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rs； or the dilatation of the capillary vessels of the skin；may all equally well serve for expression。 The three Principles are as follows。
＇1＇ Mr。 Herbert Spencer （‘Essays；' Second Series； 1863； p。 138） has drawn a clear distinction between emotions and sensations； the latter being 〃generated in our corporeal framework。〃 He classes as Feelings both emotions and…sensations。
I。 _The principle of serviceable associated Habits_。Certain complex actions are of direct or indirect service under certain states of the mind； in order to relieve or gratify certain sensations； desires； &c。； and whenever the same state of mind is induced； however feebly； there is a tendency through the force of habit and association for the same movements to be performed； though they may not then be of the least use。 Some actions ordinarily associated through habit with certain states of the mind may be partially repressed through the will； and in such cases the muscles which are least under the separate control of the will are the most liable still to act； causing movements which we recognize as expressive。 In certain other cases the checking of one habitual movement requires other slight movements； and these are likewise expressive。
II。 _The principle of Antithesis_。Certain states of the mind lead to certain habitual actions； which are of service； as under our first principle。 Now 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 are of no use； and such movements are in some cases highly expressive。
III。 _The principle of actions due to the constitution of the Nervous System； independently from the first of the Will； and independently to a certain extent of Habit_。… When the sensorium is strongly excited； nerve…force is generated in excess； and is transmitted in certain definite directions； depending on the connection of the nerve…cells； and partly on habit： or the supply of nerve…force may； as it appears； be interrupted。 Effects are thus produced which we recognize as expressive。 This third principle may； for the sake of brevity； be called that of the direct action of the nervous system。
With respect to our _first Principle_； it is notorious how powerful is the force of habit。 The most complex and difficult movements can in time be performed without the least effort or consciousness。 It is not positively known how it comes that habit is so efficient in facilitating complex movements； but physiologists admit＇2＇ 〃that the conducting power of the nervous fibres increases with the frequency of their excitement。〃 This applies to the nerves of motion and sensation； as well as to those connected with the act of thinking。 That some physical change is produced in the nerve…cells or nerves which are habitually used can hardly be doubted； for otherwise it is impossible to understand how the tendency to certain acquired movements is inherited。 That they are inherited we see with horses in certain transmitted paces； such as cantering and ambling； which are not natural to them； in the pointing of young pointers and the setting of young setters in the peculiar manner of flight of certain breeds of the pigeon； &c。 We have analogous cases with mankind in the inheritance of tricks or unusual gestures； to which we shall presently recur。 To those who admit the gradual evolution of species； a most striking instance of the perfection with which the most difficult consensual movements can be transmitted； is afforded by the humming…bird Sphinx…moth （_Macroglossa_）； for this moth； shortly after its emergence from the cocoon； as shown by the bloom on its unruffled scales； may be seen poised stationary in the air； with its long hair…like proboscis uncurled and inserted into the minute orifices of flowers； and no one； I believe； has ever seen this moth learning to perform its difficult task； which requires such unerring aim。
＇2＇ Muller； ‘Elements of Physiology；' Eng。 translat。 vol。 ii。 p。 939。 See also Mr。 H。 Spencer's interesting speculations on the same subject； and on the genesis of nerves； in his ‘Principles of Biology；' vol。 ii。 p。 346； and in his ‘Principles of Psychology；' 2nd edit。 pp。 511…557。
When there exists an inherited or instinctive tendency to the performance of an action； or an inherited taste for certain kinds of food； some degree of habit in the individual is often or generally requisite。 We find this in the paces of the horse； and to a certain extent in the pointing of dogs； although some young dogs point excellently the first time they are taken out； yet they often associate the proper inherited attitude with a wrong odour； and even with eyesight。 I have heard it asserted that if a calf be allowed to suck its mother only once； it is much more difficult afterwards to rear it by hand。＇3＇ Caterpillars which have been fed on the leaves of one kind of tree； have been known to perish from hunger rather than to eat the leaves of another tree； although this afforded them their proper food； under a state of nature；＇4＇ and so it is in many other cases。
＇3＇ A remark to much the same effect was made long ago by Hippocrates and by the illustrious Harvey； for both assert that a young animal forgets in the course of a few days the art of sucking； and cannot without some difficulty again acquire it。 I give these assertions on the authority of Dr。 Darwin； ‘Zoonomia；' 1794； vol。 i。 p。 140。
The power of Association is admitted by everyone。 Mr。 Bain remarks； that 〃actions； sensations and states of feeling； occurring together or in close succession； tend to grow together； or cohere； in such a way that when any one of them is afterwards presented to the mind； the others are apt to be brought up in idea。〃＇5＇ It is so important for our purpose fully to recognize that actions readily become associated with other actions and with various states of the mind； that I will give a good many instances； in the first place relating to man； and afterwards to the lower animals。 Some of the instances are of a very trifling nature； but they are as good for our purpose as more important habits。 It is known to everyone how difficult； or even impossible it is； without repeated trials； to move the limbs in certain opposed directions which have never been practised。 Analogous cases occur with sensations； as in the common experiment of rolling a marble beneath the tips of two crossed fingers； when it feels exactly like two marbles。 Everyone protects himself when falling to the ground by extending his arms； and as Professor Alison has remarked； few can resist acting thus； when voluntarily falling on a soft bed。 A man when going out of doors puts on his gloves quite unconsciously； and this may seem an extremely simple operation； but he who has taught a child to put on gloves； knows that this is by no means the case。
＇4＇ See for my authorities； and for various analogous facts； ‘The Variation of Animals and Plants under Domestication；' 1868； vol。 ii。 p。 304。
＇5＇ ‘The Senses and the Intellect；' 2nd edit。 1864； p。 332。 Prof。 Huxley remarks （‘Elementary Lessons in Physiology；' 5th edit。 1872； p。 306）； 〃It may be laid down as a