PHYLOGENY. 149 



others are longer, approaching the true horses. In the 

 valleys between these high cusps cement is deposited, 

 as in the true horses and other mammals with long- 

 crowned molars. There are two types of these later 

 three-toed horses. In one the posterior inner cusp is 

 not joined to the conule by a transverse crest (genus 

 Hippotherium Kaup), or it is so joined (genus Pro- 

 thippus Leidy). 



Plistocene times witnessed the perfection of the 

 horse line. The lateral toes dwindled into splints 

 concealed beneath the skin. The crowns of the molar 

 teeth became very long, and in the upper jaw the inner 

 posterior tubercle, now a column, joined the adjacent 

 conule, and became extended very much in fore and 

 aft diameter. The small anterior premolar disap- 

 peared, and the canines became the mark of the male 

 sex onl}', in general. The lower molars acquire some 

 additional complications, and the feet are longer than 

 in any of its ancestors. The genus Equus L. is fin- 

 ished, and remains a permanent member of the human 

 epoch, from which its only relatives, the rhinoceros 

 and the tapir, are gradually disappearing. 



This history may be duplicated in manner and 

 mode, by the lines of the camels, the dogs and bears, 

 the cats, the beaver, etc. 



Examination of all these genealogical lines reveals 

 a certain definiteness of end and directness of approach. 

 We discover no accessions of characters which are 

 afterwards lost, as would naturally occur as a result of 

 undirected variation. Nor do we discover anything 

 like the appearance of sports along the line, the word 

 sport being used in the sense of a variation widely di- 

 vergent from its immediate ancestor. On the con- 

 trary, the more thorough becomes our knowledge of 



