RELATIONSHIP AND DESCENT 
instructed person that this large group contains some 
exceedingly diverse types. Consider for a moment a 
monkey, a bat, a whale, a horse, and a cat. It is 
plain that though all are mammals (judged by the tests 
which we have used above), they exemplify very 
extreme types, radiating, it may be, from a common 
centre. Now a proper classification is one which 
most nearly represents true relationship; this may 
seem to bea remark hardly worth making, on account 
of obviousness. But nevertheless the history of the 
classifications of mammals and of other animals show 
that, obvious though it may be, the truth of the remark 
has not been generally laid hold of. 
Now relationship implies and is proved by a common 
descent. To apply this method to classification is of 
course a counsel of perfection not yet attainable, though 
in some cases the study of extinct forms has given us a 
fairly complete series of gradations from type to type. 
To take one example: the South American lama and 
the Old World camels can be traced back to an extinct 
form, Procamelus, which appears to combine the 
divergencies of the two, and to be thus the ancestor 
of both. The descendants of a parent form must 
clearly be related. The advance of Paleontology will 
doubtless make other relationships clear. Further- 
‘more it is plain that the degree of relationship 
depends upon the nearness or remoteness of the 
common ancestor; For instance, the lama and the 
camel soon converge in an apparently common ances- 
tor; on the other hand the two great divisions of 
the Ungulates, whose characters will be dealt with in 
succeeding pages, viz., the Artiodactyles and_ the 
Perissodactyles, retain their characters for a long 
period of time; and it is not until the early Eocene 
period that we arrive at forms, with many intervening 
lacune, unfortunately, which may possibly be looked 
14 
