174 A Review of the Modern Doctrine of Evolution. (March, 
Icticyon has but one. Now examples of Canis familiaris (domestic 
dog) with but one tubercular molar are not rare, while an individ- 
ual with three is occasionably found. | 
To take another case. The normal dentition of Homo (man) is, 
on each side, incisors, 2; canin2, 1; premolars, 2; molars, 3. It 
is very common to find in the higher races, individuals who 
have molars only two in one or both jaws; and the absence of the 
external incisors of the upper jaw is almost as frequently met with. 
Here we have two new generic variations in one and the same 
species. ; 
In specific characters variations are most familiar. Thus, the 
young of deer are generally spotted, and’ the adults are nearly 
uniform in coloration. Some deer (as the Aris) retain the spotted 
coloration throughout life, while an occasional spotted individual 
of unicolor species, is a violation of specific character by a failure 
to develop. The larvz of some salamanders are of uniform col- 
oration, and the adults spotted. The unicolor adults of the same 
species, not-uncommonly met with, present examples of the same 
kind of variation. 
Any biologist can select hundreds of similar cases from his 
special department of study. 
Il. The Laws of Evolution. 
Having reviewed the reasons why the doctrine of evolution 
should be received as truth, I desire to give attention to the laws 
which may be made out by reference to its phenomena. Progress 
‘in this direction is difficult, owing to the natural impediments in 
the way of studying the history of the growth of living beings. 
We will, however, commence by examining more fully the phe 
nomena with which we have to deal. 
It is well understood that the world of animal life is a 
nicely adjusted equilibrium, maintained between each individual 
and its environment. This environment exerts forces both purely 
physical, and those exercised by other animals. Animals an- 
tagonize each other in procuring food, whether that food con- 
sist of vegetation or of other animals, but in the latter case the 
conflict is more severe. A similar competition exists among male 
animals in the matter of reproduction. These exhibitions of 
energy constitute the struggle for existence, which is the daily 
business of the living world. It is well understood, that in this 
struggle the individuals best provided with means of self-preser- 
