A BIRD’S-EYE VIEW OF THE RISE AND PROGRESS OF BACKBONED LIFE 333
PICTURE-HEADING--Man as he lived in caves after the Glacial
Period, among animals of species many of them now
extinct--the cave bear (_Ursus spelæus_), cave lion,
cave hyæna (_Hyæna spelæa_), elk, musk-sheep (_Ovibos
moschatus_), mammoth (_Elephas primigenius_), and the
sabre-toothed tiger (_Machairodus_), fighting with the man.
LIST OF ILLUSTRATIONS.
PICTURE-HEADINGS described under “Contents.”
_Frontispiece._ FISH OF THE DEEP SEA--1. Chauliodus, one foot long;
2, 9, 10, 11. Harpodon or Bombay Duck, six inches; 3. Plagiodus,
six feet; 4. Chiasmodus, one foot, with Scopelus in its stomach;
6. Beryx, one foot and a half; 8. Scopelus, one foot.
PAGE
The Lancelet, _Amphioxus lanceolatus_ 11
Sea-Squirt or Ascidian 14
Lamprey and Ammocœtes 16
Structure of Minnow and Living Fish 23
The Blue Shark 29
The Sturgeon, _Acipenser sturio_ 31
The Sturgeon’s Head and Feelers 32
The Ceratodus of Queensland 34
Remoras clinging to a Shark 51
Flying-Fish pursued by the Dorado 54
The Fishing Frog, _Lophius piscatorius_ 59
The common Sole, _Solea vulgaris_ 61
The Hippocampus or Sea-Horse 63
Sticklebacks and their Nest, _Gasterosteus aculeatus_ 65
Metamorphosis of the Frog 72
The common smooth Newt, _Lissotriton punctatus_ 78
Proteus of the Carniola Caverns 79
The Axolotl and Amblystoma 80
The Flying Tree-Frog of New Guinea, _Rhacophorus Rheinhardti_ 86
The Tortoise 96
Carapace of the Tortoise 98
Back of a young Tortoise 99
Skeleton of a Lizard 103
Gecko and Chamæleon 105
The Nile Crocodile 108
Skeleton of a Snake 111
Common Ringed Snake 113
The Boa Constrictor 115
The Cobra di Capello 118
Jaw of a Rattlesnake 119
Common English Viper, _Pelias berus_ 121
The Sparrow 125
Skeleton of a Sparrow 126
The Adjutant Bird 128
The Ostrich at full Speed 137
The Giant Moa and Tiny Apteryx 140
A Group of Sea-Birds 144
Albatrosses and Penguins 147
A Group of Wading Birds 149
The Flamingo 150
Brush Turkeys and their Egg Mounds 158
Wood-pigeon on her Nest 160
The great green Woodpecker, _Gecinus viridis_ 163
The Kingfisher, _Alcedo ispida_ 166
Nest of the Common Wren, _Troglodytes parvulus_ 171
Nest of the Tailor-Bird 173
Eagle bringing Food to its Young 175
Jaw of Dromatherium, and Tooth of Microlestes 183
The Duck-billed Platypus and the Echidna 188
Head and Feet of Ornithorhynchus 189
Australian Marsupials 193
Tasmanian Marsupials 197
South American Marsupials and imperfect-toothed Animals 200
African imperfect-toothed Animals 202
Skulls of an Insect-Eater and a Rodent 217
A Group of Insect-Eaters 220
A Group of Rodents 221
The Pyrenean Desman 226
The Beaver 227
The Taguan and the Colugo 231
Skeleton of a Bat 233
A Bat Walking 235
Fruit-Bats hanging in a Mauritius Cave 238
Aye-aye and Lemur 244
Woolly Monkey and Child 247
The Gorilla at Home 254
The Babirusa, a double-tusked Hog 262
Skeleton of a Wild Ass 266
The Camel 270
The Red-deer with branching Antlers 272
A Buffalo Cow defending her Calf 274
The Elephant 277
The Weasel 280
The Ichneumon 281
The Wolf 283
The Tiger 287
The Claw of the Cat or Tiger 289
The Polar Bear and Walrus 294
The Sea Otter 302
Skeleton of the Sea Lion 304
Sea Lion and Seal 306
Sea Lions on the Watch for Wives 311
The Manatee 314
Skeleton of a Whale and Mouth 318
Whale suckling her Young 319
The Porpoise 324
The Sperm Whale 327
[Illustration: THE GREAT BACKBONED FAMILY]
CHAPTER I.
THE THRESHOLD OF BACKBONED LIFE.
Life, life, everywhere life! This was the cry with which we began our
history of the lowest forms of Life’s children, and although we did
not then pass on to the higher animals, is it not true that before we
reached the end we were overwhelmed with the innumerable forms of
living beings? The microscopic lime and flint builders, the spreading
sponges, the hydras, anemones, corals, and jelly-fish filled the
waters; the star-fish, sea-urchins, crabs, and lobsters crowded the
shores; the oysters, whelks, and periwinkles, with their hundreds of
companions, struggled for their existence between the tides; while in
the open sea thousands of crustaceans and molluscs, with cuttle-fish
and terribly-armed calamaries, roamed in search of food. Upon the land
the snails and slugs devoured the green foliage, while the vast army of
insects filled every nook and cranny in the water, on the land, or in
the air. Yes! even among these lower forms we found creatures enough to
stock the world over and over again with abundant life, so that even
if the octopus had remained the monarch of the sea, and the tiny ant
the most intelligent ruler on the land, there would have been no barren
space, no uninhabited tracts, except those burning deserts and frozen
peaks where life can scarcely exist.
Yet though the world might have been full of these creatures, they
would not have been able to make the fullest use of it, for all animal
life would have been comparatively insignificant and feeble, each
creature moving within a very narrow range, and having but small powers
of enjoyment or activity. With the exception of the insects, by far the
greater number would, during their whole lives, never wander more than
a few yards from one spot, while, though the locust and the butterfly
make long journeys, yet the bees and beetles, dragon-flies and ants,
would not cross many miles of ground in several generations.
What a curious world that would have been in which the stag-beetle and
the atlas-moth could boast of being the largest land animals, except
where perhaps some monster land snail might bear them company; while
cuttle-fish and calamaries would have been the rulers of the sea, and
the crabs and lobsters of the shores! A strangely silent world too.
The grasshopper’s chirp as he rubbed his wings together, the hum of
the bee, the click of the sharp jaws of the grub of the stag-beetle,
eating away the trunk of some old oak tree, would have been among the
loudest sounds to be heard; and though there would have been plenty of
marvellous beauty among the metallic-winged beetles, the butterflies,
and the delicate forms of the sea, yet amid all this lovely life we
should seek in vain for any intelligent faces,--for what expression
could there be in the fixed and many-windowed eye of the ant or beetle,
or in the stony face of the crab?
These lower forms, however, were not destined to have all the world
to themselves, for in ages, so long ago that we cannot reckon them,
another division of Life’s children had begun to exist which possessed
advantages giving it the power to press forward far beyond the
star-fish, the octopus, or the insect. This was the Backboned division,
to which belong the fish of our seas and rivers; the frogs and toads,
snakes, lizards, crocodiles, and tortoises; the birds of all kinds and
sizes; the kangaroos; the rats, pigs, elephants, lions, whales, seals,
and monkeys.
Is it possible, then, that all these widely different creatures, which
are fitted to live not only in all parts of the land, but also in the
air above, and the seas and rivers below, and which are, in fact, all
those popularly known as “animals,” only form one division out of
seven in the real animal kingdom?
Can it be true that while the chalk-builders have one division all to
themselves, the sponges forming a transition group, the lasso-throwers
another division, the prickly-skinned animals a third, the mollusca a
fourth, the worms a fifth, and the insects a sixth, yet the innumerable
kinds of birds and beasts, reptiles and fishes, are all sufficiently
alike to be included in one single division--the seventh? It seems at
first as if this arrangement must be unequal and unnatural; but let us
go back for a moment to the beginning, and we shall see that it is not
only true, but that quite a new interest attaches to the higher animals
when we learn how wonderfully life has built up so many different forms
upon one simple plan.
Starting, then, with the first glimmerings of life, we find the minute
lime and flint builders, without any parts, making the utmost of their
little lives, filling the depths of the sea, and wandering in pools and
puddles on the land; acting, in fact, as scavengers for such matter
as is left them by other animals. But here their power ends; to take
a higher stand in life a more complicated creature is needed, and the
sponge-animal, with its two kinds of cells and its numerous eggs, is
the next step leading on to the curious division of lasso-throwers.
These, in their turn, do their utmost to spread and vary in a hundred
different ways. Possessed of a good stomach, of nerves, muscles,
powerful weapons, and means for producing eggs and young ones, they
fill the waters as hydras, sea-firs, jelly-fish, anemones, and corals.
But here they too find their limit, and, without advancing any
farther, continue to flourish in their lowly fashion. Meanwhile the
tide of life is flowing on in two other channels, striving ever onwards
and upwards. On the one hand, the walking star-fish and sea-urchin push
forward into active life under the sea, forming, with their relations,
a strange and motley group, but one which could scarcely be moulded
into higher and more intelligent beings. On the other hand, the oyster
and his comrades, with their curious mantle-working secret protect
their soft body within by a shelly covering, and by degrees we arrive
at the large army of mollusca, headed by the intelligent cuttle-fish.
And here this division too ceases to advance. The soft body in its
shelly home does not lend itself to wide and great changes, and it was
left for other channels to carry farther the swelling tide of life.
These take their rise in the lowly, insignificant division of the
worms, which may, perhaps, have had something to do with the earliest
forms even of the star-fish and mollusca, but which soon shot upwards,
on the one hand along a line of its own, while, on the other, we have
seen[3] how, in its many-ringed segments, each bearing its leg-like
bristles and its line of nerve-telegraph, the worm foreshadowed the
insects and crustacea, or the _jointed-footed_ animals of sea and land,
forming the sixth division.
Here surely at last we must have reached animals which will answer
any purposes life can wish to fulfil. We find among them numberless
different forms, spreading far and wide through the water and over the
land, and it would seem as if the sturdy crab and fighting lobster
need fear no rival in the sea, while the intelligent bee and ant were
equal to any emergency on dry ground. But here the tide of life met
with another check. It must be remembered that the jointed-footed
animals, whether belonging to land or water, carry their solid part or
skeleton _outside_ them; their body itself is soft, and cased in armour
which has to be cast off and formed afresh from time to time as they
grow. For this reason they are like men in armour, heavily weighted as
soon as they grow to any size, while the body within cannot become so
firmly and well knit together as if all the parts, hard and soft, were
able to grow and enlarge in common. And so we find that large-sized
armour-covered animals, such as gigantic crabs and lobsters, are
lumbering unwieldy creatures, in spite of their strength, while the
nimble intelligent insects, such as the ant and bee, are comparatively
small and delicate.
It would be curious to try and guess what might have happened if the
ant could have grown as large as man, and built houses and cities,
and wandered over wide spaces instead of being restricted to her
ant-hills for a home, and few acres for her kingdom; but she too has
found the limit of her powers in the impossibility of becoming a large
and powerful creature. Thus it remained for Life to find yet another
channel to reach its highest point, by devising a plan of structure in
which the solid skeleton should be--not a burden for the soft body to
carry, as in the sea-urchins, snails, insects, and crabs--but an actual
support to the whole creature, growing with it and forming a framework
for all its different parts.
This plan is that of the backboned animals. They alone, of all Life’s
children, have a _skeleton within their bodies embedded in the muscular
flesh, and formed, not of mere hardened, dead matter, but of bones
which have blood-vessels and nerves running through them, so that they
grow as the body grows, and strengthen with its strength_. This is a
very different thing from a mere outer casing round a soft body, for
it is clear that an animal with a living growing skeleton can go on
increasing in size and strength, and its framework will grow _with_ the
limbs in any direction most useful to it.
Here, then, we have one of the secrets why the backboned animals have
been able to press forward and vary in so many different ways; and
especially useful to them has been that gristly cord stretching along
the back, which by degrees has become hardened and jointed, so as to
form that wonderful piece of mechanism, the _backbone_.
Look at any active fish darting through the water by sharp strokes
of its tail,--watch the curved form of a snake as it glides through
the grass, or the graceful swan bending his neck as he sails over the
lake,--and you will see how easily and smoothly the joints of the
backbone must move one upon the other. Then turn to the stag, and note
how jauntily he carries his heavy antlers; look at the powerful frame
of the lion, watch an antelope leap, or a tiger bound against the bars
of his cage, and you will acknowledge how powerful this bony column
must be which forms the chief support of the body, and carries those
massive heads and those strong and lusty limbs.
