AUXILIARY SENSES
When we come to examine the methods by which, or through which, many of
the lower animals protect themselves from their enemies, we soon
discover that some of these means are very wonderful indeed. It is not
my purpose to discuss instinctive protective habits in this chapter; I
wish rather to call attention to two _senses_,[93] which are to be
observed in certain of the lower animals, and which man and some of the
higher animals have lost in the process of evolution. I refer to
tinctumutation, the "color-changing" sense, and the sense of direction,
or, as it is commonly and erroneously termed, the "homing instinct."
Neither of these faculties is instinctive, but they are, on the
contrary, true senses, just as hearing, or taste, or smell is a sense.
Careful dissections and repeated experiments have shown me, beyond
peradventure, that these two psychical habitudes have their centres in
the brains (ganglia) of animals which possess them.
[93] I believe that I am the first to claim the _sensual_ importance
of tinctumutation and the sense of direction or the "homing sense."
Heretofore they have been regarded, by all authorities as far as I
know, as instinctive in character.--W.
The chromatic function--and I use this term to designate the faculty of
changing color according to surroundings--is possessed by a number of
the lower animals. The chameleon is the best known of all the
tinctumutants (_tinctus_, color, and _mutare_, to change), though many
other animals possess this faculty in a very marked degree. In order to
understand the manner in which these changes or modifications of color
take place, one must know the anatomy of the skin, in which structure
these phenomena have their origin. The frog is a tinctumutant, and a
microscopic study of its skin will clearly demonstrate the structural
and physiological changes that take place in the act of tinctumutation.
The skin of a frog consists of two distinct layers. The epidermis or
superficial layer is composed of pavement epithelium and cylindrical
cells. The lower layer, or _cutis_, is made up of fibrous tissue,
nerves, blood-vessels, and cavities containing glands and cell elements.
The glands contain coloring matter, and the changes of color in the
frog's skin are due to the distribution of these pigment-cells, and the
power they have of shrinking or contracting under nerve irritation. The
pigment varies in individuals and in different parts of the body. Brown,
black, yellow, green, and red are the colors most frequently observed.
The color-cells are technically known as _chromatophores_. If the web
of a frog's foot be placed on the stage of a microscope and examined
with an achromatic lens, the chromatophores can readily be made out.
Artificial irritation will immediately occasion them to contract, or, as
is frequently the case, when contracted, will occasion them to dilate,
and the phenomena of tinctumutation may be observed _in facto_. Under
irritation the orange-colored chromatophores, when shrunk, become brown,
and the contracted yellow ones, when dilated, become greenish yellow.
When all the chromatophores are dilated, a dark color will predominate;
when they are contracted, the skin becomes lighter in color. Besides the
pigment-cells just described, Heincke discovered another kind of
chromatophore, which was filled with iridescent crystals. They were only
visible, as spots of metallic lustre, when the cells were in a state of
contraction. He observed these latter chromatophores in a fish belonging
to _Gobius_, the classical name of which is _Gobius ruthensparri_.[94] I
have seen this kind of color-cell in the skin of the gilt catfish, which
belongs to a family akin to _Gobius_. The skin of this fish retains its
vitality for some time after its removal from the body of the living
animal, and the chromatophores will respond to artificial irritation for
quite a while. In making my observations, however, I prefer to dissect
up the skin and leave it attached to the body of the fish by a broad
base. A few minims of chloroform injected hypodermatically rendered the
animal anæsthetic, and I could then proceed at my leisure, without being
inconvenienced by its movements. The causation of tinctumutation is now
definitely known. The theory that light acts directly on the
chromatophoric cells has been proved to be incorrect. Even the theory
that light occasions pigmentation is no longer tenable. I have, time and
again, reared tadpoles from the eggs in total darkness, yet they differ
in no respect from those reared in full daylight. The chromatophores
were as abundant and responded to irritation as promptly in the one as
in the other. The distinguished Paul Bert declared that the young of the
axolotl could not form pigment when reared in a yellow light. Professor
Semper, on the contrary, declares Bert's axolotls to be albinos, and
states that albinism is by no means infrequent in the axolotl; also that
Professor Kölliker, of Würtzburg, reared a family of white axolotls in a
laboratory where there was an abundance of light, and that he (Semper)
never succeeded in rearing an albino, though there was less light in his
laboratory than in that of Kölliker, and his axolotls came from the same
stock. Bert made the mistake of confounding albinism with the phenomenon
of etiolation in plants; in fact, he gives the name "etiolation" to the
albinism noticed in his axolotls.[95]
[94] Semper, _Animal Life_, p. 93.
