76 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
Were there time enough to do so, we might trace the development 
of this family backward, step by step, through all the many stages 
between the Pleistocene and the Upper Eocene in quite as unbroken 
sequence and in as full detail as can be done for the horses. We must, 
however, pass over all the intermediate steps and consider the ances- 
tral camels of the Upper Eocene. These were very little animals, 
hardly larger than a jack rabbit, which had the full complement of 
teeth, 44 in total number, and all with very low crowns. The limbs, 
and especially the feet, are relatively short, the ulna is complete and 
separate, as is also the fibula; there are four toes in each foot, though 
the lateral pair of the hind foot are extremely slender, and there is no 
co-ossification to form cannon-bones. The hoofs are well developed, 
in form like those of an antelope, so that there can have been no pad. 
For the present, the line cannot be carried back of the Upper Eocene, 
the probable ancestors from the middle and Lower Eocene being, as 
yet, represented only by fragmentary specimens. 
In addition to this main stem of cameline descent which resulted 
in the modern species, there were two short-lived side branches which 
should be mentioned. One, ending in the Lower Miocene, was the 
series descriptively called “‘gazelle-camels,’’ small animals with very 
long and slender legs, evidently swift runners. The other series, the 
so-called ‘‘giraffe-camels,’’ terminated in the Upper Miocene; these 
were browsers and display an increasing stature, especially in the 
length of the neck and fore limbs. They adapted themselves to the 
growing aridity of the western plains. 
EVOLUTION OF THE ELEPHANTS* 
A. FRANKLIN SHULL 
The mastodon-elephant series shows a larger number of obvious 
changes than most of the other series named, all of these changes 
except that of the body having to do with features of the head. 
From the numerous specimens of elephant-like forms available, the 
following are selected (following Lull) as probably representing a 
direct line of evolution: Moeritherium from the Upper Eocene of 
Egypt; Palaeomastodon from the Lower Oliogocene of Egypt, also 
from India; Trilophodon from the Miocene of Europe, Africa, and: 
North America; Mastodon from the Pliocene and Pleistocene of 
1 From A. F. Shull, Principles of Animal Biology (copyright 1920). Used by 
special permission of the publishers, The McGraw-Hill Book Company. 
