312 OUTLINES OF EVOLUTIONARY BIOLOGY 



however, by no means vitiates the general argument, for it is usually 

 possible to pick out pretty accurately those which come into or 

 near to the direct line, and even the collaterals afford valuable 

 evidence as to the general course of evolution. We may safely 

 say that the palaontological evidence amounts to a clear 



demonstration of the evolution 

 of the horse from a five-toed 

 ancestor along the lines indi- 

 cated above. 



The ancestry of the elephants 

 is less well known than that 

 of the horses, but recent dis- 

 coveries in the Egyptian Ter- 

 tiary formations, which we owe 

 especially to the investigations 

 of Dr. Andrews, have done 

 much to elucidate the history 

 of this remarkable group of 

 mammals, and there can now 

 be no doubt as to the main 

 line of evolution which has led 

 up to the existing Proboscidea. 

 In some respects the elephants 

 have remained in a some- 

 what primitive condition, as 

 is indicated very clearly by the 

 fact that all the digits remain 

 well developed in both fore and 

 hind feet. It is in the struc- 

 ture of the head that they 

 exhibit a high degree of special- 

 ization, marked particularly by 

 the elongation of the snout to 

 form a long prehensile trunk, by the enormous development of 

 the occipital region of the skull, by the enlargement of the 

 incisor teeth to form great tusks, by the shortening of the jaws 

 and by the increase in size and complexity and the reduction in 

 number of the cheek teeth. These changes have been accom- 

 panied by a huge increase in the size of the entire body, so that 

 most of the elephants are amongst the largest of known land 

 mammals, whether fossil or recent. 



FIG. 156. a, Fore Foot and b, Hind 

 Foot of Mesohippus celer, X i. 

 (From Lull, after Marsh.) 



