78 UNDERHILL : 



ent adaptation, the influence of a similar mode of life bring- 

 ing about changes in these marine forms along converging 

 lines from divergent land-dwelling origins. 



The early Tertiary progenitors of the now widely distinct 

 orders of land mammals were probably all five-toed Planti- 

 grades with the tuberculated molars of Omnivora. There was 

 as yet very little modification among them for any special kind 

 of food or for differing climatic and physical conditions. The 

 incisors were not specially adapted for grass cropping, nor the 

 tuberculate molars for either flesh or herb eating ; the feet 

 were neither clawed and retractile like those of the cat, nor 

 hoofed like those of the later deer and horse families. There- 

 fore we see no particular adaptation of the feet for seizing 

 prey, climbing, or running; the type is a synthetic one — a 

 stem — which is later, under changing environment and adapt- 

 ive specialization, to unfold a tree of separate and separating 

 branches. 



One of the most fascinating chapters for study in tracing 

 these fossil relationships is furnished by the branching from 

 this generalized stock of the hoofed animals, which constitute 

 the most numerous and comprehensive group of larger mam- 

 mals, including those formerly classed as Ruminants, Solid- 

 ungula, and all of the Pachyderms with the exception of the 

 Proboscideans. To the reader not already acquainted with the 

 outline characteristics which distinguish this order it will be suf- 

 ficient for this article to say that the Ungulates walk upon the 

 extremities of the toes, the third phalanx of each digit being 

 encased in a nail so modified as to form a hoof. The first digit 

 (thumb) is always obliterated, therefore the number of devel- 

 oped digits in the true Ungulate never exceeds four. The 

 limbs are used only for locomotion, never for prehension, 

 and clavicles are wanting. Although the primitive Ungulates 

 had the tuberculated molar teeth of Omnivora (buuodont) 

 like a few living forms, as the pigs, the typical molar of living 

 members of the order is massive with a broad, flat crown 

 (selenodont), suited for crushing and grinding vegetable 