Nor is it only by its flexibility and strength that this jointed
column is such an advantage to its possessors; the backbone has a
special part to play as the protector of a most valuable and delicate
part of the body. We have already learnt in _Life and her Children_
to understand the importance of the nerve-telegraph to animals in the
struggle for life. We found its feeble beginnings in the jelly-fish
and the star-fish; we saw it spreading out over the body of the snail;
we traced it forming a line of knots in the worm, with head-stations
round the neck, which became more and more powerful in the intelligent
insects. But in all these creatures the stations of nerve-matter from
which the nerves run out into the body are merely embedded in the soft
flesh, and have no special protection, with the exception of a gristly
covering in the cuttle-fish. We ourselves, and other backboned animals,
have unprotected nerve-stations like these in the throat, the stomach,
and the heart, and cavity of the body. But we have something else
besides, for very early in the history of the backboned animals the
gristly cord along the back began to form a protecting sheath round the
line of nerve-stations stretching from the head to the tail, so that
this special nerve-telegraph was safely shut in and protected all along
its course.
A careful examination of the backbone of any fish, after the flesh
has been cleared off, will show that on the top of each joint (or
_vertebra_) of the backbone is a ring or arch of bone; and when all the
joints are fastened together, these rings form a hollow tube or canal,
in which lies that long line of nervous matter called the _spinal
cord_, which thus passes, well protected, all along the body, till,
when it reaches the head, it becomes a large mass shut safely in a
strong box, the skull, where it forms the brain.
Here, then, besides the unprotected nerve-stations, we have a much
more perfect nerve-battery, the spinal cord, carried in a special
sheath formed of the arches of the backbone, which is at once strong
and yielding, so that the delicate telegraph is safe from all ordinary
danger. Now when we remember how important the nerves are,--how they
are the very machinery by which intelligence works, so that without
them the eye could not see, the ear hear, nor the animal have any
knowledge of what is going on around it,--we see at once that here
was an additional power which might be most valuable to the backboned
division. And so it has proved, for slowly but surely through the
different classes of fish, amphibia (frogs and newts), reptiles, birds,
and mammalia, this cord, especially that larger portion of it forming
the brain, has been increasing in vigour, strength, and activity, till
it has become the wonderful instrument of thought in man himself.
We see, then, that our interest in the backboned or _vertebrate_
animals will be of a different kind from that which we found in the
boneless or _invertebrate_ ones. There we watched Life trying different
plans, each successful in its way, but none broad enough or pliable
enough to produce animals fitted to take the lead all over the world.
Now we are going to trace how, from a more promising starting-point, a
number of such different forms as fish, reptile, bird, and four-footed
beast, have gradually arisen and taken possession of the land, the
water, and the air, pressing forward in the race for life far beyond
all other divisions of animal life.
On the one hand, these forms are all linked together by the fact that
they have a backbone protecting a nerve-battery, and that they have
never more than two pair of limbs; while every new discovery shows how
closely they are all related to each other. On the other hand, they
have made use of this backbone, and the skeleton it carries, in such
very different ways that out of the same bones and the same general
plan unlike creatures have been built up, such as we should never think
of classing together if we did not study their structure.
What the lives of these creatures are, and what they have been in past
time, we must now try to understand. And first we shall naturally ask,
Where did the backboned animals begin? Where should they begin but in
the water, where we found all the other divisions making their first
start, where food is so freely brought by passing currents, where
movement from place to place is much easier, and where there are no
such rapid changes as there are on the land from dry to damp, from heat
to cold, or from bright leafy summer, with plenty of food, to cold
cheerless winter, when starvation often stares animals in the face?
It is not easy to be sure exactly how the backboned animals began,
but the best clue we have to the mystery is found in a little
half-transparent creature about two inches long, which is still to be
found living upon our coast. This small insignificant animal is called
the “Lancelet,”[4] because it is shaped something like the head of a
lance, and it is in many ways so imperfect that naturalists believe it
to be a degraded form, like the acorn-barnacle; that is to say, that it
has probably lost some of the parts which its ancestors once possessed.
But in any case it is the most simple backboned animal we have, and
shows us how the first feeble forms may have lived.
[Illustration: Fig. 1.
The Lancelet, _the lowest known fish-like form_.
_m_, mouth. _e_, eye-spot. _f_, fin. _r_, rod or notochord, the
first faint indication of a backbone. _nv_, nerve cord. _g_, gills.
_h_, hole out of which water passes from the gills. _v_, vent for
refuse of food.
]
Flitting about in the water near the shore, eating the minute creatures
which come in his way, this small fish-like animal is so colourless,
and works his way down in the sand so fast at the slightest alarm,
that few people ever see him, and when they do are far more likely to
take him, as the naturalist Pallas did, for an imperfect snail than a
vertebrate animal. He has no head, and it is only by his open mouth
(_m_), surrounded by lashes with which he drives in the microscopic
animals, that you can tell where his head ought to be. Two little
spots (_e_) above his mouth are his feeble eyes, and one little pit
(_n_) with a nerve running to it is all he has to smell with. He has
no pairs of fins such as we find in most fishes, but only a delicate
flap (_f_) on his back and round his tail; neither has he any true
breathing-gills, but he gulps in water at his mouth, and passes it
through slits in his throat into a kind of chamber, and from there out
at a hole (_h_) below. Lastly, he has no true heart, and it is only by
the throbbing of the veins themselves that his colourless blood is sent
along the bars between the slits, so that it takes up air out of the
water as it passes.
But where is his backbone? Truly it is only by courtesy that we can
call him a backboned animal, for all he has is a cord of gristle, _r
r_, pointed at both ends, which stretches all along the middle of his
body above his long narrow stomach, while above this again is another
cord containing his nerve-telegraph (_nv_.) All other backboned animals
that we know of have brains; but, as we have seen, he has no head, and
his nerve-cord has only a slight bulge just before it comes to a point
above his mouth. Now when the higher backboned animals are only just
beginning to form out of the egg, their backbone (which afterwards
becomes hard and jointed) is just like this gristly rod or _notochord_
(_r r_) of the lancelet, with the spinal cord (_nv_) lying above it;
so that this lowest backboned animal lives all his life in that simple
state out of which the higher animals very soon grow.
This imperfect little lancelet has a great interest for us, because
of his extremely simple structure and the slits in his throat through
which he breathes. You will remember that when we spoke of the
elastic-ringed animals in _Life and her Children_, we found that the
free worms were very active sensitive creatures, whose bodies were
made up of segments, each with a double pair of appendages; the whole
being strung together, as it were, upon a feeding tube and a line of
nerve-telegraph, but without any backbone. Now among these worms we
find many curious varieties; some have the nerve-lines at the sides
instead of below, and one sea-worm, instead of breathing by outside
gills like the others, has slits in its throat through which the water
can pass, and so its blood is purified.
You may ask, What this has to do with backboned animals? Nothing
directly, but these odd worms are like fingerposts in a deserted
and grass-grown country, showing where roads may once have been.
The lancelet, like the worm, has a line of nerve-telegraph and a
feeding-tube, only with him the nerve-telegraph lies above instead of
below. He has also slits in his throat for breathing, only they are
covered by a pouch. Thus he is so different from the worms that we
cannot call them relations; but at the same time he is in many ways
so like, that we ask ourselves whether his ancestors and those of the
worms may not have been relations.
[Illustration: Fig. 2.
Diagram of the growth of a Sea-Squirt or Ascidian.
A a, Young free swimming stage. a², Intermediate stage when first
settling down. B b, Full-grown Sea-Squirt.
_m_, mouth; _e_, hollow brain with eye; _g_, gill slits; _h_,
heart; _r_, rod of gristle in free swimming form; _nv_, nerve cord
in same; _t_, tail in process of absorption in intermediate form.
]
But you will say he is quite different in having a gristly cord.
True--but we shall find that even this does not give us a sharp line of
division. By looking carefully upon the seaweed and rocks just beyond
low tide, we may often find some curious small creatures upon them,
called Sea-Squirts or Ascidians (B, Fig. 2).[5] These creatures are
shaped very like double-necked bottles, and they stand fixed to the
rock with their necks stretching up into the water. Through one neck
(_m_) they take water in, and after filtering it through a kind of net
so as to catch the microscopic animals in it and taking the air out
of it, they send it out through the other neck, thus gaining the name
of sea-squirts. So far, they are certainly boneless animals. But they
were not always stationary, as you see them fixed to the rock. In their
babyhood they were tiny swimming creatures with tails (A and a), and in
the tail was a gristly cord (_r_), with a nerve cord (_nv_) above it,
like those we find in the lancelet. For this reason we were obliged to
pass them by among the lower forms of life, because, having this cord
(_r_), they did not truly belong to the animals without backbones; and
yet now we can scarcely admit them here, because when they are grown
up they are not backboned animals. They belong, in fact, to a kind of
“No Man’s Land,” behaving in many ways like the lancelet when they are
young, as if they had once tried to be backboned; and yet they fall
back as they grow up into invertebrate animals.
So we begin to see that there may have been a time when backbones had
not gained quite a firm footing, and our lancelet, with his friends the
sea-squirts, seems to lie very near the threshold of backbone life.
And now that we are once started fairly on our road, let us turn aside
before beginning the history of the great fish-world and pay a visit
to a little creature whose name, at least, we all know well, and which
stands half-way between the lancelet and the true fish. This is the
Lamprey, represented by two kinds; the large Sea-Lamprey, caught by the
fishermen for bait as it wanders up the rivers to lay its eggs, and the
true River-Lamprey or Lampern, which rarely visits the sea.
What country boy is there who has not hunted in the mud of the rivers
or streams for these bright-eyed eel-like fish, with no fins, and a
fringe on back and tail? If you feel about for them in the mud they
will often come up clinging to your hand with their round sucker-mouth,
while the water trickles out of the seven little holes on each side of
their heads. The small river-lampreys do not hurt in the least as they
cling, though the inside of their mouth is filled with small horny
teeth. But the larger sea-lamprey uses these teeth as sharp weapons,
scraping off the flesh of fish for food as he clings to them.
[Illustration: Fig. 3.
Figure of a full-grown Lamprey[6] and of the young Lamprey, formerly
called Ammocœtes.
_Showing the seven holes through which it takes in water to breathe._]
These Lampreys, together with some strange creatures, the “Hags”
or “Borers,”[7] belong to quite a peculiar family, called the
Round-mouthed fishes,[8] and, though they stand much higher in the
world than the lancelet, yet they are very different from true fish.
Like the lancelet they have only a gristly cord for a backbone, but
this cord has begun to form arches over the nerve battery, and it
swells out at the end into a gristly skull covering a true brain. They
have clear bright eyes too, and ears, which if not very sharp, are
at least such as they can hear with; they have only one nostril, and
their mouth is both curious and useful. When it is shut it looks like
a straight slit, but when it is open it forms a round sucker with a
border of gristle, and this sucker clings firmly to anything against
which it is pressed, so that a stone weighing twelve pounds has been
lifted by taking a lamprey by the tail. Inside the mouth the palate and
tongue are covered with small horny teeth, and these are the lamprey’s
weapons.
Salmon have been caught in the rivers with lampreys hanging to them,
and where the mouth has been the salmon’s flesh is rasped away, though
he does not seem much to mind it.
Lastly, the lamprey has a peculiar way of breathing. He has seven
little holes on each side of his head, reminding us of the slits in
the worm’s throat and those hidden under the skin of the lancelet, and
behind these holes are seven little pouches lined with blood-vessels,
which take up air out of the water. These pouches are all separate, but
they open by one tube into his throat. When the lamprey is swimming
about it is possible that he may gulp water in at his mouth and send
it out at the slits. But when he is clinging to anything he certainly
sends water both in and out at the slits, so that he can still breathe,
though his mouth is otherwise occupied.
And now, what is the history of his life? For three years he lives
as a stupid little creature, with a toothless mouth surrounded by
feelers, and tiny eyes covered over with skin, and he is so unlike a
lamprey that for a long time naturalists thought he was a different
animal and called him _Ammocœtes_. But at the end of the three years he
changes his shape, and then he is as bright and intelligent as he was
dull and heavy before. His one thought is to find a mate and help her
to cover up her eggs. To do this a number of lampreys find their way
up the river and set to work. Sometimes one pair go alone, sometimes
several together, and they twirl round and round so as to make a hole
in the sand, lifting even heavy stones out with their mouths if they
come in the way. Then they shed the spawn into the hole, where it is
soon covered with sand and mud, to lie till it is safely hatched, and
when this is done the marine lampreys swim out to sea to feed on the
numberless small creatures in it, or to fasten upon some unfortunate
fish.
