We are becoming terribly learned, my poor child, and I am half afraid
you will be getting tired of me. When I was little myself, I had rather
a fancy for breaking open those barking pasteboard dogs you know so
well; to see what was inside them. Why should you not, then, feel a
certain amount of interest in looking with me into the insides of real
animals? Still I cannot conceal from myself that the subject grows
very serious at last, and that while I am busied in struggling to make
myself intelligible through the endless crowd of facts which surround
me, I am apt to neglect chatting with you as we go along. Happily,
however, here is an opportunity for so doing.
Up to the present time we have lived, as it were, upon the explanations
I gave you whilst studying the action of life in yourself, and all the
organs we have met with since, have been only, properly speaking,
reproductions, more or less exact, of those which you yourself possess.
But, in passing over into the kingdom of fishes, we find ourselves in
the presence of something altogether new, and I must go back to our
old familiar style of talking to open the subject.
Take a water-bottle half-filled with water, and shake it well, and you
will see a quantity of white froth come to the surface of the liquid.
This is the air which having been drawn in by the water, as it went
up and down in the bottle, is now struggling to fly off again in bubbles
as fast as it can. But the whole of it does not get away; a small
portion remains behind, and melts, as it were, into the water, as a
morsel of sugar would do, taking up its abode therein. This seems odd
to you, but I will tell you how you may convince yourself of the fact.
Get a small white glass bottle, slightly rounded, and thin at the
bottom, if possible; fill it with water, and hold it for a short time
over a lighted taper. If you do this carefully there is no danger. You
will soon see tiny little balls, looking like drops of silver, rise
from the bottom of the bottle, come up to the surface, and burst. This
is the air which was installed in the water, as I described above, and
which is now running away from the heat of the candle, as the
inhabitants run away from a house on fire. After a time the whole will
have passed off, and the little balls will cease to rise.
But what has all this to do with fishes? you ask.
A very great deal, I assure you, dear child. If there had been a little
fish in your bottle, before it was exposed to the flame, it would have
found means to make use of that air, whose original presence in the
water you cannot refuse to believe after having seen it come out. It
is with this air that fishes breathe in the water. They do so rather
feebly, I admit; but, as if to make up to them for the small amount
of the air placed at their disposal, it contains more oxygen than that
we breathe ourselves, because oxygen, dissolving more readily in water
than nitrogen, is there in greater proportion. Of course, you do not
suppose that fishes have lungs like ours? I dare say you know the two
large openings on each side of their head, called _gills_, by which the
fishermen string them together to carry them away more easily? It is
there you will find their lungs, to which the name of _branchiae_, or
gills, has been given, because they are so different from other organs
of respiration that it was impossible to use one word for the two. The
arrangement of the gills varies considerably in the different species,
but their general form is the same everywhere. They are composed of a
number of plates, consisting of an infinitude of leaflets, arranged like
a fringe, and suspended by bony arches, into which plates and leaflets
the blood pours from a thousand invisible canals.
First of all, then, we must see how blood circulates in fishes.
Like reptiles, their heart has only one ventricle, and yet the arterial
and venous blood go each its separate way without the slightest risk
of being mixed; but this is because fishes have not that double system
of veins and arteries which hitherto we have always met with. The
venous blood goes to the heart, which drives it into the gills, from
whence it passes forward of its own accord, as arterial blood into the
organs, under the remote influence of the original impetus from the
heart, the newly-arrived blood incessantly driving the other before
it into the vessels of circulation. It does not flow very quickly, as
you may suppose; and as the heart is close to the head, its action is
but very feebly felt at the extremity of the body, when this happens
to be very long. Nature has, in consequence, taken pity on the eel,
whose tail is so far from its heart, and provided accordingly. Dr.
Marshall Hall has discovered near the tip a second, reinforcing heart,
so to speak, which has its own pulsations, independent of the pulsations
of the one above, and gives a fresh impetus to the sluggish blood,
[Footnote: Many observers refer this to the lymphatic system.--TR.]
which otherwise, as it would seem, would scarcely be able to accomplish
the long return journey. Finally, even with an additional heart in
thetail, the circulation among fishes is quite on a par with their
respiration. They have a melancholy steward, whose legs are very heavy,
and his pockets very light, and their life comes down a peg lower in
consequence. It is always the same life nevertheless--you must never
lose sight of that fact: it gets low in consequence of the imperfection
of the machine, but without changing its nature, any more than the
light in our different sorts of lighting apparatus. You remember that
comparison of the lamp with which I began my story, and which you could
not at the time see the full value of? From a dungeon lamp up to a
candle, you have always grease burning in the air at the end of the
threads of a wick. It does not burn equally well everywhere, and does
not always give the same amount of light; but that is all the
difference. From the mammal to the fish, it is always hydrogen and
carbon (as we have said of the grease) which oxygen sets on fire in
the human body at the fine-drawn extremities of the blood-vessels;
only the fire is lower in some than others, and the life with it. Let
us now look at the circulation of water in the fish's body.
