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 be a 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 Palaeontology 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 

 lacunae,, unfortunately, which may possibly be looked 



14 



