276 THE STRUCTURAL EVIDENCE OF EVOLUTION. 



in both the horse and the ox. In the foetus these bones are well 

 distinguished. 



The dentition tells as clearly as possible the same story. Here, 

 again, as I have pointed out in a paper on the ** Homologies and 

 Origin of the Types of Dentition of the Mammalia Educahilia,^'' 

 the most specialized forms of dental structure are presented by the 

 horse, the ox, and the tiger. But they are all modifications of a 

 single type of tooth, viz., an oval crown supporting four tubercles 

 on the summit, in the lower jaw, and three or four in the upper 

 jaw. In the lower cutting molar of the cat but one of these tu- 

 bercles remains, forming with another in front of it a double shear 

 blade, whose development may be traced from its earliest begin- 

 nings in the genera of the Eocene. In the odd-toed forms (tapir, 

 rhinoceros, etc.) the tubercles become connected transversely, 

 forming cross-crests, and the outer ones are generally flattened on 

 the outer side. In the horse the tubercles have a very complex 

 form, and the spaces between them filled by a peculiar substance, 

 the cementum. In the ruminants the tubercles come to have a 

 crescent-shaped section and are drawn out to an enormous length, 

 forming a prismatic tooth : here, also, the intervening deep valleys 

 are filled with cementum. . In the third series, that of the ele- 

 phant, the original tubercles (permanently separate in the masto- 

 dons) are connected into cross-crests, which are drawn out to a 

 great length, and as in the other series are supported by a deposit 

 of hard cementum in the intervening valleys. The transitions be- 

 tween these and the primitive four-tubercled molar are numerous 

 and direct. 



There is not now opportunity to consider the question of tran- 

 sition from type to type by descent, further than to indicate by a 

 few examples the manner in which it has evidently occurred. 

 This has been by unequal growth of parts during foetal life, ac- 

 cording to the laws of acceleration and retardation. The union 

 of the two basal bones of toes into a single one (the "cannon- 

 bone ") in the ruminants, is accomplished by the more and more 

 rapid completion of the process of ossification in the growth of 

 those bones ; the confluence of the various carpal and tarsal bones 

 in various orders has the same history. In many genera it has 

 been observed that the milk dentition has resemblances to other 

 and older dentitions, which entirely disappear in the permanent 

 teeth. This is the case with Coryphodon and Equus ; while it 

 may be observed in the kitten, whose sectorial milk-tooth has the 