[95] _Ibid._, p. 88 _et seq._
There is a marked difference between the functions of the chlorophyll
bodies found in plants and the chromatophores found in animals. The
former play one of the most important rôles in the drama of plant life,
inasmuch as they subserve a vital function, while the latter act a minor
part, because they serve only as an instrument or means of protection.
Light is of great importance in its influence on chlorophyll, which is a
microscopic, elementary body on which the vital strength of the plant
depends, while it is not at all necessary to the chromatophores,--cell
bodies secreting pigmentary matter for the purpose of protection. Of
course, when animals are subjected to darkness for very long periods of
time, the chromatophores are modified, and, sometimes, are wholly
obliterated. They follow a well-known natural law, which declares that,
when a function of an organ is no longer of any use to an animal, both
organ and function become rudimentary, and finally disappear.
Many animals live for generations in total darkness before losing their
pigment. I, myself, have seen black beetles in Mammoth Cave, Kentucky,
in the neighborhood of Gorin's Dome, which is far within the depths of
the cave. As beetles rarely range over a hundred yards from their place
of birth, these insects must have been born in the cave and reared in
the dark.
When speaking of light, if not otherwise specified, I mean diffused
daylight which carries no heat rays. I believe that heat is a prominent
factor in the production of color; the discussion of this point,
however, does not properly belong to the subject under consideration.
Some experiments on newts, made by myself several years ago, show that
the absence of light does not influence pigmentation,--that is, through
several generations. My animals were kept under observation from the
extrusion of the eggs until full maturity had been reached, and great
care was taken to make experiments as accurate and as conclusive as
possible.
Those reared in total darkness or in a red light were always
dark-colored; those reared in a yellow light[96] were almost but not
quite as dark; while those reared in white ironstone crocks and in
diffused daylight were very much lighter, being pearl-gray in color.
This apparent (for the microscope showed that it was only apparent)
absence of color in the last-mentioned specimens was due to
tinctumutation.
[96] Vide Dewar, "The Physiological Action of Light," _Nature_, p.
433, 1877; quoted also by Semper, _loc. cit. ante_, Notes, p. 423. I
do not think that the absence of the slight amount of color in the
animals reared under the yellow light was due to the "optic current"
of Dewar. The microscope showed that the chromatophores were just as
large and just as numerous, and that they contained as much pigment,
as those reared under the red light. The apparent absence of color was
due to tinctumutation.--W.
In most viviparous animals the embryo is developed in almost or absolutely
total darkness, yet when it is born it has bright colors. Kerbert has
found in the cutis of the embryonic chick, about the fifteenth day,
certain pigment-cells. These cells have entirely disappeared by the
twenty-third day. It is probable that little, if any, light can reach the
chick through the shell and membranes, yet pigment-cells develop and
disappear again.[97]
[97] Karl Semper, _Animal Life_, p. 422.
A butterfly emerges from the cocoon arrayed in all the colors of the
rainbow; yet it was developed, while in the _pupa_ state, in total
darkness. It is not necessary to mention further instances; we readily see
that pigmentation in animals is not necessarily dependent on light.
Neither is tinctumutation the result of the direct influence of light on
the chromatophores. Light, however, if not the direct, is the indirect
cause of this phenomenon. Lister, in 1858, showed that animals with
imperfect eyesight were not good tinctumutants, notwithstanding the fact
that they had the chromatophoric function. He showed, by his experiments
on frogs, that the activity of the chromatophores depended entirely on the
healthy condition of the eyes,--that is, so far as the phenomenon of
tinctumutation was concerned. So long as the eyes remained intact and
connected with the brain by the optic nerve, the light reflected from the
surrounding objects exerted a powerful influence on the chromatophores.
