4.54 THE CAUSES OF THE XI 
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 unaccount- 
able and inexplicable, and I may say incompre- 
hensible. 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 examin- 
ation and comparison of the different members 
of the animal kingdom one with another. I men- 
tioned 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 aman anda fish are members of 
the same sub-kingdom Vertebrata, because they are 
much more like one another than either of them 
