(14) This at any rate seems certain - they are not in either case the product of disease, as we use the word. There is nothing pathological about them. They are physiological in their nature. But they are so strange, irrational, and disorderly as to be readily and properly taken as manifestations of disordered mind. Loud boisterous laughter with its purposeless movements - the dancing about, the clapping of hands, the stamping with the feet, and the shouting - it is difficult to regard all this as a manifestation of sober sanity. And in all I have said I have not so regarded it. I am treating it as an indication of a certain transient condition of mental disorder, which has about it no trace of disease. Man wants to laugh - he greatly desires it -he spends large sums of money in the purchase of laughter, and he is conscious of its doing him good. If it represents fits of unreason, he feels that he comes out of those fits into a stronger reason, and he laughs at those who counsel him not to laugh, as he does at those who counsel him not to sleep and dream.

(15) I have endeavoured to show that dream-thinking may correctly be regarded as a state of mental disorder - a form of delirious thinking, which is physiological and not pathological, and which is beneficial to the mental health. "We may as correctly advise men to avoid sleep, as to avoid laughter. It is during sleep that dreams occur, but no one doubts that sleep yields health of mind and body, and laughter 1 think has been as rightly called ' a promoter of sanity.' Yet, in my opinion, dreaming and laughing both appear to be states of passing mental disorder.

(16) To bring this view under consideration is my chief object, and I might stop here. But I think I can usefully go further, and state various things about laughter, which, by more fully disclosing its curious and complex character, may give support to the opinion.

(17) It is frequently said that man is the only animal that laughs. Emerson puts this common belief prettily when he says: 'A taste for fun is all but universal in our species, which is the only joker in nature.' So far as is yet known, it is true that no other animal laughs exactly as man usually does. It is necessary to say as man usually does, because men, women, and children laugh differently, and the laughter of one race is not the same as that of another race. These differences are considerable, yet the laughter of all of them has a complete sameness in essentials.

The reiteration that takes place in the sounds attending laughter may be regarded as of the nature of an essential, and this reiteration Darwin saw in the tittering of monkeys, as the expression of pleasurable feelings. He also says that the anthropoid apes utter a reiterated sound, corresponding with the laughter of man, when they are tickled, especially under the armpits. Why the laughter of man and its analogue the tittering of monkeys should be a rapidly reiterated sound, we do not know. When the armpits of the Chimpanzee are tickled, there follows a chuckling or laughing sound, with a wrinkling of the face. So Darwin says, and he says also that, if the Orang is tickled, it grins and makes a chuckling sound, and when that ceases the expression of the face has been held to resemble in some degree that of smiling in man. The muscles of the jaws and lips are chiefly affected in the Baboon and other monkeys in what has been regarded as their laughter, but in man the muscles chiefly affected are those of the chest. No peals of loud laughter, with rapid and violent spasmodic expirations and inspirations, have been observed in the apes or in any other animal. The apes appear to show what may be considered an imperfect laughter, but no fully formed and fully voiced laughter occurs among them. Perhaps what has been seen to occur in the apes may be properly accepted as their form of man's laughter. It has been objected that Darwin sometimes saw what he desired to see, but no weight can here attach to this objection. The different races of man, all over the world, express their emotions and feelings with remarkable uniformity, and there is no uncertainty or hesitation in saying that all of them laugh. There are differences in their laughter, but all the essentials are present, and they always constitute unmistakable laughter - beyond doubt or question. But as regards the movements, which in apes seem to answer to the laughter of man and to resemble it, there is difficulty in showing in what the resemblance to man's laughter consists, and that there is no straining of the evidence. A man, who chuckled or tittered like a monkey, could scarcely be said to be laughing; nevertheless this tittering may be the monkey's way of laughing, and may represent nothing more than a great increase of the difference that exists between the laughter of one man or one race of men, and that of another man or another race.

Except as regards the apes or monkeys, no serious effort, so far as I am aware, has been made to show that other animals, either wild or domesticated, express pleasurable feelings by movements and sounds which can be regarded as representing, or as analogous to, the voiced convulsive laughter of man, and we may therefore conclude that, if they do so, it must be in a very obscure manner. Though I say this, I shall afterwards have to state that many believe that dogs have the power both to laugh and joke.

(18) Without definitely holding that man is the only animal that laughs, it may be confidently said that he is the only animal that has bursts of loud laughter, with reiterated sounds, and extensive commotion of the muscles of the chest and face; that possibly the anthropoid apes have both a voiced and silent laughter, somewhat resembling, but at the same time greatly differing from, the laughter and smile of man; and that no other animal shows anything which can be properly accepted as either sonorous or silent laughter - unless it be the dog, to which reference will be made. With these qualifications man may, perhaps, be correctly enough spoken of as the only animal that laughs.