But there are round-mouthed fishes even more greedy than these. It is
not only among the lower forms of life that some creatures, such as
worms, which are driven from the outer world, find a refuge inside
other animals. But here again we meet with the same thing, for those
relations of the lampreys, the hags or borers, which we mentioned
above, use their sharp teeth to bore their way into other fish so as to
feed upon them. These greedy little creatures actually drill holes in
the flesh of the cod or haddock and other fish, and eat out the inside
of their bodies, so that a haddock has been found with nothing but the
skin and skeleton remaining while six fat hags lay comfortably inside.
So the round-mouthed fishes, feeble though they are, hold their own
in the world. How long ago it is since they first began the battle of
life we shall probably never know for certain; but if some little horny
teeth[9] found in very ancient rocks belong to their ancestors, they
were most likely among the first backboned animals on our globe. At any
rate they are very interesting to us now, for they have wandered far
away from the true fishes, and give us a glimpse of some of the strange
by-paths which the backboned animals have followed in order to win for
themselves a place in the race for life.
[Illustration: THE ANCIENT FISH & THEIR HUGE RIVAL]
NOTE.--For description of the Picture-Headings see the Table of
Contents.
CHAPTER II.
HOW THE QUAINT OLD FISHES OF ANCIENT TIMES HAVE LIVED ON INTO OUR DAY.
Who is there among my readers who wishes to understand the pleasures,
the difficulties, and the secrets of fish life? Whoever he may be he
must not be content with merely looking down into the water, as one
peeps into a looking-glass, or he may, perchance, only see there the
reflection of his own thoughts and ideas, and learn very little of how
the fishes really feel and live. No! if we want really to understand
fish-life we must forget for a time that we are land and air-breathing
animals, and must plunge in imagination into the cool river or the
open sea, and wander about as if the water were our true home. For
the fish know no more about our land-world than we do about their
beautiful ocean-home. To them the water is the beginning and end of
everything, and if they come to the top every now and then for a short
air-bath they return very quickly for fear of being suffocated. Their
great kingdom is the sea--the deep-sea, where strange phosphorescent
fish live, lying in the dark mysterious valleys where even sharks and
sword-fish rarely venture;--the open sea, where they roam over wide
plains when the ocean-bottom makes a fine feeding-ground, or where
they thread their way through forests of seaweed, while others swim
nearer the surface and come up to bask in the sun or rest on a bank of
floating weed;--and the shallow sea, where they come to lay their eggs
and bring up their young ones, and out of which many of them venture up
the mouths of rivers, while others have learnt to remain in them and
make the fresh water their home.
The tender little minnows that bask in the sunny shallows of the river
have never even seen the sea, their ancestors left it so long long
ago; yet to them, too, water is life and breath and everything. The
green meadow through which the river flows is just the border of their
world and nothing more, and the air is boundless space, which they
never visit except for a moment to snap at a tiny fly, or when they
jump up to escape the jaws of some bigger fish. Every one knows the
minnow, and we cannot do better than take him as our type of a fish
in order to understand how they live and move and breathe. Go and lie
down quietly some day by the side of the clear pebbly shallows of some
swiftly-flowing river where these delicate little fish are to be seen;
but keep very still, for the slightest movement is instantly detected.
There they lie
“Staying their wavy bodies ’gainst the streams
To taste the luxury of sunny beams
Tempered with coolness. How they ever wrestle
With their own sweet delight, and ever nestle
Their silver bellies on the pebbly sand!
If you but scantily hold out the hand,
That very instant not one will remain;
But turn your eye and they are there again.”
[Illustration: Fig. 4.
The structure of the Minnow and the living fish.
A _n_, nose-pit; _e_, eye-nerve; _ea_, ear-nerve; _g_, gills; _h_,
heart; _t_, food-tube; _s_, stomach; _k_, kidney; _v_, vent; _da_,
dorsal-artery. _a_, air-bladder; _b_, backbone; _nv_, nerve cord or
spinal cord.
B _n_, nose; _gc_, gill cover; _af_, arm-fin; _lf_, leg-fins; _sf_,
single fins; _ms_, mucous scales.
]
If you can be motionless and not frighten them you may see a good deal,
for while some are dashing to and fro, others, with just a lazy wave of
the tail and the tiny fins, will loiter along the sides of the stream,
where you may examine their half-transparent bodies. Look first at one
of the larger ones, whose parts are easily seen, and notice how every
moment he gulps with his mouth, while at the same time a little scaly
cover (_g c_, B, Fig. 4) on each side of the head, just behind the eye,
opens and closes, showing a red streak within. This is how he breathes.
He takes in water at his mouth, and instead of swallowing it passes
it through some bony toothed slits (_g_, A Fig. 4) in his throat into
a little chamber under that scaly cover; in that chamber, fastened to
the bony slits, are a number of folds of flesh full of blood-vessels,
which take up the air out of the water; and when this is done he closes
the toothed slits and so forces the bad water out from under the scaly
cover back into the river again. It is the little heart (_h_), lying
just behind the gills, which pumps the blood into the channels in
those red folds, and as it keeps sending more and more, that which is
freshened is forced on and flows through the rest of the body. It goes
on its way slowly, because a fish’s heart has only two chambers instead
of four as we have, and these are both employed in pumping the blood
_into_ the gills, so that for the rest of the journey through the body
it has no further help. For this reason, and also because taking up air
out of the water is a slow matter, fish are _cold-blooded_ animals, not
much warmer than the water in which they are.
But while our minnow breathes he also swims. He is hardly still for a
moment, even though he may give only the tiniest wave with his tail
and fins, and he slips through the water with great ease, because his
body is narrow and tapers more or less at both ends like a boat. At
times, too, if he is frightened, he bounds with one lash of his tail
right across the river; and if you look at one of the small transparent
minnows you will see that he has power to do this because his real
body, composed of his head and gills, heart and stomach, ends at half
his length (see Fig. 4, A), and all the rest is tail, made of backbone
and strong muscles, with which he can strike firmly. This is one
great secret of fish strength, that nearly one half of their body is
an implement for driving them through the water and guiding them on
their way. Still although the tail is his chief propeller, our minnow
could not keep his balance at all if it were not for his arm and leg
fins. You will notice that it is the pair of front fins (_af_) which
move most, while the under ones (_lf_) are pressed together and almost
still. Besides these two pairs he has three single fins (_sf_), one
under his body, one large V-shaped one at the end of his tail, and
another single one upon his back. All these different fins help to
guide him on his way; but while the single ones are fish-fringes, as
it were, like the fringe round the lancelet’s body, only split into
several parts, the two pair under his body are real limbs, answering to
the two pair of limbs we find in all backboned animals, whether they
are all four fins, or all four legs, or wings and legs, or arms and
legs.
These paired wings are most important to the minnow, for, if his
arm-fins were cut off, his head would go down at once, or, if one of
them was gone he would fall on one side, while, if he had lost his fins
altogether, he would float upside down as a dead fish does, for his
back is the heaviest part of his body. It is worth while to watch how
cannily he uses them. If you cannot see him in the stream you can do so
quite well in a little glass bowl, as I have him before me now. If he
wants to go to the left he strikes to the right with his tail and moves
his right arm-fin, closing down the left, or if he wants to go to the
right he does just the opposite; though often it is enough to strike
with his tail and single fin below, and then he uses both the front
fins at once to press forward.
But how does he manage to float so quietly in the water, almost without
moving his fins? If your minnow is young and transparent you will
be able to answer this question by looking at his body just under
his backbone, and between it and his stomach. There you will see a
long, narrow, silvery tube (_a_, Fig. 4) drawn together in the middle
so that the front half near his eyes looks like a large globule of
quicksilver, and the hinder half like a tiny silver sausage. This
silvery tube is a bladder full of gas, chiefly nitrogen, and is called
the _air-bladder_. Its use has long been a great puzzle to naturalists,
and even now there is much to be learnt about it. But one thing is
certain, and that is, that fish such as sharks, rays, and soles, which
have no air-bladders, are always heavier than the water, and must make
a swimming effort to prevent sinking. Fish, on the contrary, which have
air-bladders, can always find some one depth in the water at which they
can remain without falling or rising, and we shall see later on that
this has a great deal to do with the different depths at which certain
fish live. Our minnow floats naturally not far from the top, and, even
if he were forced to live farther down, the gas in his bladder would
accommodate itself after a few hours if the change was not too great,
and he would float comfortably again.
And now the question remains, What intelligence has the minnow to
guide him in all these movements? If you will keep minnows and feed
them yourself every day you will soon find out that they see, smell,
and feel very quickly, though their hearing and taste are not so
acute. They are cunning enough too, and will often steal a march upon
heavier and slower fish, snatching delicate morsels from under their
very noses. For our little minnow can boast of a real brain, though
it is a small one in comparison with his size. All along, above his
delicate backbone, the thread of nerve telegraph (_nv_, Fig. 4) runs
under protecting bony arches, and sends out nerves on all sides to the
body and fins; and when it reaches the head it swells out, under a
bony covering, into a small brain, sending out two nerves to the ears
(_ea_), in front of which is a second part, with two nerve-stations
(_e_) for the eyes, and beyond this a third part, with two more for the
nostrils, besides others which go to the face. Look on the top of a
minnow’s head and you will see two little raised bumps (_n_). These are
its nostrils, but remember _they have nothing to do with breathing_;
they have not even any connection with the mouth, but are simply little
covered cups, each with two openings for water to flow in and out, and
they are lined with nerves, which, tickled by good or bad scents in the
water, carry to the brain a warning, or a promise of good things.
Such, then, is our little minnow, and the different parts of his body
are supported by a slender bony-jointed backbone, with ribs growing
from it, supporting a strong mass of flesh on his sides. He is a
delicate tender creature, but is protected and buoyed up by the water,
out of which he never attempts to go. The thin, rounded, transparent
scales which cover his body, growing out of little pockets in his skin,
just like our nails on the tips of our fingers, protect this skin from
the water and from rough treatment; while they themselves are kept
soft by a slimy fluid which oozes out from under them, and especially
through the dark line of larger scales (_ms._ Fig. 4) running along his
body.
* * * * *
Now the minnow is a bony fish, and from it we can learn very fairly
what bony or modern fishes are like. But these fish were not the
founders of the race; long before they existed there was another very
ancient group of fishes in the world, which were in many ways more like
the lancelet and the lamprey; and to find such descendants of this
ancient group as are now living we must leave the river and find our
way into the open sea.
If we do this, we shall travel not many miles from the shore in summer,
wending our way through shrimps and lobsters, gurnards, cod-fish,
soles, and turbot, before we may chance to come across a great Blue
shark, with his slaty-coloured back and fins, swimming heavily but
strongly through the water, and turning sharply from time to time to
seize a passing fish, his white belly gleaming like a flash of light as
it comes uppermost, and then disappearing again in the dark water.
“His jaws horrific, armed with threefold fate,
Here dwells the direful shark.”
Or if this formidable monster does not happen to be in the
neighbourhood another kind, the Dogfish, may cross our path, perhaps
the Smooth hound, crushing the crabs and lobsters in his tooth-lined
mouth, or the Rough hound fastening her purse-like egg to the seaweed
by its long string-like tendrils; or, farther out still, we may perhaps
see the Thresher shark lashing the water with his long pointed tail, to
drive the frightened fish within his reach; or, if we were off the west
coast of Ireland, the huge but harmless Basking shark might be floating
calmly by in the warm sunshine. For sharks travel all over the ocean,
and though they prefer the warm seas, where they sometimes reach a size
of forty feet long and more, yet many of the smaller kinds visit our
coasts in summer.
[Illustration: Fig. 5.
The Blue Shark[10] (_from Brehm_).
To show the five slits in the neck, the uneven tail, and the mouth
opening under the pointed snout.]
Now, at first sight we might imagine that these huge monsters, the
terrible tyrants of the sea, must be the last and most finished
production of fish-life; but if we look a little closer we shall be
undeceived. Examine a shark in any good museum, and you cannot fail to
be struck with his strange form. Look first at his tub-like body, so
different from the narrow wedge-shape of the minnow, the herring, or
the salmon. Then observe his skin, which is either tough, more like
that of other animals, or thickly covered with short blunt teeth, which
sometimes, especially in front of the fins, become long pointed spines.
