ON VARIATION. 69 



well defined as closely allied genera in most natural 

 divisions. 



It is seldom that so many stages of developmental 

 series survive so as to be contemporaries, as in this 

 case of the Batrachia Salientia. In order to obtain 

 such series we usually have to explore the ages of the 

 past. In the higher groups this is also the case, but 

 here we have also occasional examples of the persis- 

 tence of fairly complete series. Such a one is pre- 

 sented by the suborder Artiodactyla of the Diplarth- 

 rous Ungulate Mammalia. I give the definitions of 

 the succession of the existing families. 



I. Molars bunodont (tubercular) ; superior incisors generally pres- 

 ent. No cannon or naviculocuboid bones. 



Lateral toes well developed ; Hippopotamid(E. 



Lateral toes rudimental ; Suidie. 



II. Molars selenodont (crescent-bearing). (Lateral toes rudimental 

 or wanting). 



A. Premolars with one row of lobes. 



No naviculocuboid bone ; one superior incisor ; a can- 

 non bone ; Camelidce. 

 A naviculocuboid bone ; no superior incisor ; (cannon 

 bone variable) ; Traguiidcs. 

 AA. Premolars with two rows of tubercles ; a naviculocuboid 

 and cannon bones ; no incisors above. 



Premolar iii with only one row of lobes ; canine teeth, 

 no horns ; RIoschid.c. 



Premolar iii with two rows of lobes ; fixed horns ; no 

 canines above ; Bovidce. 



Premolar iii with two rows of lobes ; horns decidu- 

 ous ; Cervid<c. 



In this suborder we see a gradual complication of 

 the structure of the molar teeth, and a loss of the in- 

 cisors. In the limbs we observe the successive loss of 

 the lateral digits, and the fusion of elements, — as the 

 metapodials into cannon bones, and the elements of 