As soon as the optic nerve was severed, the chromatophores ceased to
respond to the influence of light and color, no matter how bright and
varied they were. The deductions drawn from these experiments are not to
be controverted or denied. The chromatophores are influenced by light
reflected from objects and transmitted _via_ the optic nerve to the brain;
from this organ the impression or irritation goes to the nerve governing
the contractile fibres of these pigment-holding glands.[98]
[98] Karl Semper, _Animal Life_, p. 95.
Pouchet followed Lister, and confirmed his conclusion by experiments on
fishes and crabs. He remarked that the plaice--a fish with a white
under-surface and a party-colored back--had the chromatophoric function
highly developed. Among a number of specimens which appeared pale on the
white, sandy bottom, he met "one single dark-colored fish, in which, of
course, the chromatophores must have been in a state of relaxation; and
this specimen was as distinct from its companions as from the bottom of
the aquarium. Closer investigation proved that the creature was totally
blind,[99] and thus incapable of assuming the color of the objects
around it, the eyes being unable to act as a medium of communication
between them and the chromatophores of the skin."[100] Thus far Pouchet
had only confirmed Lister's observations, although it is highly probable
that he was unaware of Lister's experiments. But he went a step further.
There are two ways in which cerebral impressions may be transmitted from
the brain to the skin: one, by way of the spinal cord and the pairs of
nerves arising from it and known as spinal nerves; the other, by two
nerves running close to the vertebral column--the sympathetic nerves.
[99] Mr. Gordon Rett has recently called my attention to a blind
"angel fish" which shows, most conspicuously, a lack of
tinctumutation. This fish was made blind for experimental
purposes.--W.
[100] Karl Semper, _Animal Life_, pp. 95, 96.
Pouchet cut the spinal cord close to the brain, yet the chromatophores
still responded to light impression, showing that they did not receive
the message through the cord and spinal nerves. He then divided the
sympathetic nerves, and the chromatophores lost at once the power of
contraction; he thus demonstrated that the sympathetic nerves were the
transmitters of the optical message, and not the cord.
This discovery of Pouchet is, psychologically, of great importance,
though he failed to recognize it as such. He was satisfied with its
anatomical and physiological significance.
When we remember that the actions of the sympathetic nerves are almost,
if not entirely, reflex in character, we at once see the psychological
importance of this discovery. This fact makes the phenomenon of
tinctumutation an involuntary act on the part of the animal possessing
the chromatic function, and thus keeps inviolate the fundamental laws of
evolution, which, were the facts otherwise, would be broken.[101]
[101] This simple fact of involuntary action renders the sensual
nature of the function all the more apparent.--W.
By a series of experiments on frogs I have confirmed the conclusion of
Pouchet _in toto_, and have even solved, so I believe and unhesitatingly
assert, the puzzling problem of the physiological _modus operandi_ of
the wonderful phenomenon of tinctumutation.
For a very long time I believed that this function was a distinct sense,
and, five years ago, I set to work in search of the sense's centre.
After many dissections I found it (in the frog) lying immediately below
the optic centres and closely connected with them. Nerve-fibres of the
sympathetic can easily be traced and can be seen to penetrate this
centre. When this centre is artificially stimulated either with the
point of a needle or with a mild electric current, tinctumutation can be
incited at will.
Again, when this centre is destroyed (which can be done without injury
to the optic centres), the chromatophoric function ceases--the
phenomenon of tinctumutation is no longer observable.
That the sympathetic nerves are the carriers of the messages from the
optic nerve and the color-changing centre, can be demonstrated by other
means than by excision of the nerve. Atropine, to a certain extent,
paralyzes the sympathetic when given in sufficiently large doses, and
injections of this drug beneath the skin of a frog render the division
of the sympathetic unnecessary. The chromatophores will not respond to
light impressions if the animal be placed thoroughly under the influence
of atropine.
A large number of the lower animals possess the chromatophoric function.
