12 INTRODUCTION. 



the features in different passions shows that, according 

 to the kind of feeling excited, entirely different groups 

 of the fibres of the facial nerve are acted on. Of the 

 cause of this we are quite ignorant." 



No doubt 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; and it 

 has proved as pernicious with respect to Expression 

 as to every other branch of natural history. With 

 mankind some expressions, such as the bristling of the 

 hair under the influence of extreme terror, or the un- 

 covering of the teeth under that of furious rage, can 

 hardly be understood, except on the belief that man once 

 existed in a much lower and animal-like condition. The 

 community of certain expressions in distinct though 

 allied species, as in the movements of the same facial 

 muscles during laughter by man and by various mon- 

 keys, is rendered somewhat more intelligible, if we be- 

 lieve 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. 



The study of Expression is difficult, owing to the 

 movements being often extremely slight, and of a fleet- 

 ing nature. A difference may be clearly perceived, 

 and yet it may be impossible, at least I have found it 

 so, to state in what the difference consists. When we 

 witness any deep emotion, our sympathy is so strongly 

 excited, that close observation is forgotten or rendered 

 almost impossible; of which fact I have had many curi- 

 ous proofs. Our imagination is another and still more 

 serious source of error; for if from the nature of the 



