INTELLECTUAL FUNCTIONS OF ANIMALvS. 



II 



These remarkable changes are not peculiar to Insects, for all living 

 beings are more or less metamorphosed during the period of their 

 growth; that is to say, they lose certain parts altogether, and develop 

 others which were formerly less considerable. Thus, the antennte, the 

 wings, and all the parts of the butterfly, were concealed under the skin of 

 the caterpillar; and, when the insect cast off its skin, the- jaws, the feet, 

 and other organs, which belong not to the butterfly, ceased to form a 

 part of its body. Again, the feet of the frog are inclosed within the skin 

 of the tadpole ; and the tadpole, in order to become a frog, loses its tail, 

 mouth, and gills or branchias. 



Even the infant, before its birth, at that period, and during its progress to maturity, 

 undergoes several metamorphoses. In the earher periods of development, the em- 

 bryo corresponds, in some of its parts, with certain of the lower animals. At first, 

 it seems destitute of a neck, and the heart is situate in the place where a neck after- 

 wards appears, an arrangement which is found to exist permanently in fish. There is 

 also a striking resemblance between the lower extremity of the vertebral column in 

 the embryo, and the tail of the fish. About the end of the fifth month, it is covered 

 all over with a yellowish white silk, like the down of a young duck, which entirely 

 disappears in six or seven weeks. The hmbs are formed under the skin, and reaching 

 it, gradually shoot out into their permanent position ; yet, even when fully developed 

 in other respects, the shoulders and thighs arc still concealed under the skin. In this 

 respect, the embryo resembles the horse and other animals, which have the shoulders 

 and thigiis permanently enveloped under a thick covering of muscle. The fingers, 

 when first formed, are surrounded by a skin, which entirely covers them, like the 

 niitten-gloves used for an infant. This covering is gradually absorbed, when it takes 

 the form of a duck's web, and finally disappears. JI. Tieddeman and M. Serres, 

 have shown that the brain of the foetus, in the highest class of animals, assumes in 

 succession the various forms which belong to Fishes, Reptiles, and Birds, before it ac- 

 quires those additions and modifications which are pecuhar to the Mammalia. " If 

 you examine the brain of the Mammalia," says M. Serres, " at an early stage of ute- 

 rine life, you perceive the cerebral hemispheres consolidated, as in Fish, in two vesicles 

 isolated one from the other; at a later period, you see them affect the configuration 

 of the cerebral hemispheres of Reptiles; still later again, they present you with the 

 forms of those of Birds; finally, they acquire, at the era of birth, and sometimes later, 

 the permanent forms which the adult Mammalia present." 



As the infant grows towards manhood it loses, at a certain age, the 

 thymus gland; by degrees it acquires hair, teeth, and beard; the relative 

 size of its organs changes ; the bodj' increases at a much greater rate than 

 the head, and the head more rapidly than the internal part of the ear. 



Le lieu oil les germes se montrent, I'assemblage de ces gerrnes se nomme 

 Vovaire; le canal, par ou les germes une fois detaches se rendent au de- 

 hors, Voviductus; la cavite oil ils sont obliges, dans plusieurs especes, de 

 sejourner un temps plus ou moins long avant de naitre, la malrice ou 

 Vuterus; I'orifice e.\terieur par lequel ils sortent, la vulve. Quand il y a 

 des sexes, le sexe male est celui qui feconde; le se.xe femelle celui dans 

 lequel les germes paraissent. La liqueur fecondante se nomme sperme; 

 les glandes qui la separent du sang, iesticules; et, quand il faut qu'elle 

 soit introduite dans le corps de la femelle, I'organe qui I'y porte s'appcUe 

 vcme. 



SECT. VII. — A BRIEF NOTICE OF THE INTELLECTU.\L FUNCTIONS OF .\NIM.\L5. 



Miad — Matter — Sensation — Illusions — Perception — Memory — Association nf Ideas 

 ■ — Abstraction — Judgment — Faculties of Man and other animals compared — In- 

 stinct — Connexion between the Brain and Intellectual Faculties, 



We have already explained, when treating of the nervous system, that before the 

 mind can perceive an object, an impression must be made upon an organ of sense, 

 either immediately, or through some materi-al medium ; and that this impression must 

 be transmitted through the nerves to the brain. 



But the manner in which sensation, and its consequent perception, are 

 produced, is a mystery impenetrable to the human understanding; and, 

 since philosophy is unable to jjrove* the existence of matter, it is only ha- 

 zarding a gratuitous hypothesis to attempt to explain mind by materialism, 

 [or by analogies borrowed from the quaUties of matter. The considera- 

 tion of the Physiology of the Human Mind, or Metaphysics, forms the 

 subject of another science.] But it is the province of the naturalist to 

 ascertain the conditions of the body attendant on sensation, — to trace the 

 extreme gradations of intellect in all living beings, — to investigate the 

 precise point of perfection attainable by each animal, — and, finally, to 

 ascertain whether there be not certain modifications of the intellectual 

 powers, occasioned by the peculiar organization of each species, or by 

 the momentary state of each individual body. 



It has been already explained, that, to enable the mind to perceive. 



