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by their often at the same time taking a bit of the shawl into their mouths and sucking it； generally closing their eyes and purring from delight。 This curious movement is commonly excited only in association with the sensation of a warm soft surface； but I have seen an old cat； when pleased by having its back scratched； pounding the air with its feet in the same manner； so that this action has almost become the expression of a pleasurable sensation。
＇16＇〃Dr。 Darwin； ‘Zoonomia；' 1794； vol。 i。 p。 160。 I find that the fact of cats protruding their feet when pleased is also noticed （p。 151） in this work。
Having referred to the act of sucking； I may add that this complex movement； as well as the alternate protrusion of the fore…feet； are reflex actions； for they are performed if a finger moistened with milk is placed in the mouth of a puppy； the front part of whose brain has been removed。＇17＇ It has recently been stated in France； that the action of sucking is excited solely through the sense of smell； so that if the olfactory nerves of a puppy are destroyed； it never sucks。 In like manner the wonderful power which a chicken possesses only a few hours after being hatched； of picking up small particles of food； seems to be started into action through the sense of hearing； for with chickens hatched by artificial heat； a good observer found that 〃making a noise with the finger…nail against a board； in imitation of the hen…mother； first taught them to peck at their meat。〃＇18＇
＇17＇ Carpenter； ‘Principles of Comparative Physiology；' 1854； p。 690； and Muller's ‘Elements of Physiology；' Eng。 translat。 vol。 ii。 p。 936。
＇18＇ Mowbray on ‘Poultry；' 6th edit。 1830； p。 54。
I will give only one other instance of an habitual and purposeless movement。 The Sheldrake （_Tadorna_） feeds on the sands left uncovered by the tide； and when a worm…cast is discovered； 〃it begins patting the ground with its feet； dancing as it were； over the hole；〃 and this makes the worm come to the surface。 Now Mr。 St。 John says； that when his tame Sheldrakes 〃came to ask for food； they patted the ground in an impatient and rapid manner。〃＇19＇ This therefore may almost be considered as their expression of hunger。 Mr。 Bartlett informs me that the Flamingo and the Kagu （_Rhinochetus jubatus_） when anxious to be fed； beat the ground with their feet in the same odd manner。 So again Kingfishers； when they catch a fish； always beat it until it is killed； and in the Zoological Gardens they always beat the raw meat； with which they are sometimes fed； before devouring it。
We have now； I think； sufficiently shown the truth of our first Principle； namely； that when any sensation； desire； dislike； &c。； has led during a long series of generations to some voluntary movement； then a tendency to the performance of a similar movement will almost certainly be excited； whenever the same； or any analogous or associated sensation &c。； although very weak； is experienced； notwithstanding that the movement in this case may not be of the least use。 Such habitual movements are often； or generally inherited； and they then differ but little from reflex actions。 When we treat of the special expressions of man； the latter part of our first Principle； as given at the commencement of this chapter； will be seen to hold good； namely； that when movements； associated through habit with certain states of the mind； are partially repressed by the will； the strictly involuntary muscles； as well as those which are least under the separate control of the will； are liable still to act； and their action is often highly expressive。 Conversely； when the will is temporarily or permanently weakened； the voluntary muscles fail before the involuntary。 It is a fact familiar to pathologists； as Sir C。 Bell remarks；＇20＇ 〃that when debility arises from affection of the brain； the influence is greatest on those muscles which are； in their natural condition； most under the command of the will。〃 We shall； also； in our future chapters； consider another proposition included in our first Principle； namely； that the checking of one habitual movement sometimes requires other slight movements； these latter serving as a means of expression。
＇19＇ See the account given by this excellent observer in ‘Wild Sports of the Highlands；' 1846； p。 142。
＇20＇ ‘Philosophical Translations；' 1823； p。 182。
CHAPTER II。
GENERAL PRINCIPLES OF EXPRESSION_continued_。
The Principle of AntithesisInstances in the dog and cat Origin of the principleConventional signsThe principle of antithesis has not arisen from opposite actions being consciously performed under opposite impulses。
WE will now consider our second Principle； that of Antithesis。 Certain states of the mind lead； as we have seen in the last chapter； to certain habitual movements which were primarily； or may still be； of service； and we shall find that when a directly opposite state of mind is induced； there is a strong and involuntary tendency to the performance of movements of a directly opposite nature； though these have never been of any service。 A few striking instances of antithesis will be given； when we treat of the special expressions of man； but as； in these cases； we are particularly liable to confound conventional or artificial gestures and expressions with those which are innate or universal； and which alone deserve to rank as true expressions； I will in the present chapter almost confine myself to the lower animals。
When a dog approaches a strange dog or man in a savage or hostile frame of mind be walks upright and very stiffly； his head is slightly raised； or not much lowered； the tail is held erect； and quite rigid； the hairs bristle； especially along the neck and back； the pricked ears are directed forwards； and the eyes have a fixed stare： （see figs。 5 and 7）。 These actions； as will hereafter be explained； follow from the dog's intention to attack his enemy； and are thus to a large extent intelligible。 As he prepares to spring with a savage growl on his enemy； the canine teeth are uncovered； and the ears are pressed close backwards on the head； but with these latter actions； we are not here concerned。 Let us now suppose that the dog suddenly discovers that the man he is approaching； is not a stranger； but his master； and let it be observed how completely and instantaneously his whole bearing is reversed。 Instead of walking upright； the body sinks downwards or even crouches； and is thrown into flexuous movements； his tail； instead of being held stiff and upright； is lowered and wagged from side to side； his hair instantly becomes smooth； his ears are depressed and drawn backwards； but not closely to the head； and his lips hang loosely。 From the drawing back of the ears； the eyelids become elongated； and the eyes no longer appear round and staring。 It should be added that the animal is at such times in an excited condition from joy； and nerve…force will be generated in excess； which naturally leads to action of some kind。 Not one of the above movements； so clearly expressive of affection； are of the least direct service to the animal。 They are explicable； as far as I can see； solely from being in complete opposi