INTELLECTUAL FUNCTIONS OF ANIMALS. 
11 
These remarkable changes are not peculiar to Insects, for all living 
beings are more or less metamorphosed during the period of their 
growth j that is to say, they lose certain parts altogether, and develop 
others which were formerly less considerable. Thus, the antennas, 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 branchia-. 
Even the infant, t)eforc its birth, at that period, and during its progress to maturity, 
undergoes several metamorphoses. In the earlier 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 wdiero a neck after- 
wards appears, an arrangement, whicli 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 tho end of the fifth month, it is covered 
■all over with a yellowish white silk, like the down of a young duck, ndiich entirely 
disappears in six or seven weeks. The limbs arc fonnod under the skin, and reaching 
it, gradually shoot out into their fiermancnt position; yet, even wlien fully developed 
in other respects, tho shoulders tuid thighs are still concealed under the skin. In this 
respect, the embryo resomhles the horse and other animals, which have the shoulders 
and thighs permanently enveloped under a thick covering of muscle. Tho fingers, 
when first formed, are .surrounded by a skin, which entirely covers them, like the 
mitten-gloves used for an infant. This covering i.s gradually absorbed, when it takes 
the form of a duck’s web, and finally disappcai-s. M. Tieddeman and M. .Serres, 
have shown that tho brain of the foitus, in the highest class of animals, assumes in 
succession the various forms which belong to Fishes, Ueptiles, and Birds, before it ac- 
quires those additions and modifications which are peculiar to the hfammalia. “ If 
you exainiue the brain of tho Wiraimalia,” says M. Serres, “ at an early stage of ute- 
rine life, you perceive the cerebr.al hemispheres consolidated, as in Fish, in two vesicles 
isolated one from the other ; at a later period, yon see them atfoct the configuration 
of tho cerebral hemispheres of Beptilcs ; still later again, they present you with the 
forms of those of Bird,s; finally, they acquire, at the era of birth, and sometimes later, 
the permanent forms which the adult Mammalia present." 
As the infant grows tow.ards manliood 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 body' increases at a much greater rate than 
the head, and tho head more rapidly than the interna! part of the ear. 
Le lien oh los germes se montrent, rassemblage de cos gerrnos se nomme 
I’ot’nu'c; le canal, par oh les gerincs une fois detaches se rendont an dc- 
liors, Voviductius : la cavite oh ils sont obliges, dans plusicurs especcs, de 
sejourner un temps plus on moins long avant de naitro, la matricc on 
I’affru.!; rorificc exterieur par lequel ils sortent, la vidoc. Quand il y a 
ties sexes, le sexc mfile est, cehii qni fcconde; le sexe femcllo celni dans 
lequel les germes paraissent. La liqueur fccondante se iiommo sperme; 
les glandes qni la sSparent du sang, tcslicules; ct, quand il faut qu’elle 
sou introduite dans le corps do la femelle, I’organe qni I’y portc s'ap}>cl!e 
verge. 
SECT. VII A BRIEF NOTICE OF THE INTEI.I.ECTOAL EITNCTIONS OF ANIMAI.S. 
^Hnd — Matter — Setisftiiou — Tllusious — Perception — Memory — Amoe.iatloti of Ideas 
— Abst ractian — Jvdymeni — Faculties of Man and other animals compared — In- 
stinct — Connexion heticecn the Brain and Intellectual Faculties. 
have already cxplaimul, when treating of the nervous system, that before the 
ttend can perceive an object, an impression must bo made upon an organ of sense, 
Bither immediately, or through some material medium ; and that this impression must 
ke transmitted through the nerves to the brain. 
Hut 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 prove* the existence of matter, it is only ha- 
zarding u gratuitous hypotho.sis to attempt to explain mind by materialism, 
[or by analogies borrowed from tlie qualities 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 
^certmn the conditions of the liody attendant on sensation, — to trace the 
Oktreme gradations of intellect in all living beings, — to investigate the 
precise point of perfection attainable by each animal, — and, finally, to 
■tscortain whether there be not certain modifications of tlie intelleetual 
powers, occasioned by' tlie peculiar organization of each species, or by 
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. IVe cannot prove the cx- 
stenee of mind, but we are conscious of its existence; and we cannot prove the ex- 
*stcncc of matter, for wo perceive it. 
there must be an uninterrupted communication of nerves between the 
external organ of sense, and the central masses of the nervous system. 
The mind is, tbcroforc, 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 the nervous chain of com- 
munication, or in the central masses themselves. Tins 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 arc referred by the rahid 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 art of sneezing, gleams of light 
are emitted from each eye, both during the inhalation of the air, and during its sub- 
sequent protrusion ; and in blowing air violently through the nostrils, two patches of 
light appear above the axis of tho eye and in front of it, while other two luminous 
spots unite into one, and appear as it were about tho point of tho 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. W'hen tho stomach is under a 
temporary derangement, accompanied by headache, the pressure of tho blood-vessels 
upon the retina shows itself, in total darkness, by a faint blue liglit 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 biu-ning fire, or upon an irregu- 
larly shaded surface, it is easy to conceive huw 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 conch 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, whicii imparts a new character to the phantasms of the senses.”* 
Many cireum.staneos render it extremely probable, that the pictures drawn in tlie 
mind by memory, or created Ijy imagination, do not merely exist “ in tho mind’s 
eye,” hut are actually figured on the retina. During health, and in ordinary ca.sps, 
these images are faint, and are easily didingnished from the sensations resulting from 
real perception. It is only wlnm the body is affected by certain diseases, or during 
sleep, that tho impressions on tho retina appear to proceed from objects in actual 
existence. 
Several instances might be brought forward to illustrate the illusiuns of the senses. 
By the well -known experiment of raiddng 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 tho nerve on ils passage to the 
central mas.s, the mind will bo affected in the same maimer as if the impression had 
been made on the external organ. 
By the terms central massc.t, we understand a certain portion of tlie 
nervous system, which is always more circumscribed as tlie animal is 
more perfectly constructed. In Man it is exclusively a limited portion of 
the brain. On tlie contrary, in lleptilos the central mass may include 
either tiie brain, tlio entire marrow, or any portion of them taken sepa- 
rately; so that the absence of 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 
diflitscd. 
We arc 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 tlie cause of the sensation to some external object. 
This constitutes perception ; and the mind is said to form an idea of the 
oliject. By a necessary law involved in the constitution of tho mind, all 
the ideas of material objects are in time and in space. 
When an impression has once been made tlirongh the medullary 
masses upon the mind, it jiossesses the pow'or of recalling the impression 
after the exciting cause has been removed. This is memory, a faculty 
which varies much with the age and health of tho individual. 
During childhood, and in youth, tho memory is very vivid. Accordingly, this 
period of life Ls 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; but the memory of a former 
mental impression is in general more faint. 
Certain diseases, such as apoplexy, destroy tho memory, either entirely or partially. 
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 st.ages of fevers. 
* Letters on Natural Magic, by Sir David Brewster. 
