1880.] A Review of the Modern Doctrine of Evolution. 173 
which these conditions bear to geologic time is displayed in the 
following table, commencing with the lowest horizon: 
No cannon bone. Cannon bone present. 
A 
Incisor lia present. Incisors one and two wanting. 
EEY zh os 
4 premol ts. 3 prem’rs. 2 prem’s. I prem’r. 
a 
Lower Miocene. f z Poëbrotherium. 
Procamelus. 
Protolabis. 
| Pliauuchenia. 
Upper Miocene. 
Blisdene and Recent. ie Aakn 
This table shows that geological time has witnessed, in the his- 
tory of the Camelidæ, the consolidation of the bones of the feet 
and a great reduction in the numbers of the incisor and premolar 
teeth. The embryonic history of these parts is as follows: In 
the foetal state all the Ruminantia (to which the camels belong) 
have the cannon bones divided as in Poébrotherium ; they exhibit 
also incisor teeth, as in that genus and Protoladis. Very young 
recent camels hive the additional premolar of Pliauchenia. They 
shed this tooth at an early period, but very rarely a camel is found 
in which the tooth persists. The anterior premolar of the normal 
Camelus is in like manner found in the young lama (Auchenia), 
but is shed long before the animal attains maturity. I may add 
that in some species of Procamelus caducous scales of enamel 
and dentine i in shallow cavities represent the incisive dentition of 
Protolabis 
It remains to shows that characters of the kind above mention- 
ed are sometimes inconstant; that they may or may not appear 
in individuals of a species. Under such circumstances it is evi- 
dent that their origin does not imply m break in the line of de- _ 
scent, 
First, as to a family character. It is well known that the : 
deer differ from the giraffes in the presence of a burr or ring of . 
osseous excressences surrounding the base of the horn. Now - 
which possess or lack this ie indifferently. Why. some joe ae 
viduals should, and others should not possess it, is not known, — 2 
cond, as to a generic character. The, genus Canis. is 
defined by the presence of two tubercular molars in the inferi 
sne. ‘The Tin genus Thous, prane three such teet 