Several years ago, I placed in a large cistern several specimens of gilt
catfish. This is a pond fish and is quite abundant throughout the middle
United States. It is of a beautiful golden yellow color on the belly and
sides, shading into a lustrous greenish yellow on the back and head.
Several months after these fish had been placed in the cistern, it
became necessary to clean the latter, and the fish were taken out. They
were of a dusky drab color when first taken out, but soon regained their
vivid tints when placed in a white vessel containing clear water. They
had evidently changed color in order to harmonize with the black walls
and bottom of the cistern.
Certain katydids are marked tinctumutants. I took one from the dark
foliage of an elm and placed her on the lighter-colored leaves of a
locust. She could be easily seen when first placed on the locust; in a
few moments, however, she had faded to such an extent that she was
barely noticeable.
The larvæ of certain moths, beetles, and butterflies also possess the
chromatophoric function. The chromatophores in the larva of _Vanessa_
are very numerous, and this grub is a remarkably successful
tinctumutant; the same can be said of the larvæ of certain varieties of
_Pieris_.
The power of changing color so as to resemble, in coloring, surrounding
objects is evidently one of Nature's weapons of defence. In some animals
it is developed in a wonderful manner. Wherever it is found it becomes
to the animal possessing it a powerful means of defence by rendering it
inconspicuous, and in some instances wholly unnoticeable.
After nine years of careful, systematic, and painstaking investigation,
I am prepared to affirm that, besides the senses, sight, smell, taste,
touch, hearing, and tinctumutation, certain animals have yet another
sense, the sense of locality, or of direction, commonly called the
"homing instinct." This remarkable function of the mind is not an
instinct any more than the sense of sight or smell is an instinct, but
is, on the contrary, a true sense; for I have demonstrated by actual
experiment that it has a centre in the brains (ganglia) of some of the
animals possessing it, just as the other senses have their centres. And,
since this centre has been found in certain species, and that, too, in
creatures very low in the scale of animal life, it is reasonable to
infer that it is present in the brains (ganglia) of all those animals
which evince the so-called "homing instinct."
In the process of civilization certain of the five senses in man become
dull and blunted; thus, the sense of smell in the Tagals of the
Philippine Islands is much more acute than it is in the civilized
European, and what is true of the sense of smell is also true of the
other senses, save that of touch, in all primitive peoples. This last
sense seems to be much more acute in civilized man than it is in
savages. This, for certain psychical reasons, unnecessary to detail
here, is a necessary result of evolutionary growth and development.[102]
[102] Compare Tyler, _Anthropology_; De Quatrefages, _The Human
Species_; Peschel, _The Races of Man_; Lombroso, _L'Uomo Delinquente_;
Ellis, _The Criminal_; the writer, "Criminal Anthropology," _N. Y.
Medical Record_, January 13, 1894.
As far as I have been able to learn, after much research in natural
history, the anthropoid apes do not show that they possess the sense of
direction in a marked degree; thus we see that the immediate ancestors
of pithecoid man had already begun to lose this sense, which in man is
entirely wanting, and the absence of which should not be a matter of
surprise in the slightest degree, but rather a result that should be
expected.
Evidences of this sense are to be observed in animals of exceedingly low
organization. On one occasion, while studying a water-louse, as I have
already described elsewhere in this book, I saw the little creature swim
to a hydra, pluck off one of its buds, then swim a short distance away
and take shelter behind a small bit of mud, where it proceeded to devour
its tender morsel. In a short while, much to my surprise, the louse
again swam to the hydra, again procured a bud, and again swam back to
its hiding-place. This occurred three times during the hour I had it
under observation. The louse probably discovered the hydra the first
time by accident; but when it swam back to the source of its food-supply
the second time and then returned again to its sheltering bit of mud, it
clearly evinced conscious memory of route and a sense of direction.
The common garden-snail is a homing animal, and it will always return to
a particular spot after it has made an excursion in search of food. In
front of my dwelling there is a brick wall capped by a stone coping; the
overhanging edge of this coping forms a moist, cool home in summer for
hundreds of snails. Last summer I took six of these creatures, and,
after marking their shells with a paint of gum arabic and zinc oxide, I
set them free on the lawn some distance away from the wall. In course of
time, four of them returned to their homes beneath the stone coping; the
other two were probably killed and eaten by blackbirds, numbers of which
I noticed during the day feeding on the sward.