There is no trace of fish-scales here. Look at his mouth opening under
his pointed snout, and you will see that as the skin turns over the
lips these blunt teeth line his mouth, so that he has several rows fit
for biting, and they are sometimes so formidable that they can cut a
man in two at one snap. Then look more especially at the sides of his
throat, and there you will see on each side from five to seven slits,
reminding you at once of the slits of the lamprey, though they are long
instead of round. For the shark has never arrived at having true gills
under a horny cover like the minnow, but still breathes by pouches
and slits somewhat after the way of the lowly round-mouthed fishes.
Lastly, observe his curious tail. In nearly all living fish the tail is
even[11] or V-shaped, but in the sharks the top point is usually longer
than the lower one,[12] and in some, such as the Thresher, it is very
remarkably so.
This uneven tail is the badge of a very ancient race; out of the shark
family we scarcely find it anywhere now except among the sturgeons,
who, we shall see, are old-fashioned too.
And now when we inquire into the growth of the shark and the kind of
backbone he has, we find that he has still more links with the lower
fish-like animals. For when he is young he has nothing but a rod of
gristle or cartilage running between the long narrow feeding-tube
and the spinal cord; but this rod is flattened in front, and as the
young shark grows up the flat part enlarges so as to form a boat-like
box--the skull, round the swollen end of the nerve telegraph--the
brain. Meanwhile the rod becomes divided into rings, and from each ring
an arch of gristle growing upwards surrounds the nerve cord so as to
protect it from injury, and the whole skeleton becomes firm and strong.
But though the shark is one of the strongest of sea-animals _he never
loses this gristly state of his backbone or his skeleton_; however much
he may strengthen it by hard matter it never becomes true bone, but
remains quite distinct from the skeleton of the bony or osseous fish.
[Illustration: Fig. 6.
The Sturgeon[13] entering a Russian river.]
We see, then, that there is a race of gristly or _cartilaginous_
fishes, which, though they have grown strong and powerful, still hold
to many primitive habits in forming both their body and skeleton. Nor
do the sharks stand alone, for the large sturgeons, which live partly
in the sea and partly in fresh water, crowding up the rivers of Russia
and America to grope in the river mud for food, and to lay their
millions of eggs, are also remnants of the ancient type. It is true
that with them the slits in the neck are covered by a horny flap like
the bony fish, and like them too they have an air-bladder under the
backbone.[14] But they too have a gristly skeleton, and the gristly rod
more or less hardened runs right along their back. In other respects
they are perhaps even more peculiar than the sharks; for the sturgeon’s
head is covered with hard bony shields, and five rows of bony bucklers
are arranged along his body. We seem almost to have got back among the
armour-covered animals as we look at his shiny plates, reminding us
that with a mere gristly skeleton within, it may have been wise for
the early types of fish to wear some outward protection. His snout is
long and pointed, with four delicate feelers hanging down from it, and
his mouth, which is quite under his head, is a soft open tube without
teeth, which he can draw up or push out to suck up fish or any animal
matter he finds in the mud.
[Illustration: Fig. 7.
The Sturgeon’s head seen from below, showing the tube-like mouth and
the four barbels or feelers.]
Clearly the sturgeon is an old-fashioned fellow, as you may see for
yourself, when specimens caught at the mouths of our rivers are shown
in the fishmongers’ shops. I have often wondered, when standing looking
at him and at the sharks in the British Museum, whether the people who
stroll by have any idea what a strange history these quaint old fishes
have, or how they stand there among the scaly and bony fishes lying in
the cases around, just as an Egyptian and a Chinaman might stand in an
English crowd, descendants of old and noble races of long long ago,
whose first ancestors have been lost in the dim darkness of ages, whose
day of strength and glory was at a time when the modern races had not
yet begun to be, and whose representatives now live in a world which
has almost forgotten them.
In the silent depths of the large lakes of North America there is a
fish called the Bony pike,[15] a huge fellow often six feet long,
with a long beak-shaped mouth, which he snaps as he goes, devouring
everything that comes in his way. This fish has his body covered with
lozenge-shaped, bony, enamelled scales, like the fish of long ago, and
so too has the strange Bichir,[16] which wanders above the cataracts of
the Nile, with its row of eight to eighteen fins raised upon its back
like tiny sails. Then again there are the curious calf-fish of North
America,[17] of the Amazons,[18] of the Nile,[19] and of the rivers
of Queensland in Australia.[20] These all have gristly skeletons,
and together with the sharks and sturgeons make up all that remains
of those strange shadows of the past moving among the bony fishes of
to-day.
The mud-fishes are indeed the most curious of all, for they breathe
both water and air, and in the Nile and Gambia often coil themselves
round in the mud when the water goes down, and, lining their bed
with slime, sleep comfortably till the rains refill the pools with
water.[21] The fact is they have two quite separate ways of breathing.
They have gills with which they can take air out of the water like
other fish, and these they always use when they can. But they have a
tube in their throat leading into the air-bladder lying under their
backbone, and through this they can breathe in air when they cannot get
it from the water. In _Amia_ especially, which is a true enamel-scaled
fish, this air-bladder is divided into numerous cells, and it breathes
with it just as with a lung.
[Illustration: Fig. 8.
The Ceratodus of Queensland, an air-breathing and water-breathing
mud-fish of the ancient type, with paddle-fins.]
It was in the year 1870 that the _Ceratodus_, or the “Barramunda,”
as the Australian natives call him, was discovered in the rivers of
Queensland; and since then he has become very famous, for, more than
any of the others, he is like the fishes of long ago. He is a lumpy
fish, sometimes as much as six feet long, with a gristly cord for a
backbone. His body is covered with large rounded scales, and he has a
broad fringe round his pointed tail. His fins are more like paddles
than fish fins, having several joints, and he uses them, together with
his fringed tail, to flap along in the water, or even to wander over
the reedy flats at night, chewing the weeds with his broad ridged
teeth. And as he flaps along, from time to time, when the water is too
muddy for his liking, he comes up to the top, and with a great gulp
swallows air into the air-chamber. But before he can do this he must
send out the bad air within, and in doing so he gives a grunt which
is often heard far away at night in those still Australian wilds. He
need not come up for air-breathing, however, if the water is pure, for,
strange to say, the whole course of his blood can be altered to suit
his wants. When he can get clear water to breathe through his gills the
blood flows to them to be freshened, and his air-bladder simply takes
in gas from the body as it does in other fishes, and wants feeding with
good blood. But when he comes up to breathe then the blood is carried
the other way, and comes to the air-bladder to be freshened.
And now if we want to read the history of all these strange forms, you
must let me take you by the hand and lead you in imagination back, back
through millions of years, to a time so long ago that we cannot even
count the ages between. As we recede from our own day we shall leave
behind us all the kinds of plants and animals we now know so well, and
meet with strange kinds only bearing a general resemblance to them.
After a long journey of thousands and thousands of years, in which
the plants and animals, and even the very shape of the continents and
islands, have gone through many changes, we shall get back to the time
when the lime-builders were forming thin layers of chalk at the bottom
of the sea, which were afterwards to become our enormous chalk hills.
Still backwards we must go through all that long period, and then
through three others quite as long, with ever-changing scenes of life
and climate and geography, till we find ourselves in those grand old
forests whose trees and plants we now dig out as coal.
Even then we must not stop to rest, though we are getting back to
the dim ages of the world, for the journey is not yet ended. On,
on, backwards through countless years, till we lose sight not only
of beasts and birds and reptiles, but even of insects and flowering
plants, which, at the time we are reaching, had not yet begun to be. At
last we lose almost all life upon the land, so far as we can tell, and
after another long period has passed before us we find ourselves in a
scene of _water, water everywhere._
True, there is a line of shore where strange ferns and unknown
club-mosses and reed-like plants are growing; but these only border the
vast water-world, and we have reason to believe that no living animal
wanders over that wild and barren country. But the water itself is full
of life, though its inhabitants are of low kinds, as if Nature herself
was as yet only half-awake.
Rich and rare seaweeds carpet the floor of the ocean, mingled with
delicate flint-sponges and old massive corals; beautiful feather-stars
in the form of rooted stone-lilies wave their slender arms; greedy
star-fish, grazing sea-urchins, and all their many relations, grope
upon the rocks; and sea-snails crawl or float in countless numbers. The
Nautilus, too, is there, with curious half-uncoiled companions of forms
we have never seen before; and huge sea-woodlice, the Trilobites of
olden time with their three-ridged shields, burrow in the sand, or roll
themselves up at the bottom of the water. And above all these, among
many kinds of armour-covered animals, a huge form, nine feet long, like
a lobster, with an imperfect head, rows himself along with his oar-like
hind feet, seizing the smaller creatures with his long nipping claws
in front. For we have travelled back to a time when the crustaceans
were the most powerful animals in the world, and the huge lobster-like
Pterygotus was the monarch of the seas.
It was in the midst of a scene such as this that we first find the
feeble ancestors of the Sturgeon and the Shark beginning to make their
way in the world. It may be that creatures such as the sea-squirts,
the lancelet, and the lamprey, were there to bear them company, but
these soft animals could leave no trace behind except the tiny teeth
of the lampreys; for they had no enamelled plates like the plated
fish, no hard teeth-spines like the sharks, which could become buried
in the soft mud when they died, and remain, together with the hard
shell of their enemy the Pterygotus, to be dug out now in our day and
bear witness to the fight they fought. But the plated-scaled fish
had something to leave behind, and from their remains we can picture
to ourselves a group of clumsy fish scarcely a foot long, with hind
fins like paddles and single-fringe fins on their back, with enamelled
lozenge-shaped plates on their bodies and unevenly pointed tails.
These fish would keep well out of the way of the Pterygotus, because
they were small and weak and he was large and strong. We may imagine
them gliding among the seaweeds, and hugging the shore as they chewed
the plants with their flat-ridged teeth, for their skeletons were
probably feeble and their armour-like shields were heavy, and they
would not be so active as the little shark-like animals, not bigger
than a half-pound perch, with tough skins and sharp spines, which swam
more boldly out to sea. These more active fish were the founders of
the shark group, and those sharp spines, together sometimes with the
tough skin, remained buried in the mud, and have come down to us as
fossils.[22]
We should find it difficult to say exactly to what class all these
early fish belonged, for there were very few kinds, and therefore fewer
distinctions, between them in those days; and many peculiarities which
afterwards appear in different groups either did not exist or were
united in one fish. It is enough for us that they were the ancestors
of our sharks and sturgeons and mud-fish of to-day; and though they
were but small and weak, yet they were the beginning of a powerful race
of creatures, for they had the great advantage of a growing inside
skeleton, which could vary and strengthen with their bodies from
generation to generation, while their rivals, the Pterygotus and his
companions, had only their heavy cumbrous armour with a mass of soft
flesh inside, and were but lumbering creatures at best.
And so we find that as thousands and thousands of years rolled by, the
descendants of the enamel-shielded fish began to improve, and became
larger and more powerful as the generations passed on, till they became
masters of the shallow seas, and after awhile of the rivers and lakes.
By the time that the first air-breathing creatures, the May-flies and
Dragon-flies, had found their way out of the water into the forests
of pines and tree-ferns on the land, and left their tender wings in
the soft ground of the ponds and lakes, large fishes[23] whose tails
were uneven-pointed like the sturgeon,--whose bodies were covered with
lozenge-shaped enamelled scales and their heads with shields,--were
grazing along the shores and in the rivers and bays, with probably
swarms of smaller kinds which have left no traces behind.
These were peaceable fish which fed upon plants, and among them were
some curious forms with paddle-like fins and broad-ridged teeth, which,
as they swam under the shade of the huge forest trees, would come to
the top and take in air through their mouth. These were the distant
ancestors of our present mud-fishes, and through all the passing ages,
from the time of the coal forests till now, they have kept their
fish-like form, so that we have their descendants among us now in the
Australian _Ceratodus_ and the mud-loving _Protopterus_ of the Nile.
But besides these gentle vegetarians there were in the sea huge
enamel-scaled monsters, with terrible jaws and gigantic teeth,
floundering about and making great havoc among the crab-like animals.
