432 Mental Factors in Evolution 
treatment than Darwin’s conception of an impelling force, and that 
it is in line with the main trend of Darwin’s thought. 
In a characteristic work,—characteristic in wealth of detail, in 
closeness and fidelity of observation, in breadth of outlook, in 
candour and modesty,—Darwin dealt with The Expression of the 
Emotions in Man and Animals. Sir Charles Bell in his Anatomy 
of Expression had contended that many of man’s facial muscles had 
been specially created for the sole purpose of being instrumental in 
the expression of his emotions. Darwin claimed that a natural 
explanation, consistent with the doctrine of evolution, could in many 
cases be given and would in other cases be afforded by an extension 
of the principles he advocated. “No doubt,” he said’, “as long as 
man and all other animals are viewed as independent creations, an 
effectual stop is put to our natural desire to investigate as far as 
possible the causes of Expression. By this doctrine, anything and 
everything can be equally well explained....With mankind, some 
expressions...can hardly be understood, except on the belief that man 
once existed in a much lower and animal-like condition. The com- 
munity of certain expressions in distinct though allied species...is 
rendered somewhat more intelligible, if we believe in their descent 
from a common progenitor. He who admits on general grounds that 
the structure and habits of all animals have been gradually evolved, 
will look at the whole subject of Expression in a new and interesting 
light.” 
Darwin relied on three principles of explanation. “The first of 
these principles is, that movements which are serviceable in gratifying 
some desire, or in relieving some sensation, if often repeated, become 
so habitual that they are performed, whether or not of any service, 
whenever the same desire or sensation is felt, even in a very weak 
degree.” The modes of expression which fall under this head have 
become instinctive through the hereditary transmission of acquired 
habit. “As far as we can judge, only a few expressive movements 
are learnt by each individual; that is, were consciously and voluntarily 
performed during the early years of life for some definite object, or 
in imitation of others, and then became habitual. The far greater 
number of the movements of expression, and all the more important 
ones, are innate or inherited; and such cannot be said to depend on 
the will of the individual. Nevertheless, all those included under 
our first principle were at first voluntarily performed for a definite 
object,—namely, to escape some danger, to relieve some distress, or 
to gratify some desire®.” 
“Our second principle is that of antithesis. The habit of volun- 
1 Expression of the Emotions, p. 13. The passage is here somewhat condensed. 
2 Ibid. p. 368, 3 Ibid. pp. 373, 374, 