The gills communicate with the mouth by a sort of grating, formed by
the bony arches to which the gill-plates are suspended. The fish begins
by swallowing water, which then passes through the grating and
circulates round the innumerable leaflets of which each plate is
composed, and among which creep the blood-vessels. It is through the
thin coats of these leaflets that the mysterious exchange is made of
the unemployed oxygen in the water and the carbonic acid in the blood.
When this is over, the cover which closes the gills opens to let out
the water, and a fresh gulp takes its place; and so on continually.
When the fish is out of the water its gills fall together and dry up;
the course of the blood, already so weak, is interrupted by the breaking
down and shrinking of the vessels, and the animal can no longer breathe;
so that we have here the curious instance of a creature breathing
oxygen like ourselves, who is drowned, if we may use the expression,
in the air in which we find life, and lives in the water in which we
are drowned. While he is in the water matters take another course, and
his gills, moistened and supported, accommodate themselves perfectly
to the contact of the air, which desires nothing better than to give
up its oxygen to the blood, through the coats of the capillaries.
Accordingly you will often see fishes--carps, for example--come to
the surface of the water to inhale the air like a mammal or a reptile.
This is a valuable resource, which supplements the parsimonious
allowance of air given out to them by the water. There are even certain
fishes whose gills, more firmly closed than those of others, have, in
addition, a number of cells, which retain for a considerable time a
sufficient quantity of water to preserve the gills in their natural
state. These fishes can easily take an airing on land, where they
breathe the air as you or I do, and are downright amphibians.
The most celebrated of these is the _Anabas_, or "climbing-fish."
an Indian fish, which not only can remain many days out of the water,
but also amuses itself by climbing up the palm trees--it is hard to
say how--and establishing itself in the little pools of water left by
the rain at the roots of the leaves. But we need not go to India to
find those wandering fishes. There is one of them living among ourselves
who can walk about in the grass, and I was talking to you about him
only just now--that is the eel. If you ever put eels in a fish-pond
you must, I assure you, try to make it agreeable to them, otherwise
they will have no scruple in setting politeness at defiance and moving
off to seek their fortune elsewhere. In a country walk, when the dew
is on the ground, you yourself may chance to come across one or two
of these gentlemen, who have had their reasons for changing their
residence, and whom you will see gliding so briskly along that they
will deceive you into taking them for snakes if you have not a very
experienced eye; so much so, that in certain parts of France where the
peasants ate snakes formerly, they reconciled themselves to the sickly
idea by christening them _hedgerow-eels_.
On the other hand, fishes may be drowned in water just as easily as
ourselves if it does not contain air. The little fish who could have
lived very well in the bottle we were just now talking about before
you exposed it to the flame of the taper, would have died in it after
all the air-bubbles had gone off; and I hope I need not tell you why.
In the same way, if you leave fishes too long in a small quantity of
water without renewing it, they suffer exactly as we do if the air
which we breathe is not changed often enough. As soon as they have
consumed what oxygen is in the water, it can no longer keep them alive.
It is then, especially, you will see them come gasping to the surface
to call upon the air for help. Those who keep gold fish in a glass
bowl ought to know this, and to change their water oftener than is
generally done. When we take poor little creatures from their natural
way of life, and set a human providence over them in the place of the
Divine one which has hitherto been their safeguard, the least we can
do is to acquaint ourselves with the laws of their existence, so that
we may not expose them to the risk of suffering by our ignorance.
Finally, there are fishes whose gills, still more greedy of oxygen,
will not act well except in thoroughly aerated water, and who would
soon die in our tanks. This is the case with the trout, who is only
happy in the waters of hilly countries; rich with all the air they
have carried along with them as they fell from rock to rock. Now that
people are beginning to do with fishes what has long since been done
with sheep and oxen--keep them in flocks to have them always ready for
use--you may perhaps hear a good deal said about vessels made expressly
for the carriage of trout, with a thousand inventions besides for
sending air into the water, and you will not have to ask the meaning
of this now.