One of these, whose head-shield has been found in the ancient rocks of
Ohio in America,[24] must have been at least fifteen feet long, with a
huge head, three feet long and a foot and a half broad; and no doubt
there were many others like him, having a fine time of it now that they
were the strongest creatures living. For this was the Golden Age of
fishes, just before the time when the coal-forests grew; and the clumsy
crab-like animals, and the trilobites, which had had their innings when
the fish were small, now began gradually to be exterminated by their
powerful enemies. Little by little they gave up the battle of life,
and the larger ones died out altogether, leaving only those smaller
crustaceans which did not clash with the fish.
So time passed on. The coal-forests grew, and died away and were
buried; and as the ages rolled by a still stronger class of animals
began to grow up which was to pay back upon the enamel-scaled fish
the vengeance they had wreaked upon the crustaceans. For in the coal
forests we first meet with creatures like our newts and salamanders,
and after these came the true air-breathing reptiles (see Chap. v.),
which swarmed over land and sea. There were the fish-lizards, with
their strong swimming paddles and sharp teeth, and the swan-like
lizards, with their long necks, which enabled them to strike their prey
in the water; and these, together with the flying-lizards, and the huge
dragon-like reptiles which haunted the shore, made the life of the
heavily-moving enamelled fish a burden to them. So they, in their turn,
began to give way, and became smaller and rarer as the history went on,
till at the time when the chalk-building animals were at work at the
bottom of the sea we begin to lose sight of all but those few forms
which linger still. It was about this time that the Sturgeon, as we now
know him, became the chief representative of these old cartilaginous
fishes, and to this day he and his children go on travelling up the
rivers of Europe, Asia, and America, or crossing from sea to sea--a
living example of those ancient races which ruled the seas of long ago.
The history of the small shark-like animals was rather different. They
too grew strong and powerful before the reptiles came, and they did
not afterwards lose much of their greatness. With the wide ocean for
their home, and not troubled with the heavy enamelled plates of their
companions, they kept clear of the monster reptiles, or struggled with
them bravely. Some took to the open sea, and from them are descended
the giant sharks of to-day which still remain masters of the ocean.
Others still lingered near the shore, where we find quite new forms
springing up; some, like the Chimæra or “King of the Herrings,” formed
a group of their own, half-way between sharks and sturgeons; and
some, slightly flattened like the huge Monk-fish, hide themselves in
the loose sand when seeking their prey. Others, the Skates and Rays,
with flat bodies, and long tails serving as rudders, shoved smoothly
along with a wavy flapping motion of their broad arm-fins. These too
lie chiefly at the bottom of the sea, where their dusky colour hides
them both from the fish they would wish to attack and those that
would attack them; for while the sharks trust to their strength, the
skates and rays trust to stratagem, and, coming along stealthily in
the shadow, flap rapidly over their prey and suck them into their
open mouth below. And for further protection we find some of them,
such as the Sting-rays, armed with barbed spines; others, such as
the Torpedo-fish, with electric batteries in their heads, which they
can use to stun and kill their enemies; while others again, such as
the Saw-fishes of the Tropics, have the front part of their skull
lengthened out in a long bony weapon, armed with teeth, which they use
to rip open the bodies of their prey.
All these formidable fish are descendants of the shark family, which,
with powerful gristly backbones, strong fins and tails, and highly
developed brains, refused to be suppressed as their plated companions
were, but found room in the wide ocean to do battle for themselves,
and improve in many ways upon their ancestors. They do not, like the
sturgeon and the bony fish, lay their thousands of eggs, but are
content with one or two at a time, such as the leathery purse-eggs of
the skate and the rough hound shark; or give birth to a dozen or twenty
living young ones. Yet they are so well fitted for their life that they
flourish and keep their ground, so that while the enamel-scaled fish
and the mud-fish are small groups, many of them fading away, the sharks
and rays bid fair to be the race which will keep up the traditions of
those quaint old Fishes of ancient times, which were once the masters
of the world.
[Illustration: THE BONY FISH IN THE EARLY DAYS]
CHAPTER III.
THE BONY FISH, AND HOW THEY HAVE SPREAD OVER SEA AND LAKE AND RIVER
When the palmy days of the enamel-scaled fish had passed away, and
the sharks and rays had taken up their various quarters in different
parts of the sea, there still remained vast tracts and many snug nooks
and bays admirably fitted for fish-life. But these were not empty,
for long before this time another order of fish--light, strong, and
active,--had been pressing forward to take possession of every vacant
space.
If we could dive under the water and watch the fishes at home we should
see at once how much more agile and easy the bony fish are in their
movements than their gristly companions. Look at a shoal of silvery
herrings as they swim and leap and gambol, or a fine salmon sailing up
the river or springing over a waterfall, or a tiny stickleback darting
across the stream, and compare their graceful motion with the ponderous
though powerful movements of an unwieldy shark. Any one who has done
this will feel at once that though the sharks have still kept their
power as tyrants of the sea, because they are so strong and big, yet
these light skirmishers are much more at their ease, and move with much
less effort in the water, so that it is natural they should have made
their way into all parts of the rivers and seas. But where have they
come from? We know very little of their early history, but what little
we do know leads us to think that long ago they branched off from the
enamel-scaled fish, and struck out a path of their own to make the most
of the watery world.
Turn back for a moment to our little minnow, and recall his tender
backbone made of joints hollowed out before and behind, with cushions
of gristle between; those cushions, when the minnow was growing out
of the minnow egg, were one long gristly cord, like the cord of the
sturgeon, and it was only as the minnow grew that the bony joints
hardened round it and separated it. Moreover, that huge bony pike
which we find now wandering in the American lakes has bony joints
hollowed out like the minnow’s, although by his enamel-scales and
uneven tail we know him to be one of the ancient fishes. Some time
or other, then, the sturgeon, the bony pike, and the modern minnow,
must have had a common ancestor, though we should have to reach him
through millions of generations. In the same way, too, we find the
red-folded gills covered by a scaly lid, both in the sturgeon and the
minnow, though in other ways they are not exactly alike; while even
the V-shaped tail of the modern fish is not so different from the
ancient shape as it seems, for the end of the backbone runs up into
the top branch of the fork as it does in the uneven tails of the olden
fish. Lastly, the delicate rounded scales on our minnow’s body are not
entirely the property of bony fishes, for we find such scales on the
mud-fishes, the _Amia_ and _Ceratodus_ (see p. 33); while the little
modern stickleback, on the other hand, has bony plates, reminding us of
those of olden times. We see, then, that the bony fish still carry upon
them many signs of their origin from the older fish, and when once the
coast was left clear, and they got a fair start, we can easily imagine
that the fish of this younger race which was still in its childhood,
and easily moulded to suit different kinds of life, would press forward
in every direction and make the most of every chance.
And so we find that little by little, from the time of those chalk seas
till now, the remains of enamel-scaled fish grow rarer and rarer in the
hardened mud, and the bones and scales of modern fish take their place,
till this bony race has spread so far and wide that in our own day, if
we were to start from the head of a river and swim down into the open
sea of the Atlantic or Pacific, we should meet on our way bony fish of
all shapes and sizes and habits of life. River-fish and lake-fish and
sea-fish; shore-fish, surface-swimming fish, and fish of the deep sea;
flat-fish like the sole, half hidden in the sand, and long rounded fish
like the eel, threading their way through holes and passages all over
the world; flying fish with long arm-fins, and clinging fish whose fins
form a sucking disk; nay, even so strange a thing as an angling-fish,
whose back fin is turned into a fishing-rod to attract his prey.
All these, during the long ages since they first started in life, have
been learning to make use of some area in the wide expanse of water
spread over our globe, and it remains for us now to see how they have
succeeded. Where shall we make our start? If we begin at home in the
rivers we should have to work, as it were, backwards, for the sea is
the chief home of fishes, and the rivers only the refuge of a few stray
kinds. The sea-shore would be, perhaps, our truest starting-point, but
then we should have to travel two different ways. Will it not be best
to dive down first into the silent depths of the ocean, and learn what
little is known of those which have taken refuge there? Thence we can
rise up to the open sea, from there swim in to the shore, and then up
the rivers and back to our own land-home.
It makes but little difference where we take our plunge into the deep
sea, for changes of climate are scarcely or not at all known there,
and the fish seem to wander over every part. Wherever it may be,
then,--let us say in the seas of the Tropics, which have given us most
of our specimens--let us dive down, down, till we reach about 1800 feet
(300 fathoms).
... “For who can know
What creatures swim in secret depths below,
Unnumber’d shoals glide thro’ the cold abyss
Unseen, and wanton in unenvied bliss.”
We shall be groping more and more in darkness as we go, for the
sunlight scarcely reaches beyond 1000 feet, and we have left its last
rays behind us, and the water is growing icy cold. How strange, then,
that the first fish we meet should have large wide-open eyes! This is
the Beryx,[25] shaped something like a perch, but about a foot and a
half long, and genealogists ought to look at him with respect, for his
ancestors (see heading of Chapter) are almost the oldest known bony
fish, and lived in the chalk seas.
Has he come down here because the upper world was too rough for him?
If so, he has found comparative stillness, for he is far beneath the
turmoil of the waves, and only the slowly creeping currents make any
movement around him. But he has not escaped from the struggle for life,
for not only is a good-sized shark coming his way, but a huge monster
of the bony race, six feet long,[25] with wide-opened jaw, sharp
pointed teeth, and large keen eyes, is wandering near in search of
prey, devouring large and small fish with great impartiality.
Still in the dense darkness the Beryx must surely escape? No! for,
strangely enough, lights are travelling about in this midnight region.
The monster himself carries lamps upon his body, and a shoal of
small oblong fish, something of the size and shape of a gudgeon, come
swimming by, carrying on their sides whole rows of shining spots
giving out phosphorescent light; while not far off another fish,
called in India the Bombay duck, glows all over, as if his whole
body had been rubbed in phosphorus. Nay! so far as we know the Beryx
himself is probably gleaming with light, for his body is covered with
a large quantity of the same slimy fluid which makes the “Bombay duck”
phosphorescent when he is freshly drawn out of the sea.
So these curious fish, living in eternal darkness except when they make
an expedition to the surface, carry many of them their own lights; and
as we go deeper still more and more of them are found with shining
mother-of-pearl-like spots on their head, or sides, or tail, so that
the very darkness is alive with light. What slaughter and hunting
there is among them! for they all eat each other, and even their own
young, there being no plants for any of them to feed on. There are
the deep-sea cod-fish; strange forms with large heads, long tapering
tails, and thread-like fins, chasing the smaller fish, and falling
victims themselves to the fierce _Stomias_ which comes sailing along
with its row of glowing lights, and its huge sharp teeth, ready to
seize its prey. Both these fish go down as deep as ten thousand feet
and more, accompanied by another fish quite as ferocious, though only
a foot long, with large teeth sticking out of its mouth like the tusks
of a boar, and curious round spots, with lenses in them, on its side,
which may be eyes, or may be lanterns to light it on its road; and
among these luminous fishes are wriggling along the deep-sea Conger
eels, with toothless mouths and elastic stomachs, swallowing large
fish whole; while another curious cod-like fish, whose stomach can
stretch to more than four times its natural size, draws itself over
its prey just as a snake does, and carries it in the hanging bag till
it is digested. And deeper yet in the dead calm water roam many fishes
with delicate feelers hanging from their mouths, while their fins are
slender and tapering, so that they feel their way along the still
depths. Among these are the Ribbon-fish, twenty feet long but only a
foot deep, and never more than two inches thick in any part, with their
long rosy fins floating like ribbons back from their heads and from
under the body.[26]
Strange monsters are all these deep-sea fish, some of them living
as much as 16,000 feet under the surface of the sea, so that if
Switzerland were turned upside down in mid-ocean, the peak of Mont
Blanc would not reach down to where they swim. Yet they are only
modified forms of ordinary fish from the world above, which have become
fitted to live under that vast pressure of water. Their skeletons,
though bony and well-knit together at that depth, are fibrous and
slight compared to those of their surface relations, for although
they have to resist a weight of from two to sixteen tons pressing
all round them, a ton weight being added for every thousand feet, no
special strength is required, because the dense water permeates their
whole structure, and the pressures are everywhere equal. It is the
same with them as with the most delicate and fragile insects living
in our atmosphere, the pressure of which would tear them to pieces if
unbalanced by equal pressures within and without.
