Classification. 25 
for instance, has a large number of teeth, and in this 
feature resembles most fish, while it differs from all’ 
mammals. But it also givessuck to its young. Now, 
looking to these two features alone, should we say | 
that a porpoise ought to be classed as a fish or as a | 
mammal? Assuredly as a mammal; because the 
number of teeth is a very variable feature both in fish , 
and mammals, whereas the giving of suck is an in-_ 
variable feature among mammals, and occurs nowhere 
else in the animal kingdom. This, of course, is chosen 
as a very simple illustration. Were all cases as 
obvious, there would be but little distinction between 
natural and artificial systems of classification. But it 
is because the lines of natural affinity are, as it were, 
so interwoven throughout the organic world, and 
because there is, in consequence, so much difficulty in 
following them, that artificial systems have to be made 
in the first instance as feelers towards eventual dis- 
covery of the natural system. In other words, while 
forming their artificial systems of classification, it has 
always been the aim of naturalists—whether con- 
sciously or unconsciously—to admit as the bases of 
their systems those characters which, in the then state 
of their knowledge, seemed most calculated to play an 
important part in the eventual construction of the 
natural system. If we were dealing with the history 
of classification, it would here be interesting to note 
how the course of it has been marked by gradual 
change in the principles which naturalists adopted as 
guides to the selection of characters on which to found 
their attempts at a natural classification. Some of 
these changes, indeed, I shall have to mention later 
on; but at present what has to be specially noted is, 