The centre of the sense of direction in snails is located at the base of
the cephalic ganglion (brain); this ganglion lies immediately between
and below the "horns" (eye-stalks), and is composed of several
circumscribed and well-marked accumulations or corpuscles of nerve-cells
and nerve-filaments.
This sense centre can easily be destroyed without inflicting injury on
the circumjacent sense centres. Whenever this is done, the snail loses
its sense of direction and locality, and cannot find its way back to its
home when it is carried thence, and deposited amid new surroundings. It
is not killed by the mutilation, for I have seen marked snails in which
this sense centre had been destroyed, alive and apparently in good
health, several weeks after having undergone this operation; they found
temporary homes wherever they chanced to be.
The limpet is likewise a homing animal, and invariably returns to its
home after journeys in search of food. Lieutenant L----, an officer in
the British navy, once told me that he had repeatedly had specimens of
this animal under observation for months at a time, and that they always
had particular spots, generally depressions in rocks, which they
regarded as homes, to which they would always return after excursions in
search of sustenance. Romanes makes a similar statement.[103]
[103] _Animal Intelligence_, pp. 28, 29.
Some beetles have their homing sense highly developed; thus, in Mammoth
Cave, the blind beetle (_Adelops_) has its particular home, and will
always return to it even when it is set free at a considerable distance.
Notwithstanding the fact these insects are blind, and that darkness
reigns in this immense cavern, they have periods of rest corresponding
with the diurnal rest-periods of kindred species living in daylight;
hence, it is easy to study their habits at home and abroad.
I have frequently marked these beetles and then set them free some
distance away from their domiciles; they would hide themselves at once
beneath stones or clods of earth, but as soon as they had recovered from
their fright they would turn towards home, and would not stop, if left
unmolested, until they arrived at their particular and individual homing
places. Truly a most wonderful exhibition of the homing sense!
At first, these beetles are, probably, directed and governed by their
sense of direction alone, but as soon as they arrive among familiar
surroundings, memory comes to their aid.
The agile flea is another "homesteader," and if marked, its favorite
resting-place on a dog or cat can easily be determined. After feeding,
it will invariably return to a certain spot in order to enjoy its nap in
peace; for, strange as it may seem, fleas are sound sleepers, and, what
is more, seem to require a great deal of sleep.[104]
[104] All insects have periods of rest, during which they seem to be
in a state of slumber. Their sleep may not be the physiological
slumber of mammals, yet it effects a like purpose in all
probability.--W.
Ants are, of the entire insect world, probably the most gifted
home-finders. Time and again have I tested them in this, sometimes
taking them what must have been, to these little creatures, enormous
distances from their nests before freeing them. Of course the ants
experimented with were marked, otherwise I could not have watched them
successfully. When an ant is taken into new surroundings and set free,
it at first runs here and there and everywhere. As soon, however, as it
regains its equanimity and recovers from its fright, it turns toward
home. At first it proceeds slowly, every now and then climbing tall
blades of grass, and from these high places viewing the surrounding
country in search of landmarks. As soon as it arrives among scenes
partially familiar to it, it ceases to climb grass-blades or weeds, and
accelerates its pace. When it arrives among well-known and accustomed
surroundings it runs along at its utmost speed, and fairly races into
its nest.
The burying beetle has a regular abode, to which it invariably returns
after performing the offices of mortician to some defunct bird, beast,
or reptile. This insect grave-digger, by the way, is remarkably expert
at its business, and will bury a frog or a bird in a very short time.
As soon as it has buried the dead animal and deposited its eggs, it
returns to its domicile beneath some log or stone.
Some snakes likewise are exceedingly domestic, and have their regular
dens, to which they resort on occasions. The homing sense seems to be
rather highly developed in them, for they can find their way back to
their dens from great distances. I have had under observation for the
past three years a garden snake, locally known as a "spreading viper";
this snake was brought to me by a friend[105] when it was only a foot
long, so I have known her (for it is a female) ever since her infancy.
