom in virtue of
certain apparently slight though really fundamental resemblances which
they present. But a man and a fish are members of the same sub-kingdom
'Vertebrata', because they are much more like one another than either
of them is to a worm, or a snail, or any member of the other
sub-kingdoms. For similar reasons men and horses are arranged as
members of the same Class, 'Mammalia'; men and apes as members of the