But man has a much less exclusive possession of laughter than he has of blushing. Indeed, of all the expressions of emotion or feeling, blushing, I think, has been correctly described as the most strictly human. The beasts may, perhaps, join man at a great distance in laughing, but as yet we have no reason to think that any of them can join him even distantly in blushing.

(19) Man's erect position, with the alleged consequent special place of the diaphragm in relation to the heart, can but very lamely account for his being the only animal that laughs. I do not think that the opinion merits consideration. Nor is it better accounted for by his possession of reason, for it is scarcely possible to hold that reason is the mother or begetter of laughter, since (1) there is so much laughter which reason condemns or of which at least it cannot approve, (2) since the young in whom reason is not ripe laugh most, and since (3) there occurs a laughter - that from tickling - in which reason plays no part.

(20) I have lately had frequent interviews with a young smooth-haired retriever. When he is lying half asleep beside me, I often lightly touch the tuft of hair between the pad and the toes of a hind foot. This is always followed by a sudden and forcible retraction and extension of his leg. If I repeat and repeat the touching, the same thing happens, but he then gets up, and stretches himself out for another sleep at some distance from me, evidently in order to avoid what he finds disagreeable.

The same thing happens if I lightly touch the hair between the pad and toes of a fore foot, but in that case the jerk or kick is much less forcible.

A rough-haired Aberdeen terrier, when I treat him in the same way, gives the same kicks - perhaps less vigorously - and, as he hails from Aberdeen, he gets sooner out of the way of a repetition.

Something of a like kind happens, I believe, in the case of all dogs, though many persons seem ignorant of the fact. Mr. Charles Cook, an authority on matters relating to the habits and character of dogs, writes me that he has 'often experimented' in the way just described, and he gives the following as his general conclusion: 'If the hair between the pads of the toes of the hind foot be tickled, the dog's leg at once responds by spasmodic kicks.' He adds that ' if the fore feet are tickled, the dog shows discomfort by withdrawing the foot, but I have not noticed the same spasmodic action in the fore legs as in the hind legs,' and with reference to this he says that he fancies that' humans' are in this respect the same as dogs, namely, that they are more 'tickly' on the soles of their feet (their hind legs) than on the palms of their hands (their fore legs).

I have not been able to detect that these jerks in dogs are accompanied by any action either of the muscles of the face or of those of respiration. The jerks appear to be uncomplicated reflexes. Mr. Cook confirms this. He says that, so far as he has been able to make out, dogs never smile when thus tickled - they only show signs of discomfort.

The tickling of the sole of the foot of a quite young child seems to be exactly of the same character. Neither laughter nor smiling attends it. Darwin writes: 'I touched with a bit of paper the sole of the foot of one of my infants, when only seven days old, and it was suddenly jerked away, and the toes curled about,' and he regarded it as manifestly a simple reflex action.

(21) Mr. Cook says: ' I have never tied a dog up, and continued to tickle him - I never will - and so I cannot say what he would do in such circumstances, but I am sure he would act similarly to a man (persistently tickled), namely, he would first whine (protest and implore), then growl (swear), and finally howl (in agony).'

It has been very frequently alleged that the prolonged tickling of children has done serious harm, and cases in illustration could be easily adduced. The evidence of its having been carried so far in young adults as to cause permanent injury is not so clear, but there is no reason to doubt that such may be the case, I give one instance. A gentleman, thirty-one years old, recently consulted Dr. Graham-Brown for heart trouble - dilatation and irritability -which he - the patient - attributed to a tickling of the axilla; and the sides of the thorax, when his age was twenty-five, which was so prolonged as to make him practically speechless - unable even to beg that the tickling should be discontinued. Almost immediately thereafter, and more or less at intervals from that time down to the present time, the action of his heart has given him distress, and Dr. Graham-Brown thinks it probable that his patient is right as to the origin of the mischief.

(22) This simple reflex in a dog - best seen if he is young - appears to be on all fours with what happens to a very young child when the sole of its foot is tickled. The 'jerking away' follows in both, and in both no laughter or smiling follows. In the case of the child, however, when it grows older, but while it is still young, a difference appears. It still jerks its foot away, but there also occurs a commotion of the muscles of the face, with something like a smile. As it grows still older, voiced laughter follows the tickling. In the dog the thing appears to stop at the spasmodic jerk. In the child it passes on to something more. If the two things, in the dog and the infant, are at their beginning the same, it may be held to mean that dogs thus show the starting-point of laughter from tickling, which never gets in them beyond that point; yet, to that small extent, they may be said to exhibit the faculty of laughing.

(23) Firm pressure on the foot of the dog has not the effect of light touching or tickling. When firm pressure is made on the hair between the pads no jerk follows. The dog walks on its feet - puts the whole of its weight on them - and no jerk follows. So also it is with man, from the earliest age all through life. He can have the soles of his feet pressed firmly by the palm of another's hand without any tickling, or any tendency to a jerk or to laughter. He walks on his feet, and nothing of this kind takes place.