EXTINCT ANIMALS 



horse and the knee of the man, correspond. The 

 man has a short foot, the horse a long one. The 

 upstanding bit at the back of the horse's leg 

 called the " hock " is really the heel, and cor- 

 responds to the heel bone 'which you can dis- 

 tinguish in the man's skeleton. So also the 

 fore-arm and shoulder-blade correspond in the- 

 two skeletons. 



Accordingly, as animals are alike or unlike 

 in the details of their structure, so we can 

 group them into divisions and sub-divisions (see 

 the Hst of classes at the end of Chapter I). There, 

 are certain marks by which it is easy to 

 recognize fragments of bone, dug it may 

 be out of a quarry or railway cutting, and 

 to know at once the division or kind of 

 animals to which the owner of the fragments 

 belonged. I have already alluded to the fact 

 that the strata of the earth are revealed to us 

 by cliffs on the sea-shore, by exposed rocks and 

 by river banks ; and I would add by such 

 activities of man as the digging of quarries and 

 railway cuttings. Suppose that you find a 

 skull in such a digging — there are marks by 

 which you can tell whether it belongs to a 

 mammal or reptile. 



72 