But when these deep-sea fishes are brought up quickly to the surface,
the outside pressure no longer balances that inside, and so their
tissues loosen and their whole framework starts apart, so that they
almost fall to pieces at a touch; and their air-bladder, if they have
one, expands so much as to force the stomach out of the mouth, turning
them almost inside out. Neither are their lanterns a special creation
for their use, but merely adaptations of that slimy fluid which we
saw oozing from the scales of the minnow. In some of the deep-sea
fish even the outer bones are filled with this fluid, and the line of
scales along the side has large openings, so that the body is bathed in
glowing slime. In others it collects in glands on the sides, making the
phosphorescent spots.
In this way the deep-sea fish have become fitted to make a home in the
very heart of the ocean. Some with large eyes, seeing by means of their
own and their neighbours’ light, others with small eyes and delicate
feelers, testing each step as they go, and feeding, probably, on the
shower of minute sea-animals that falls continually from above; while
some, like the Beryx, the Bombay Duck, and the light-carrying Scopelus,
which live nearer the top, come up on still nights to feed at the
surface of the sea.
[Illustration: Fig. 9.
Remoras[27] clinging by their sucking-disk to the under part of a
shark.--(_Adapted from Brehm._)]
And now, as we rise again from the dark still depths up to warm layers
of the tropical seas into which the sun is pouring his penetrating
rays, it may happen that a large dark body moves between us and the
surface, as the Great Blue Shark, or one of his smaller relations,
ploughs his way through the water. But what are these little dark brown
fish, with round gaping mouths, which are hanging by the top of their
head and back from under the shark’s belly? (see Fig. 9). Where he goes
they go with him, and, as they are borne along, they feed upon the
tiny sea-animals among which they are carried so easily. These cunning
passengers, of whose very existence the shark seems unconscious, are
the Remoras, or sucking-fish. You would scarcely think that they
belong by descent to the mackerel tribe, a strong-swimming, active,
and almost warm-blooded group of fish, with a large supply of nerves
and blood-vessels to their muscles, so that they swim boldly out to
sea, and make more use of the open ocean than almost any other group.
But among all tribes there will be some weak members, and these must
live by stratagem. The little remora is a feeble swimmer, and, having
to live out at sea, has acquired a curious sucker by which he clings
to sharks, and whales, and even ships, so that he is carried along
without exertion. Yet this sucker, again, is only a special adaptation
of the back-fin, which, instead of being single, as in other mackerel,
has its spines divided and bent, one set to the left, the other to the
right, and joined by a double set of plates, surrounded by a fringe of
skin. This forms an oval disk, and, as the remora glides along under
the shark’s belly, he presses the damp membrane against the fish, and,
drawing together the muscles of the plates, clings as firmly as a
limpet to a rock.
Nor is the remora the only companion of the shark--
“Bold in the front the little Pilot glides,
Averts each danger, every movement guides;”
for the little steel-blue striped Pilot-fish,[28] another distant
connection of the mackerel tribe,[29] is hovering around, feeding
upon the scraps of the shark’s food, and finding protection in his
neighbourhood, though in olden times he was supposed to protect the
shark. A brave little fish this, which has succeeded in making the
shark his friend: while near him he is safe from other fishes.
And now, as we continue our way in the open sea, it is nearly always
forms more or less related to the mackerel tribe which cross our
path. The slender Bonito[30] and the heavier Tunny[31] sometimes ten
feet long, are hunting below or on the surface, and the beautiful
Dorados,[32] or gold-mackerel, as the Germans call them, with their
silvery blue backs tinged with a sheen of gold, their dull-coloured
fins, and their golden eyes, are driving by in large shoals in pursuit
of the flying-fish. All these are powerful swimmers, and they have no
air-bladder, which is an advantage to such active hunters which wish
to turn rapidly, to go down deep or rise to the top, and change their
position at every moment; for in all these movements a natural float
inside is a hindrance to be overcome. And so we find that in fish, even
of the same family, some have lost the air-bladder, while others have
it enlarged to meet their wants, as in the case of the lovely blue and
silver sun-fish[33] for example, which, though quite near relations
of the dorado, have very large air-bladders, enabling them to float
quietly on the top of the water, waving their deep scarlet fins.
[Illustration: Fig. 10.
Flying-Fish[34] pursued by the Dorado.[35]]
But while we are watching all these large and strong swimmers an active
and bloodthirsty struggle is going on, for the bonitos and the dorados
are looking to make their meal upon the little Flying-fish, which are
straining every nerve to escape them, while here and there one drops
down into their very mouths. Lovely little creatures these are, of
the Pike family, which have taken to the open sea, where they rise
with a stroke of the tail many feet out of the water, their bright
purple backs and silvery sides gleaming in the sun, as, with their
long transparent arm-fins outspread, they float for as much as two
hundred yards before they fall back, to spring up again with another
stroke. Their air-bladder, which is half as long as their body, and
contains in a six-inch fish as much as three and a half cubic inches
of gas, stands them in good stead, and they rise and fall with quick
rapid flights out of the reach of their foe, so that in the open sea
they do fairly well on the whole, though, if they venture near land,
the sea-birds persecute them in the air. Nor do they stand alone in
this curious habit of flying, or rather floating, in the air, for a
larger fish of quite another family, the “Flying Gurnards,”[36] with
a smaller but still ample air-bladder, and long arm-fins, may also be
seen rising in the Mediterranean and tropical seas, out of reach of the
fish-hunters of the water.
And now we must leave the open sea and steer for the shore. It is true
that many other fish are wandering in the broad watery main, but many
of them, such as the globe-fish, feeding on the small crustaceans
and the sea-horses,[37] whom we shall meet nearer shore, are feeble
forms carried hither and thither by currents or on floating banks
of seaweed, while others have no special interest. The sharks, the
mackerel, and the flying-fish, are the most remarkable colonisers of
the ocean-surface, for even the enormous Sword-fish,[38] which attacks
the bonitos and whales with its long wedge-shaped bony jaw, and is said
to sail by raising his back-fin, is a distant off-shoot of the mackerel
tribe.
So we cannot do better than follow our own common Mackerel, as they
migrate in shoals out of the deep sea to feed on the fry of the herring
or the pilchard in shallower water, or to leave their eggs floating not
many miles from land, so that the tiny mackerel, when hatched, may live
in the quiet bays till their strength comes.
But stop! Long before we have come so far as this, and while we are
still a hundred miles or more from the shore, let us peep down into the
sea-valleys, where forests of seaweed and marine plants are growing,
and myriads of tiny sea-lice and crustaceans throng the water. What
is that army of thin spindle-shaped forms rising and falling in such
numbers? It is a shoal of herring, which have come there to feed upon
the sea-animals, keeping out of sight of the sea-birds above, and the
cod and sharks and ravenous fish which hunt them without mercy, so that
they only venture to come to the surface on calm dark nights. It was
in valleys such as these that the herrings were living when the older
naturalists thought they were gone away to the Polar Seas, because they
only saw them in spring and autumn, when they come into shallower water
to drop their myriads of eggs,[39] which sink down, and stick to the
seaweed and stones below.
But now they are revelling in the deep ocean, rising and falling with
ease, for their air-bladder has two openings, one to the stomach and
one to the outside of the body, so that the gas can adjust itself
to their movements; and surely if the shark is the type of the old,
lumbering, powerful, slow-breeding fish, the herring, with its narrow
lissome body, light playful movements, and myriads of young, is the
type of the new and active race. They are as truly social animals
as any herds on land, for they travel in shoals of many hundreds of
millions; and as they can squeak, and have a very good apparatus for
hearing, it is more than likely that they call to each other. They
make both the salt and fresh water their own; for when the eggs are
hatched at the mouths of rivers the tiny fish take refuge there from
the violent persecutions of the cod and mullet and haddock, flat-fish
and whiting, and, together with the small fry of other fish, stroll up
the rivers, where we call them “white-bait.”
And now, as we come nearer to the shore, where countless numbers of
small fry are filling the water, and all creatures are struggling
together to accomplish three objects, namely, to get food, to avoid
being turned into food, and to lay their eggs, we find many strange
weapons and devices adopted by the different fish for protection and
attack.
... “Each bay
With fry innumerable swarms, and shoals
Of fish, that with their fins and shining scales
Glide under the green waves, in sculls that oft
Bank the mid sea.”
There are the Mullets,[40] with tender feelers under their chin, with
which they brush the ground lightly as they swim, feeding on the tiny
creatures. There are the walking fish, the Gurnards,[41] which have
three of the spines of their arm-fins separate, and moved by strong
muscles and nerves, so that they can walk on the sea-bottom, feeling
their way, while the stiff, spiny rays of their back-fin stand up to
wound any enemy attacking them from above. There are the tiny Blennies
which walk too, but by means of the few rays which alone remain of
their leg-fins growing close under the head. Then there are the
clinging-fish, the Gobies,[42] living on the rocky shores, where the
waves beat and roar, and they have their leg-fins joined together, so
as to form a kind of funnel under their throat, with which they cling
to the rocks and then dart across the waves to feed, coming to anchor
again out of the dash of the water; some of these little fellows make
nests and guard their eggs after the mother has left them, till the
young can shift for themselves. More curious still, the Lumpsucker[43]
has its arm-fins and leg-fins all joined together into a round disk
under the throat, and so holds on bravely against the dashing tide,
defending the eggs which have been laid in the seaweed near the shore,
and even remaining to take up the young ones when hatched, and carry
them safely back into deep water as they cling to his sides.
Meanwhile, close down upon the sand are the hiding-fish, the Weevers,
the Anglers, and the Flat-fish.
The weevers[44] are the most dangerous. Their shaded yellow colour
hides them from view, while the sharp spines of their back-fins, which
they keep raised, will inflict very severe, if not poisonous, wounds
on any creature striking against them. Nor is this all, for behind the
cheeks, fastened on to the horny gill cover, are daggers with which
they can strike, deliberately jerking them back so as to give a sharp
blow. These are fighting aggressive fish, waging the war that goes on
so sharply all round our coasts.
[Illustration: Fig. 11.
The Fishing Frog.[45]]
But there is one even more cunning than they, lying hidden in the
seaweed or the sand--a large, flat, soft fish, about three feet in
length, and quite half as broad as he is long, with a soft stumpy
tail, stretching out behind, and a kind of wrist-joint to arm and leg
fins, by which he can creep noiselessly along. His wide mouth is gaping
open, so that a two-foot rule could be passed crossways into it, and
his pointed teeth are bent back to allow his prey to enter. But how is
this prey to be caught, for he is not going to move to fetch it? Notice
all round his head and his body, the skin is fringed like blades of
seaweed and plays about in the water; while above his head and back
the spines of his fin stand up quite separate, and the front one is
tapering and long like a fishing-rod, with a lappet at the end like a
bait. And now, as the shallow water ripples over his head, the lappet
plays to and fro, and the unwary fish come up to nibble at it, lower
and lower he waves it, and the nibblers follow, till, opening his wide
gape, he gulps them down, even if they are as large as himself, and
lies passive with his swollen stomach till they are digested. This is
our own Fishing-frog,[46] of which one was once found with seventy
herrings in his stomach. He has relations all over the world--in the
open sea and down in its depths, and all of them more or less follow
his fishing habits. Yet there is no creation of special parts for these
strange weapons; the altered back-fin and the jagged skin do all the
work, just as in some curious fish of the weever family in the tropics,
called the Stargazers,[47] the feelers on their lips, longer than those
of other fishes, and a lengthened thread from below the tongue, play in
the watery currents and attract the small animals, while the fish with
upturned eyes watches them as they are lured to destruction.
Lastly, among all these curious forms upon our shores there is an
abundance of flat-fish--soles and turbot, brill and plaice--flapping
along at the bottom, covering themselves with sand, or rising up with
that strange wavy movement of the whole body in which they use what
look like long side-fins, but which are really the back-fin and the
belly-fin.
[Illustration: Fig. 12.
The Common Sole.[48]
Above are two small soles as they swim when young. At that time
they are not larger than a grain of rice.--(_Adapted from Figuier
and Malm._)
]
If we wanted to pick out the strangest and strongest proof of how the
shape of fish is altered to suit their wants, we need seek no further
than the flat-fish.
