18 INTRODUCTION. 



causes combined, the observation' of Expression is by 

 no means easy, as many persons, whom I have asked to 

 observe certain points, have soon discovered. Hence 

 it is difficult to determine, with certainty, what are the 

 movements of the features and of the body, which com- 

 monly characterize certain states of the mind. Never- 

 theless, some of the doubts and difficulties have, as I 

 hope, been cleared away by the observation of infants, 

 — of the insane, — of the different races of man, — of 

 works of art, — and lastly, of the facial muscles under 

 the action of galvanism, as effected by Dr. Duchenne. 



But there remains the much greater difficulty of 

 understanding the cause or origin of the several ex- 

 pressions, and of judging whether any theoretical ex- 

 planation is trustworthy. Besides, judging as well as 

 we can by our reason, without the aid of any rules, which 

 of two or more explanations is the most satisfactory, 

 or are quite unsatisfactory, I see only one way of test- 

 ing our conclusions. This is to observe whether the 

 same principle by which one expression can, as it ap- 

 pears, be explained, is applicable in other allied cases; 

 and especially, whether the same general principles can 

 be applied with satisfactory results, both to man and 

 the lower animals. This latter method, I am inclined 

 to think, is the most serviceable of all. The difficulty 

 of judging of the truth of any theoretical explanation, 

 and of testing it by some distinct line of investigation, 

 is the great drawback to that interest which the study 

 seems well fitted to excite. 



Finally, with respect to my own observations, I may 

 state that they were commenced in the year 1838; and 

 from that time to the present day, I have occasionally 

 attended to the subject. At the above date, I was al- 

 ready inclined to believe in the principle of evolution, 

 or of the derivation of species from other and lower 