I promised last time that I would revert in the chapter of fishes to
that marvellous transformation of the crocodile which has been explained
by the torrent of water he draws into his stomach. You could understand
nothing about it the other day; but after what we have just seen the
explanation suggests itself. Just as the extraordinary activity of
life in birds is explained by that double oxygenization of blood, of
which part takes place in the lungs and part in the reservoirs of air
placed everywhere in the way of the capillaries, so this sudden increase
of energy in the crocodile the moment it plunges into water may be
explained by a second respiration suddenly established in the vast
cavity of the abdomen, by the contact of the capillaries with the water
which penetrates there. Hence the crocodile would then have, like the
bird, a double respiration: only with him the one would be permanent
and from the lungs, the other temporary and from the stomach. By this,
on the one hand, he would rise up to the birds, since the blood
encounters air twice over in its course, while, at the same time, he
would plunge into the world of fishes, since the blood has to seek air
in the water. The above, be it remembered, is only a supposition, and
I ought to add that in this case there would be a good deal of danger
in observing nature at work, for in front of the laboratory, where she
is toiling in secret, stands on guard a row of teeth, by no means
encouraging to indiscreet intruders. At the same time, if there ever
were a legitimate conjecture, this is it. Everything seems to confirm
it; and if it be true, we should have in the crocodile a specimen of
each of the four systems adopted by nature for the mammal, the bird,
the reptile and the fish. At first I spoke of two, then of three; so
that even in my addition I was modestly below the mark, and had really
some grounds for recommending our friends the classifiers to beware
what they asserted in this case.
Talking of puzzling classifications, this is just the place for
mentioning the _batrachians_, who have been made into a class by
themselves, but who most distinctly belong to two classes at the same
time; not like the crocodile by details borrowed from each, but by a
fundamental change which takes place at a certain period in their
organization. The batrachians are in reality reptiles, but they are
reptiles which begin by being fishes, and real fishes too.
If you have ever strolled about in the country, you must have often
come across those great pools of water which collect at rainy seasons
in the ruts of deep lanes. Amuse yourself by looking into them in
early summer, and unless the land is too parched and dry, the chances
are that you will see quantities of little black fishes, almost entirely
composed of a long tail joined to a large head, playing jovially in
the muddy waters, and looking as if they had dropped there from the
skies. These are young frogs--_tadpoles_, as we call them--and
they are beginning their apprenticeship of life. Enclosed in each side
of those great heads, they have gills, and they breathe in the same
manner as fishes. Presently the two hind feet begin to bud out and
grow, little by little; then the fore feet; finally, the tail wastes
away till it disappears; and thus insensibly the tadpole is transformed
into a frog. Observe here that the tadpole's gills share the same fate
as his fish-tail; they wither and disappear by slow degrees, and
gradually as they do so, his lungs are developed. The animal changes
his class very quietly, and without ceasing to be genuinely the same,
although it would be impossible at last to recognize the old individual
in the new if you had not heard its history beforehand. This is one
of the most striking exemplifications I know of the mysterious process
by which nature has insensibly raised animals from one class to another,
always improving upon her original plan without ever abandoning it.
On the shores of certain subterranean lakes which exist in Carniola,
a country subject at this time to Austria, there are to be found
batrachians far more ambitious than our frog--namely, the _proteans_.
These cumulate rather than change: they become reptiles without ceasing
to be fishes, if I may so express it; they develop lungs as they grow
up, and yet keep their gills. I could tell you a thousand other
particulars about these batrachians if I were to examine them all in
succession; for it is a very motley family, in the bosom of which the
transition from reptiles to fishes is in some imperceptible manner
accomplished; from the frog, which the unanimous consent of mankind has
always ranked among reptiles, to the axolotl or siren, who lives in
Mexican lakes; and who, feature for feature, is exactly like a carp,
with four little feet fastened under him. To be quite in order, the
batrachians ought to have followed the reptiles, for their interior
organization is the same; but how could I tell you about their gills
without explaining that there was air in the water? and I did not want,
for the sake of these intruders, whose babyhood-gills only just appear
and disappear, to rob the history of the fishes of its most interesting
points.
Let us be satisfied, then, with this passing glance at a dubious class,
whose history is only a repetition of two others, and let us return
to our friends the fishes. We have seen how they breathe, now let us
look how they eat.
The modifications of the digestive apparatus are endless among fishes.
The lampreys, who are placed in the lower ranks of the class, carry
out to its fullest extent the type which we have already seen indicated
in the serpent. The digestive tube is quite straight, without any
perceptible swelling, and does not even go the whole length of the
body. It comes to an end at some distance from the tail. Among some
fishes an odd tendency begins to display itself, which we shall meet
with again farther on. The digestive tube, after going downwards towards
the bottom of the body, as we have seen it do so constantly hitherto,
doubles back, and comes up again to the throat, under which it empties
itself. In most cases the stomach is distinct; but it assumes a thousand
different forms; as if nature had wished to try her hand in all sorts
of ways in the construction of these imperfect vertebrates, before
adopting the definite model which was to serve for the others.