First truths do not admit of proof ; they are assumed. "We cannot prove the ex- 

 istence of mind, but we are conscious of its existence ; and we cannot prove the ex- 

 istence of matter, for we perceive it. 



there must be an uninterrupted communication of nerves between tlie 

 external organ of sense, and the central masses of the nervous system. 

 The mind is, therefore, conscious only of some impression made upon 

 these central masses. It follows, then, that the mind may be conscious 

 of real sensations, without any corresponding affection of the external 

 organ ; and these may be produced either in tlie nervous chain of com- 

 munication, or in the central masses themselves. This is the origin of 

 dreams, and visions, and of several casual sensations. 



The various kinds of spectral illusions proceed from impressions, which, being made 

 on the retina, are thence communicated to the brain, and are referred by the mind to 

 an object in actual existence. " When the eye or the head receives a sudden blow, 

 a bright flash of light shoots from the eyeball. In the act of sneezing, gleams of light 

 are emitted from each eye, both during the inhalation of the air, and durino- its sub- 

 sequent protrusion ; and in blowing air violently through the nostrils, two patches of 

 light appear above the axis of the eye and in front of it, while other two luminous 

 spots unite into one, and appear as it were about the point of the nose, when the 

 eyeballs are directed to it. In a state of indisposition, the phosphorescence of 

 the retina appears in new and more alarming forms. \\'hen the stomach is under a 

 temporary derangement, accompanied by headache, the pressure of the blood-vessels 

 upon the retina shows itself, in total darkness, by a faint blue light floating before the 

 eye, varying in its shape, and passing away at one side. The blue light increases in 

 intensity — becomes green and then yellow, and sometimes rises to red; all these 

 colours being frequently seen at once ; or the mass of light shades off into darkness. 

 When we consider the variety of distinct forms which, in a state of perfect health, 

 the imagination can conjure up when looking into a burning fire, or upon an irregu- 

 larly shaded surface, it is easy to conceive how the masses of coloured light which 

 float before the eye may be moulded, by the same power, into those fantastic and un- 

 natural shapes which so often haunt the couch of the invalid, even when the mind 

 retains its energy, and is conscious of the illusion under which it labours. In other 

 cases, temporary blindness is produced by pressure upon the optic nerve, or upon the 

 retina; and under the excitation of fever or delirium, when the physical cause which 

 produces spectral forms is at its height, there is superadded a powerful influence of 

 the mind, which imparts a new character to the phantasms of the senses."* 



i\Iany circumstances render it extremely probable, that the pictures drawn in the 

 mind by memory, or created by imagination, do not merely exist " in the mind's 

 eye," hut are actually figured on the retina. During health, and in ordinary cases, 

 these images arc faint, and are easily distinguished from the sensations resulting from 

 real perception. It is only when the body is aflfected by certain diseases, or durin'^ 

 sleep, that the impressions on the retina appear to proceed from objects in actual 

 existence. 



.Several instances might be brought forward to illustrate the illusions of the senses. 

 By the well-known experiment of making a galvanic circuit through the tongue, a 

 piece of zinc and one of silver, there is produced a pungent metallic taste, in the same 

 manner as would have followed the real application of a sapid substance. Thus it 

 may be seen that, if we communicate an impression to the nerve on its passage to the 

 central mass, the mind will be affected in the same manner as if the impression had 

 been made on the external organ. 



By the terms central masses, we understand a certain portion of the 

 nervous system, which is always more circumscribed as the animal is 

 more perfectly constructed. In Man it is exclusively a limited portion of 

 the brain. On the contrar}-, in Reptiles the central mass may include 

 either the brain, the entire marrow, or any portion of them taken sepa- 

 rately ; so that the absence cf the entire brain does not deprive them of 

 sensation. The extension of the term, when applied to lower classes of 

 animals, is much greater, as their sensitive power is still more widely 

 diffused. 



We are hitherto completely ignorant of the nature of the changes which take place 

 in the nerves and brain during perception, and of the manner in which the process is 

 carried on. Analogies derived from matter, sensible species, images, and vibrations, 

 obscure rather than explain this mysterious subject. 



A certain state of mind follows a certain impression upon an external 

 organ. We refer the cause of the sensation to some external object. 

 This constitMles perception ; and the mind is said to form an idea of the 

 object. By a necessary law involved in the constitution of the mind, all 

 the ideas of material objects are in time and in space. 



When an impression has once been made through the medullary 

 masses upon the mind, it possesses the power of recalling the impression 

 after the exciting cause has been removed. This is incnwri/, a faculty 

 which varies much with the age and health of the individual. 



During childhood, and in youth, the memory is very vivid. Accordingly, this 

 period of life is most favorable to the acquisition of knowledge, especially of those 

 subjects involving a great extent of detail, such as languages, geography, civil his- 

 tory, and natural history. The memory fails with increasing years. 



Vivid perceptions and sensations are easily conceived; tut the memory of a former 

 mental impression is in general more faint. 



Certain diseases, such as apoplexy, destroy the memory, either entirely or p.artially. 

 A disordered state of the stomach will deprive the mind of the power of following a 

 continued train of deep thought. This is also the case in the first stages of fevers. 



• Letters on Natural ?,Iagic, by Sir David Brewster. 



