第 9 节
作者:
忙 更新:2021-02-27 02:14 字数:9322
on to most or all of the higher quadrupeds; and must therefore have been first acquired at a very remote period。 Why the act of clearing the throat is not a reflex action; and has to be learnt by our children; I cannot pretend to say; but we can see why blowing the nose on a handkerchief has to be learnt。
'13' Muller remarks (‘Elements of Physiology;' Eng。 tr。 vol。 ii。 p。 1311) on starting being always accompanied by the closure of the eyelids。
'14' Dr。 Maudsley remarks (‘Body and Mind;' p。 10) that 〃reflex movements which commonly effect a useful end may; under the changed circumstances of disease; do great mischief; becoming even the occasion of violent suffering and of a most painful death。〃
It is scarcely credible that the movements of a headless frog; when it wipes off a drop of acid or other object from its thigh; and which movements are so well coordinated for a special purpose; were not at first performed voluntarily; being afterwards rendered easy through long…continued habit so as at last to be performed unconsciously; or independently of the cerebral hemispheres。
So again it appears probable that starting was originally acquired by the habit of jumping away as quickly as possible from danger; whenever any of our senses gave us warning。 Starting; as we have seen; is accompanied by the blinking of the eyelids so as to protect the eyes; the most tender and sensitive organs of the body; and it is; I believe; always accompanied by a sudden and forcible inspiration; which is the natural preparation for any violent effort。 But when a man or horse starts; his heart beats wildly against his ribs; and here it may be truly said we have an organ which has never been under the control of the will; partaking in the general reflex movements of the body。 To this point; however; I shall return in a future chapter。
The contraction of the iris; when the retina is stimulated by a bright light; is another instance of a movement; which it appears cannot possibly have been at first voluntarily performed and then fixed by habit; for the iris is not known to be under the conscious control of the will in any animal。 In such cases some explanation; quite distinct from habit; will have to be discovered。 The radiation of nerve…force from strongly…excited nerve…cells to other connected cells; as in the case of a bright light on the retina causing a sneeze; may perhaps aid us in understanding how some reflex actions originated。 A radiation of nerve…force of this kind; if it caused a movement tending to lessen the primary irritation; as in the case of the contraction of the iris preventing too much light from falling on the retina; might afterwards have been taken advantage of and modified for this special purpose。
It further deserves notice that reflex actions are in all probability liable to slight variations; as are all corporeal structures and instincts; and any variations which were beneficial and of sufficient importance; would tend to be preserved and inherited。 Thus reflex actions; when once gained for one purpose; might afterwards be modified independently of the will or habit; so as to serve for some distinct purpose。 Such cases would be parallel with those which; as we have every reason to believe; have occurred with many instincts; for although some instincts have been developed simply through long…continued and inherited habit; other highly complex ones have been developed through the preservation of variations of pre…existing instincts that is; through natural selection。
I have discussed at some little length; though as I am well aware; in a very imperfect manner; the acquirement of reflex actions; because they are often brought into play in connection with movements expressive of our emotions; and it was necessary to show that at least some of them might have been Erst acquired through the will in order to satisfy a desire; or to relieve a disagreeable sensation。
_Associated habitual movements in the lower animals_。 I have already given in the case of Man several instances of movements associated with various states of the mind or body; which are now purposeless; but which were originally of use; and are still of use under certain circumstances。 As this subject is very important for us; I will here give a considerable number of analogous facts; with reference to animals; although many of them are of a very trifling nature。 My object is to show that certain movements were originally performed for a definite end; and that; under nearly the same circumstances; they are still pertinaciously performed through habit when not of the least use。 That the tendency in most of the following cases is inherited; we may infer from such actions being performed in the same manner by all the individuals; young and old; of he same species。 We shall also see that they are excited by the most diversified; often circuitous; and sometimes mistaken associations。
Dogs; when they wish to go to sleep on a carpet or other hard surface; generally turn round and round and scratch the ground with their fore…paws in a senseless manner; as if they intended to trample down the grass and scoop out a hollow; as no doubt their wild parents did; when they lived on open grassy plains or in the woods。 Jackals; fennecs; and other allied animals in the Zoological Gardens; treat their straw in this manner; but it is a rather odd circumstance that the keepers; after observing for some months; have never seen the wolves thus behave。 A semi…idiotic dogand an animal in this condition would be particularly liable to follow a senseless habitwas observed by a friend to turn completely round on a carpet thirteen times before going to sleep。
Many carnivorous animals; as they crawl towards their prey and prepare to rush or spring on it; lower their heads and crouch; partly; as it would appear; to hide themselves; and partly to get ready for their rush; and this habit in an exaggerated form has become hereditary in our pointers and setters。 Now I have noticed scores of times that when two strange dogs meet on an open road; the one which first sees the other; though at the distance of one or two hundred yards; after the first glance always lowers its bead; generally crouches a little; or even lies down; that is; he takes the proper attitude for concealing himself and {illust。 caption = for making a rush or FIG。 4。Small dog watching a cat on a spring; although the road table。 From a photograph taken is quite open and The distance Mr。 Rejlander。} great。 Again; dogs of all kinds when intently watching and slowly approaching their prey; frequently keep one of their fore…legs doubled up for a long time; ready for the next cautious step; and this is eminently characteristic of the pointer。 But from habit they behave in exactly the same manner whenever their attention is aroused (fig。 4)。 I have seen a dog at the foot of a high wall; listening attentively to a sound on the opposite side; with one leg doubled up; and in this case there could have been no intention of making a cautious approach。
Dogs after voiding their excrement often make with all four feet a few scratches backwards; even on a bare stone pavement; as if for the purpose of covering up their excrement with earth; in nearly the same manner as do cats。 Wolves and jackals behave in the Zoological Gardens in exactly the same manner; yet; as I am assured by the keepers; neither wolves; jackals; nor foxes; when they have the means of doing so; ever cover up their excrement; any more than do dogs。 All these animals; however; bury superfluous food。 Hence; if we rightly understand the meaning of the above cat…like habit; of which there can be little doubt; we have a purposeless remnant of an habitual movement; which was originally followed by some remote progenitor of the dog…genus for a definite purpose; and which has been retained for a prodigious length of time。
Dogs and jackals'15' take much pleasure in rolling and rubbing their necks and backs on carrion。 The odour seems delightful to them; though dogs at least do not eat carrion。 Mr。 Bartlett has observed wolves for me; and has given them carrion; but has never seen them roll on it。 I have heard it remarked; and I believe it to be true; that the larger dogs; which are probably descended from wolves; do not so often roll in carrion as do smaller dogs; which are probably descended from jackals。 When a piece of brown biscuit is offered to a terrier of mine and she is not hungry (and I have heard of similar instances); she first tosses it about and worries it; as if it were a rat or other prey; she then repeatedly rolls on it precisely as if it were a piece of carrion; and at last eats it。 It would appear that an imaginary relish has to be given to the distasteful morsel; and to effect this the dog acts in his habitual manner; as if the biscuit was a live animal or smelt like carrion; though he knows better than we do that this is not the case。 I have seen this same terrier act in the same manner after killing a little bird or mouse。
'15' See Mr。 F。 H。 Salvin's account of a tame jackal in ‘Land and Water;' October; 1869。
Dogs scratch themselves by a rapid movement of one of their hind…feet; and when their backs are rubbed with a sti