The liver is enormous, and generally contains a great quantity of oil,
the taste of which you will know if you have ever swallowed a spoonful
of cod-liver oil; but in most fishes its old companion, the
_pancreas_, has disappeared. In its stead you will find, close
by the outlet of the pylorus, the open ends of certain small tubes,
which are shut in at their upper extremity like a "blind alley," and
through which descends into the interstices a thick glairy fluid, given
out from their sides or walls. The result is the same, you see, although
the organ is different; and, remarkably enough, these little tubes are
wanting among fishes, which, like carp, have a species of salivary
glands in their mouths, of which the others show no trace; from which
one may fairly conclude that these glands and tubes mutually supply
each other's places. Here, then, you see an instance of the light which
different animal organisations throw upon each other when they are
compared together. In fact, this one establishes pretty clearly the
real office of the pancreas in the higher races, exhibiting it to us
as an internal salivary gland, intended to complete the work only begun
by those in the mouth, in the case of lazy people who swallow their
food too quickly.
There is the same diversity in the mouth as in the intestine. Some
fishes, like the skate, have no tongue at all. Others, instead of a
tongue, have a hard dry filament, very nearly immovable, and which one
would think was put there like a stake, to show the place where the
tongue is to be found in the more perfect organisations. There are
even fishes, like the perch and the pike, whose tongue is furnished
with teeth, or rather fangs; an evident sign that it has forfeited the
confidential position occupied by your own good little porter. You
must know also that the perch and the pike, like many other of their
fellows, have teeth all over the mouth. This invasion of the palate
by teeth, which began in the lizard and the serpent, assumes alarming
proportions here. It is not merely the roof of the palate which is
spiked with teeth: above, below, at the sides, everywhere to the very
limits of the oesophagus, the little fangs triumphantly stick out their
slender points. It is impossible, therefore, to state their number.
Nature has scattered them broadcast without counting, just as she has
done with the hairs of the beard round the human mouth; and the
comparison is not so impertinent as you may think. They sometimes form
an actual internal beard, even thicker than our outer one, and which
sprouts from the skin into the bargain. There is one fish whose teeth
are so delicate and so close together that, in passing your finger
over them, you would think you were touching velvet. This does not
refer to the shark, mind. His teeth are sharp-cutting notched blades,
hard as steel, arranged in threatening rows round the entrance of his
mouth, and cut a man in two as easily as your incisors do a piece of
apple. Others, such as the skate, have their mouths paved--that is the
proper term--with perfectly flat teeth. The first time your mamma is
sending to buy fish beg her to let you have a skate's head to look at.
You will be interested to see the small square ivory plates laid close
adjoining each other, like the tiles of a church floor. It is in fact
a regular hall-pavement, over which the visitors glide untouched, and
are then swallowed down in the lump; thus entering straight into the
house without having been stopped by the inscription nature has placed
over your door and mine--"Speak to the Porter."
But all this is nothing compared to the lamprey's entrance-hall, which
differs from ours in quite another way. The lamprey, as I have already
told you, ranks almost lowest among fishes, and consequently among
vertebrate animals, of which fishes form the rear-guard. Indeed, it
is almost stretching a point to consider her worthy to bear the proud
title of a vertebrate at all; for the vertebral column, so clearly
marked in other fishes, where it forms the large central bone, is only
faintly indicated in certain species of lampreys, by a soft thread (or
filament), which is rather a membrane than a bony chaplet, and at the
top of this mockery of a vertebral column is the creature's mouth. If
you ever had leeches on, you will remember the sharp sting you felt
when the little beasts bit you. Well, the lamprey feeds herself just
in the same way as the leech does. Her mouth forms a completely circular
ring, which sticks to the prey, and through which runs backward and
forward a small tongue armed with lancets. This darts out to pierce
the skin, and draws in the blood as it retreats. Round your lips well;
dip them so into a glass of water, and draw back your tongue, and you
will at once feel the water rise into your mouth. It is by a similar
sort of proceeding that leeches relieve people of the blood they want
to get rid of; and in the same way the lamprey draws out the blood of
the animals upon which she fastens.
What a long way we have come already! How very far we find ourselves
here from the little mouths we first talked about as chewing their
eatables so prettily! With the lamprey we bid adieu to the class
Vertebrata--the nobility of the animal kingdom--among whom nevertheless
we must distinguish between the peer, who approaches nearest the person
of his sovereign, and the inferior provincial lords who live at a
hundred miles' distance. There is only one step from the lamprey to
the _mollusks_ or soft-bodied animals, and this is the course
which animal organisation seems really to have taken in its progress.
But nature never moves forward in a single straight line. In passing
from the mollusk to the fish to get thence to the higher vertebrates,
she turned aside in another direction toward a class of animals which
rises far above mollusks, but which leads to nothing beyond.
One would think there had been a check here, as if the creative power,
having discovered that it was going in a wrong direction, had retraced
its steps; if it be allowable to apply common ideas and expressions
to our conceptions of that Great Intelligence which has arranged the
plan of the mysterious ladder of animal life.
The animals we must examine next, on account of their superiority to
the rest, are insects. Small as the ant is, it would not be right to
let her be preceded by the oyster.