When we were speaking of the shark order we saw that the rays and
skates are flattened forms suited to hide in the sand, and these fish
are truly spread out as if they had been squeezed under a heavy weight,
their broad arm-fins edging the sides of their body. But the bony
flat-fish, the Soles and Turbot, have a far stranger history. The young
sole, when it comes out of the egg, is not flat like the young skate,
but a very thin spindle-shaped fish, something like a minnow. He is
then about the size of a grain of rice, very transparent, and lives at
the top of the sea. He has one eye on each side, like other fish, only
one eye is higher up than the other, and the single fin on its back and
the one under its body reach almost from head to tail. In this way he
swims for about a week, but he is so thin and deep, and his fins are so
small, that swimming edgeways is an effort, and soon he falls down on
one side, generally the left, to the bottom of the sea. Many times he
rises up again, especially at first, till he has got used to breathing
at the muddy bottom, and meanwhile the eye that lies underneath is
gradually working its way round to the upper side, his forehead
wrinkles so as to draw the under eye up, while his whole head and mouth
receive a twist which he never afterwards loses. His skeleton, it must
be remembered, is still very soft, and the bones of his face are easily
bent; and at last this eye is screwed round, and as he lies at the
bottom he can look upwards with both eyes and save the under one from
getting scratched by the sand, as it must have done if it had remained
below.
Nor is this all, for while his under side, shaded from the sunlight,
remains white and colourless, his upper side gradually becomes coloured
like the sand in which he lies, and he is safely hidden from attack as
he flaps along, feeding on worms and other animals. And now when he
swims he no longer uses his arm and leg fins, which are quite small and
insignificant, but bends his whole body, using the back and belly fins
to help him. What we then call the top of the sole is really his side,
where you may see the dark line of scales running along the middle,
and one arm-fin lying close to his head. Yet he can swim strongly and
to far distances, for in the winter the soles, too, migrate into the
open sea, where they may be found in the deep water of the Silver Pit,
between the Dogger Bank and the Well Bank.
[Illustration: Fig. 13.
Hippocampus, a fish commonly called the Sea-Horse.]
And now, before we leave the shore, we must glance at a curious weakly
little fellow clinging by his curly tail to the seaweed, whom you
will certainly not take for a fish, even if you can find him out, so
entangled is he generally in weeds of the same colour as himself. Yet
the Sea-horse[49] is a true fish, covered not with scales but with
plates, with which he makes a clicking noise by scraping them together.
What look like large ears are really his arm-fins, while at the end of
his long snout is a mouth shaped like an ordinary fish’s mouth, but
toothless, and he breathes with fish’s gills arranged in round tufts
instead of folds. What the use of his strange shape is to him we cannot
tell, but at any rate his fleshless bony body must protect him from
other fish, while his power of clinging causes him to be often carried
by floating weed even into the open ocean, and make up for his feeble
powers. In one thing he surpasses most other fishes, for he is a most
careful father, carrying the mother’s eggs in a little pouch under his
body till the young ones escape. There is one form of these sea-horses
in tropical seas which has long red fringes floating from its body, so
that it cannot be distinguished from the seaweed in which it hides.
So we see that the deep sea, the open sea, and the shore, are filled so
full of different forms that there are enough not only to make use of
every part, but also to provide food for each other, and we also see
that by far the larger number even of widely-spread fish come near to
the shore to leave their spawn, while the young ones often make their
way into the brackish water at the mouths of rivers, and spend their
youth in the shelter of the still fresh water.
Now it is very natural that many such fish should learn to remain in
this quiet refuge, and in time to live there altogether. And because
fish-life in the rivers is comparatively uneventful and little varied,
we find much fewer peculiarities in river-fish. Many of them are very
near relations of sea forms. There is the salmon, a true sea-fish,
which wanders up the river to spawn in the pebbly shallows; and there
are the trout, his near relations, which have learned to live entirely
in the rivers. There are the sea-perches, large strong fish, and the
smaller river perch, which have made their homes very successfully in
the rivers, for their spines are so sharp that even the greedy pike
hesitates to swallow them. There are the sea-sticklebacks, and the
little river-stickleback.[50] This last is a very clever little fish,
which hollows out the foundation of his nest very carefully in the bed
of the river, and then builds it up for several inches with blades of
grass and weeds (Fig. 14), gumming them together with the slime of his
body. Then, when all is ready, he swims about to drive and coax the
mother to the nest, sending her in to lay her eggs, and then driving
her right through and out at the other side, so that a stream of water
flows constantly over the eggs till they are hatched. Nay, his care
does not end here, for when the young fish come out of the egg with a
bag of yelk hanging under the body, as all young fish have at first,
and so cannot swim easily and escape their enemies, the courageous
little father will defend them and fight fiercely with any fish which
thinks to make a meal upon them, not leaving them till all the yelk is
absorbed, and they are able to swim and feed themselves.
[Illustration: Fig. 14.
STICKLEBACKS AND THEIR NEST. (Gasterosteus aculeatus.)]
Besides these active river-fish there are the little stupid Miller’s
Thumbs,[51] hiding under the stones to feed on tiny animals; they are
feeble relations of the gurnards which we saw walking on the bottom
of the sea. Then there are the purely freshwater fish, the Pike and
the large Carp family, with its many branches, the Roach, and Dace,
and Gudgeon, and Minnow; and the enormous family of Cat-fish and
Sheat-fish,[52] of which we have none in England, but plenty in America
and other parts of the world, a family in which the fathers sometimes
carry the eggs in their mouths till hatched. And last but not least
among the freshwater forms is that irrepressible family of the Eels
which we saw wandering in the deep sea, and which are also to be found
near the shores all over the world. These fish will even travel through
pipes and into cisterns; and will climb up trees so as to drop into
neighbouring streams and continue their wanderings; they sleep in the
mud in winter; and even after being frozen come to life again; and in
the spring they go to the sea to spawn, giving rise to those shoals
of young ones from three to five inches long which come in incredible
numbers up the rivers in summer, making the eel-fairs,[53]--
“The silver eel, in shining volumes rolled,”
so much spoken of in old books, when the eels will often climb high
banks, nay, even pass over miles of dry land, closing down their narrow
gill-openings, and so shutting in water to serve them as they go.
All these, and many other freshwater families, show us how the fish
have wandered into every possible nook of the waters, so that even in
those inland salt lakes of North America and Asia into which no rivers
flow fish-life is abundant; and we can only suppose that the eggs must
have been carried by water-birds in their flight, or by gusts of wind,
or have arrived there in ages long ago, before these lakes were cut off
from the rest of the watery world.
Yet some few fish besides the eels have been known to travel over land
to find watery “pastures new;” the Climbing Perch[54] of India and
the Doras of Tropical America will both travel many miles when their
own ponds are dried, the perch breathing by the help of a special
apparatus, and the doras probably shutting water into its gills; for
necessity, even in fishes, proves the “mother of invention,” and in
special works on fish you will find accounts of numberless strange
devices and adaptations by which they manage to survive in the struggle
for life.
And now, collecting together all we have learned, let us in conclusion
form a rough picture of the history of the fish-world. All over our
globe, from pole to pole, and from the Indian Ocean round to the east,
back to the Indian Ocean again, is one vast world of waters, with
inlets and land-locked seas bordering its margins, and rivers pouring
into its depths. In the past ages of the world these rivers and coasts
and inlets have varied innumerable times, but the great ocean-mother
has always been there to bear the increasingly-varied forms in her
bosom, and to enable them to wander where best they could preserve life.
And so from their beginning, when they were probably as feeble as the
lancelet, these earliest and simplest backboned animals with their two
pair of limbs as yet very variable both in their position and shape,
have been spreading far and wide over the watery three-quarters of
the globe. We have seen how the enamel-scaled fish had their time of
glory, but were not able to hold their ground, because they were not
agile and fish-like enough to escape their foes; and how the sharks by
their strength and boldness remain monarchs of the sea to the present
day. Then we have seen that in old chalk seas the new and active race
of bony fish appear in force; some like the herring and the carp,
with air-bladders, which had openings like the enamel-scaled fish,
and these can dart from heights to depths; while others had closed
air-bladders, and these remain with most ease at one level, and can
sometimes, if necessary, use the gas in their bladder for breathing,
if they are oppressed with muddy water; and lastly, some, such as the
dorado, have lost their air-bladder altogether, and gain in freedom
of action what they lose in lightness and buoyancy. And during the
ages that have passed since this bony race began, different branches
each in their own way have thrown out curious weapons and developed
strange organs to help them in the battle of life, so that now we have
deep-sea fish carrying their own light; fish with distensible stomachs
swallowing prey larger than themselves; fish with large air-bladders
and long arm-fins springing out of their own element and floating in
air; angling-fish, walking-fish, clinging-fish, and hiding-fish; and
even those whose shape is distorted, like the sole, to enable them
to hide and hunt in safety; while, when the sea is full, we find new
varieties pressing their way into every river and tiny stream, and even
overland into enclosed waters. Nay! when we descend into the recesses
of the earth and visit the underground pools of the dark caverns of
Kentucky, there we come upon fish which have found a refuge in eternal
darkness, and have lost not only the power of sight but actually the
eyes themselves.
And here we must leave them to go to higher vertebrate animals.
Although but little is known of fish-life, a very small part even of
that little has been given here, and yet we take leave of it with
the feeling that its changes and chances are greater than we can ever
thoroughly learn. How much pleasure these creatures have in their
water-world it would be difficult for us to say; but since we find them
playing together, hunting together, sporting in the warm sunshine, and
diving and gambolling in the open sea, and sometimes even calling to
one another, we cannot but think that life has great charms for them in
spite of the many dangers surrounding them. And when, low though they
are in the scale of life, we find them (though curiously enough always
the fathers) carrying the eggs, building nests for them, and defending
the young, we see that even here, in the very beginning of backboned
life, we touch the root of true sympathy, the love of parent for child.
[Illustration: THE HOME OF THE EARLY AIR BREATHERS]
CHAPTER IV.
HOW THE BACKBONED ANIMALS PASS FROM WATER-BREATHING TO AIR-BREATHING,
AND FIND THEIR WAY OUT UPON THE LAND.
So the backboned animals, as fish, have peopled the seas and rivers,
and, as the ages have past on, have become more and more fitted to
their watery life, little dreaming of another and different life in
the world of air above them. And yet in the same pond with the little
stickleback, so busy building his nest, there is a creature which could
tell him that it is possible to live in both worlds, if only you have
the proper machinery to do it with.
It is clear that if the backboned animals were ever to live upon land,
after they had begun their career in the water, there must have been
some among them which learned gradually to give up water-breathing,
and to make use of free air; and we shall not have far to seek for
creatures which will help us to guess how they managed it.
From almost every country pond, or ditch, or swamp, a chorus of voices
rises up in the springtime of the year, calling to us to come and learn
how Life has taught her children to pass from the water to the air; for
it is then that the frogs lay their eggs, and every tadpole which grows
up into a frog carries us through the wonderful history of an animal
beginning life as a fish with water-breathing gills, and ending it as a
four-legged animal with air-breathing lungs.
Come with me, then, to some stagnant pool in a country lane, towards
the end of March, and there we shall no doubt find a whole company of
frogs, croaking to their hearts’ content after their long winter sleep
in the mud at the bottom of the pond. They are wide awake now, and are
actively employed laying their eggs. Look carefully around the edges of
the pond, especially in that part where the wind has driven the scum
to the side, and you will doubtless find in some still corner a gluey
mass (_e_, Fig. 15), which looks like a lump of jelly with dark specks
in it. Take this up carefully, for it is frog spawn; carry it home
together with some weeds from the pond; put it in a glass bowl with
water; and then from day to day you may study the history of a frog’s
life.
[Illustration: Fig. 15.
Metamorphosis of the Frog.
_e_. Eggs. 1. Tadpoles just out of the egg. 2. With outside gills.
3. With gills hidden, and beak-like mouth. 4. Hind legs appearing.
5. All legs grown, but fish-tail remaining. 6. Putting on Frog
appearance; tail being absorbed. 7. Young perfect Frog.
]
That jelly-like mass is a collection of frog’s eggs. When they were
laid, each egg was a small round dark body in a gluey covering, and
they all fell to the bottom of the pond, where, by degrees, the
water oozing through the envelope swelled each egg, till they clung
altogether in a mass, and, rising, floated at the top. Then very soon
each round dot lengthened out into a long streak, and in a few days an
eyeless head appeared at one end with a soft closed mouth under it,
and at the other a tail, with a soft fin round it like the tail of
the lancelet; so that by the time you find the spawn, you may, most
likely, be able to see the tiny creature wriggling every now and then
in its watery bed. This will go on for some time, and a week or two may
pass before the moving tadpole breaks through its egg skin, and coming
out into the world, fastens on to a piece of weed (1, Fig. 15) by two
little suckers behind its mouth. And now that it is out of the egg the
interest begins. Look carefully day after day and you will see some
branching tufts (2, Fig. 15) growing larger and larger on each side of
its head. What are these? We have not seen them in any fish. No! but if
you take a young hound-shark out of his leathery egg before his time,
you will find that he has outside gills much like these, only he loses
them before he comes out into the world, whereas the tadpole keeps them
to breathe with a little longer. If you put the tadpole, at this stage,
under the microscope, you can see the red blood flowing through these
gills to take up air out of the water.
