86 MEANS OF EXPRESSION CHAP. IV. will ever be given. We know that some animals, after being domesticated, have acquired the habit of utter- ing sounds which were not natural to them.1 Thus do- mestic dogs, and even tamed jackals, have learnt to bark, which is a noise not proper to any species of the genus, with the exception of the Canis latrans of North Ameri- ca, which is said to bark. Some breeds, also, of the do- mestic pigeon have learnt to coo in a new and. quite peculiar manner. The character of the human voice, under the influ- ence of various emotions, has been discussed by Mr. Her- bert Spencer 2 in his interesting essay on Music. He clearly shows that the voice alters much under different conditions, in loudness and in quality, that is,'in reso- nance and timbre, in pitch and intervals. No one can listen to an eloquent orator or preacher, or to a man call- ing angrily to another, or to one expressing astonish- ment, without being struck with the truth of Mr. Spen- cer's remarks. It is curious how early in life the modu- lation of the voice becomes expressive. With one of my children, under the age of two years, I clearly perceived that his humph of assent was rendered by a slight modu- lation strongly emphatic; and that by a peculiar whine his negative expressed obstinate determination. Mr. Spencer further shows that emotional speech, in all the above respects is intimately related to vocal music, and consequently to instrumental music; and he attempts to explain the characteristic qualities of both on physio- logical grounds—namely, on "the general law that a feeling is a stimulus to muscular action." It may be 1 See the evidence on this head in my ' Variation of Animals and Plants under Domestication,' vol. i. p. 27. On the cooing- of pig-eons, vol. i. pp. 154, 155. 2.' Essays, Scientific, Political, and Speculative,' 1858. * The Origin and Function of Music,' p. 359.