In the second edition of his 'Principles of Psychology,’ Spencer added a chapter entitled, 'The Language of the Emotions,’ which is of great value to us, as it was printed a few months before Darwin published his book, 'The Expression of the Emotions.’ One of the most important ideas, physiologically speaking, which Spencer has formulated is the following: 'The molecular motion disengaged in any nerve-centre by any stimulus, tends ever to flow along lines of least resistance throughout the nervous system, exciting other nerve-centres, and setting up other discharges. The feelings of all orders, moderate as well as strong, which from instant to instant arise in consciousness, are the correlatives of nerve-waves continually being generated and continually reverberating throughout the nervous system--the perpetual nervous discharge constituted by these perpetually-generated waves, affecting both the viscera and the muscles, voluntary and involuntary.’ The ideas developed by Darwin on the origin of the expressions have such a striking resemblance, even identity, with Spencer’s doctrine, that Darwin felt himself obliged to make the following declaration in a foot-note: 'I may state, in order that I may not be accused of trespassing on Mr. Spencer’s domain, that I announced in my “Descent of Man” that I had then written a part of the present volume.’[21] [21] Ch. Darwin: _The Expression of the Emotions_. London, 1872, p. 10. The origin of the movements of expression, as propounded by Spencer and more amply developed by Darwin in his book, does not convince me. The profound admiration which I cherish for these two great masters has made me timid in deviating from their path, but since the facts which presented themselves to me during my studies have convinced me that the same results might be obtained in another way, it is my duty to communicate those observations and experiments which point to another solution of the problem. I shall here quote a passage from Spencer’s 'Language of the Emotions,’ thus drawing, as we say, from the well of the great philosopher himself.