Owing to some antenatal accident, this reptile has a malformed head, so
that I can readily recognize her at a distance of fifteen, twenty, or
even thirty feet. Last year she reared her first brood of young, which I
was fortunate enough to see with her on several occasions. Her den is on
my lawn; and in the autumn of last year she conducted her brood to it,
where they hibernated until spring. If I remember correctly, on the 29th
of March she came out of her den accompanied by a dozen of her progeny,
all but four (two pairs) of which I killed.[106] Snakes subserve a very
useful purpose in the economy of nature, but it is well to keep them in
limits, for, when very numerous, they become dangerous to young birds,
especially after they have passed the second year.
[105] Silas Rosenfield, Esq., Owensboro, Kentucky.
[106] The above was written in the summer of 1897. This interesting
specimen was killed by a day-laborer who had been temporarily employed
to assist the gardener. An autopsy revealed a bony tumor of the right
orbital arch, which, from a little distance, looked like a horn.--W.
With the exception of the anthropoid apes all mammals possess the homing
sense in a higher or lower degree; this is true also of birds.
Experiments with the nesting robin show conclusively that this bird can
find its way back to its nest when carried fifty miles from its home and
then set free among wholly unknown surroundings. The well-known exploits
of the carrier-pigeon are so familiar that they scarcely need comment.
On May 3, 1898, two carrier-pigeons, en route for Louisville, rested for
a time at Owensboro, Kentucky; these birds had been set free at New
Orleans, Louisiana. The duck and the goose sometimes have this sense
very highly developed. I once knew a goose to travel back home after
having been carried in a covered basket for the distance of eighteen
miles. A drake and duck have been known to return to their home after
being carried a distance of nine miles by railway. Instances of
home-returning by dogs, cats, horses, etc., are of such common
occurrence that I hardly need call attention to them; the following
instance is so unique, however, that I will present it:--
In the fall of 1861, a gentleman of Vincennes, Indiana, visited his
father at Lebanon, Kentucky; when this gentleman started to return
home, his father gave him a yoke of young steers, which he drove, _via_
Louisville, Kentucky, to Vincennes.
Shortly after his arrival at this last-mentioned town, the steers made
their escape, swam the river at Owensboro, Kentucky, 160 miles below
Louisville, Kentucky, and, in a week or so, were found one morning at
the gate of their old home at Lebanon. Directed by their homing sense
alone, these animals had made a journey of several hundred miles over
a route they had never seen!
Fishermen are aware that certain fish choose localities for
lurking-places, which they will share with no other fish. The black
bass, and brook trout, and sturgeon, and goggle-eye are familiar
examples of fish which have this habit.
On one occasion, I performed the following experiment: I took a black
bass from its home near a sunken stump, and, after passing a short piece
of thread through the web of its tail and knotting it, replaced it in
the river, two miles below its lurking-place. The next day I saw it in
its old home, clearly recognizable by the bit of thread which waved to
and fro in the clear water as the fish gently moved its tail!
In an examination of phenomena such as have been discussed in this
chapter, ay, throughout this book, we must lay aside the dogmatic
assertions of our superstitious ancestors, who, to paraphrase Roscoe,
"when awed by superstition, and subdued by hereditary prejudices, could
not only assent to the most incredible proposition, but could act in
consequence of these convictions, with as much energy and perseverance
as if they were the clearest deductions of reason, or the most evident
dictates of truth."[107]
[107] Roscoe, _Life of Leo X._, p. 3.
It will take the human race many, many years to unlearn, and to recover
from the effects of the superstitious cult of the shaman, who exists,
not only among savages, but also in the most highly civilized races of
the world! Superstition is the antithesis of knowledge; in fact, it is
but another name for ignorance.
There is yet another exceedingly interesting psychical trait to be
noticed in the lower animals, especially in insects; I refer to the
instinctive habit, letisimulation (_letum_, death, and _simulare_, to
feign). The word "instinctive" must not be used, however, when this
stratagem is to be observed in the higher animals other than the
opossum; for many of these animals sometimes make an occasional and a
_rational_ use of it, as I will endeavor to show in the next chapter.