Meanwhile the tadpole’s lips are gradually forming into a round mouth,
much like the lamprey’s, and by-and-by the inner part of this mouth is
covered with two little horny jaws, forming a sharp beak (3, Fig. 15)
with which he will nip off pieces of weed for food. Meanwhile, as he
grows larger and larger, and eyes, nostrils, and flat ears form in the
head, a covering begins to grow back over the sides of the neck, and
little by little the branching tufts disappear (3, Fig. 15). How, then,
can he breathe now? Watch carefully and you will see that he gulps
every moment as we saw the minnow doing (p. 23). The fact is that the
outside tufts have faded away, and under the cover the tadpole has six
slits in his throat, like the slits of the lamprey, which are covered
in somewhat similar fashion to those of the amphioxus (see p. 11), and
he breathes through them.
Here is our tadpole, then, to all intents and purposes a fish. He
swims with a fish’s tail; he gulps in water at his mouth, passing it
out at the slits in his throat after it has poured over his fish’s
gills. Moreover, he has a fish’s heart, of two chambers only, like
the minnow’s (p. 23), which pumps the blood into these gills to be
freshened, while, like the lamprey, he has a gristly cord, enlarged
at the end to form a gristly skull, a round sucking mouth, and no
limbs. All this time, however, though he has a fish’s fin round his
tail, he has no arm or leg fins. Wait a while and you will see that
under his tender skin far more useful limbs are being prepared. As he
grows bigger and more active week by week, wriggling among the weeds
and feeding greedily, two little bumps appear one on each side of
his now bulky body, just where it joins the tail. These bumps grow
larger every day, until, lo! some morning they have pierced through the
skin, and two tiny hind legs (4, Fig. 15) are working between the body
and the tail. The two front legs are longer in coming, for they are
hidden under the cover which grew over the gills, but in about another
week they too appear, and we have a small four-legged animal with a
lamprey’s tail (5, Fig. 15). These legs are something far in advance of
fish fins, for they have shoulders and thighs, arm and leg bones, wrist
and ankle bones, hand and foot bones; and instead of the large number
of rays in a fish’s fin they have four fingers on their short front
legs, and five toes at the end of long hind ones; the toes being joined
together by a web, which helps him wonderfully in striking the water as
he swims.
The tadpole has now become fitted to jump and leap on the land or
swim by his legs in the water; and, moreover, while these legs have
been growing, another change has been taking place. You will notice
by careful watching that at first he still gulps in water as he used
to do, but he comes more often to the top, and, poising himself so
that his mouth is out of the water, gives out a bubble of bad air,
draws in some fresh, and goes down again. Why does he do this? Have
you any recollection of another fish-like animal which comes up to
take in air? Look back at our friends the mud-fishes (p. 34), and read
how the Ceratodus fills his air-bladder when he is short of good air
in the water. When you have re-read this, you will suspect that the
tadpole, too, has something like an air-bladder, which he fills from
time to time. And so he has. While his legs are growing a bag has been
forming inside at the back of his throat, which afterwards divides
into two, and he fills these by shutting his mouth, drawing air in at
his nostrils, putting up the back of his tongue to shut it in, and
then swallowing it down into the lungs; so that he is now a truly
double-breathing animal, using his gills when below water and his lungs
when above. Moreover, if you could watch inside his body, you would
now see that little by little the blood-vessels going to the gills
grow smaller and smaller, and those going to the lungs grow larger
and larger; while the fish’s two-chambered heart divides into three
chambers, one to receive the blood from the body, another to receive
it from the lungs, and one to drive this blood back again through the
whole animal. And when at last this change is so complete that all
the blood goes to the lungs to be freshened, the gills shrivel up and
disappear, and our tadpole is a true air-breathing animal.
Notice, though, that he is still cold and clammy, not warm like a mouse
or a bird. For his blood still moves slowly, and as he has only three
chambers to his heart instead of four, as warm-blooded animals have,
the good blood from the lungs and the worn-out blood from his body
become mixed each time they come round, so that his breathing work is
still of a low kind all his life. And now that he can leap and swim
with his legs, his tail is no longer of use to him, and it is gradually
sucked in, growing shorter and shorter till it disappears, and the
young frog is complete.
Thus our backboned animal has succeeded in getting out of the water
on to the land, and in doing so he has quite changed his habits. A
peaceful vegetarian before, he is now a greedy eater of insects,
slugs, and other animals. His horny beak has been pushed off; his lips
have stretched back farther and farther, till they now open right
back as far as his flat little ear; and he is a gaping, wide-mouthed,
leaping frog[55]--
... “Hoarse minstrel of a strain
Aquatic, leaping lover of the rain;”
(7, Fig. 15), with teeth in the roof of his mouth. But perhaps his
tongue is the most curious of all, for instead of being fixed at the
back, and free in the front, as in most other animals, the root of it
is fastened to the front of his lower jaw, and the tip lies back in his
mouth, so that when he wishes to catch an insect he throws his tongue
quickly forward, captures his prey on the sticky point, and flings it
back down his throat.
So he hops about the summer long, if he can only escape from ducks and
rats and other frog-eating animals. He often takes to the water, for he
can fill his lungs with air and use it very slowly, and, moreover, his
soft skin is of great use to him in still breathing in the water or in
the moist air; and when winter comes he takes refuge with many others
at the bottom of the pond, and sinks into a state of torpor, till the
spring brings croaking and egg-laying time round again.
[Illustration: Fig. 16.
The Common Smooth Newt[56]--male and young in the water; female on the
bank.]
Our little frog, then, is truly an animal with a double life, a genuine
amphibian,[57] meaning by this, not merely an animal that can swim in
the water and move on land, for seals and water-rats, white bears and
hippopotamuses, can do this, but one that in the early part of its life
would die if taken out of the water, while afterwards it lives and
breathes in the air.
[Illustration: Fig. 17.
Proteus of the Carniola caverns,[58] with its external breathing
gills.--(_Adapted from Brehm._)]
Have these double-lived creatures, then, such a great advantage over
real water animals, or how can we account for their having adopted
this strange life? If we only look upon them as they are now, we can
scarcely call them particularly successful, compared to other animals.
For though there are plenty of them, yet they are comparatively small
and insignificant; and when we find large ones like the gigantic
salamander of Japan, they are sluggish and feeble. Look at the
common newts, or water-salamanders of our ponds, with their weak
crawling limbs, as they wander round the edges of a pond, feeding on
water-insects and tadpoles, the male with his crested back, the smooth
mother, and the young eft-tadpole with its branching tufted gills (Fig.
16). They are much less active than the frog, for they never lose their
tails, and they come less often out of the water, although they are
true air-breathing animals. Then, when we go to other countries, there
is the Proteus (Fig. 17), that curious half-transparent newt, with a
round body and tiny helpless legs, which lives in eternal darkness in
the still underground pools of the Carniola caverns near Adelsberg. He
has become well fitted for his dismal life, for his tiny eyes are grown
over with skin, and he never loses the feathery gills on each side of
his neck, but lives like a tadpole all his life, although he has true
lungs. Again, in America we have the Siren, with its long snake-like
body, and only front legs, with which it cannot walk. It, too, keeps
its gills as it wanders about the stagnant waters of South Carolina,
feeding on worms and insects. Then in the Mexican lakes there are the
curious Axolotls, which also wear outside gills, as a rule, all their
lives, and fathers, mothers, and children remain breathing in the water
together, although they have real lungs. But about twenty years ago,
some of those axolotls, which were kept in the Jardin des Plantes in
Paris lost their gills, came out upon the land, and astonished people
by becoming true land salamanders, like some already well known and
called Amblystomes, breathing only with their lungs. It was difficult
for some time to make the world believe that grown-up water-breathing
creatures which could lay eggs were able to turn into other creatures
without gills. But at last a lady, Fraulein Marie von Chauvin, took
some axolotls when they were full-grown, and kept them on land in
wet moss, washing and feeding them every day, and thus succeeded in
teaching them to breathe air, so that their gills shrivelled up and
disappeared. Then there could no longer be any doubt that the axolotl
is only the lower water-form of the amblystoma, which in the Mexican
lakes, owing to the increased dryness of the surrounding country, has
lost the habit of coming out on to the land, and remains in the water
with its little ones all its life; but which, when brought to a moist
climate where it can breathe comfortably on land, sometimes returns to
its old double life.
[Illustration: Fig. 18.
Axolotl, a creature living and breeding for generations in the
water. Amblystoma coming out of the water,--an axolotl which has
lost the gills and acquired lungs.
]
We have, in fact, in Europe real land salamanders, which live in cool
damp places, looking like lumpy soft-skinned lizards, but going down
to the water to lay their eggs, that their little ones may go through
their tadpole life--and one of these, the black salamander,[59] which
lives high up in the mountains of Germany, France, and Switzerland,
does not even go to the water, but carries the young tadpoles in her
body till they can breathe air and run alone; and yet they are still
true _amphibia_, for if they are taken out of their mother and put in
water, they go through all their changes like common efts and newts.
Lastly, there is a strange group of legless creatures called Cæcilians,
which have taken refuge underground, burrowing like worms, though they
are true amphibians and their young have gills in their babyhood hidden
under a slit in the neck. These cæcilians are the only amphibians which
have scales something like fishes, yet they never live in the water,
but in the marshy ground of tropical countries, feeding on worms and
insects.
* * * * *
Now when we think that these sluggish newts, and salamanders, and
cæcilians, with their more nimble but comparatively unprotected
relations, the frogs, are all the amphibians now living, we cannot but
wonder how Life came to produce such a feeble set of creatures to fight
the battle of existence.
But if we glance back to that far-off time when the ancient fishes were
wandering round the shores and in the streams of the coal-forests, we
shall be better able to read the riddle. For in those days it was a
great step for an animal to get out of the water at all, and those that
did so had a much better time of it than our frogs and newts have now,
when the country is full of land enemies.
And so we find that the _amphibia_ were not then the small scattered
groups they are now, but strong lusty animals, with formidable weapons.
In the hardened mud, which in those days formed the soft swampy ground
of the coal-forests, but is now stiffened into the roofs and floors
of our coal-mines, footprints have been left which tell us of large
and formidable creeping animals, with toed feet and long flat tails,
dragging themselves over the marshes of the coal-forests, and finding
their way to many places which even the mud-fish with their paddles
could not reach; and from time to time, in these same roofs and floors
of our mines, both here and in America, we find the bones and coverings
of these _amphibia_, buried in Nature’s catacombs for ages, and only
brought to light by the rude hand of man.
These remains remind us that
“A monstrous eft was of old the lord and master of earth,
For him did the high sun flame, and his river billowing ran.
And he felt himself in his force to be Nature’s crowning race;”
for they show us huge and powerful creatures[60] which sported in the
water or wandered over the land with sprawling limbs, long tails, and
bones on which gills grew, while their heads were covered with hard
bony plates, and their teeth were large, with folds of hard enamel
on the surface. Some of these were fish-like, with short necks and
broad flat tails, but they had true legs and toes; others, more like
crocodiles, and sometimes ten feet long, were able to walk firmly, but
still dragging their bodies and long tails over the swampy ground on
which their footprints are still found; some were small and more like
lizards, with simple teeth, scaly armour, and light nimble bodies; and
these, probably, ran about quickly on the land, and have sometimes left
their skeletons in the hollow trunks of the old coal-forest trees.
All these plated and formidable creatures were _amphibia_ or
double-lived animals, and this was _their_ Golden Age, as they preyed
upon the fishes in the swamps and ponds, probably not sparing even
their nearest connections, the mud-fishes, who, less fortunate than
themselves, had followed the road of fish-life instead of coming out
upon the land. They lived so long ago that we can tell but little of
their daily lives, but it is clear that they played a very different
part from our small frogs and newts of to-day, and in their well-formed
limbs were worthy forerunners of land and air-breathing animals.
But like the old race of fishes these large amphibians were only to
have their day, for as other branches of the family tree grew up, and
reptiles grew strong and mighty, and other true land animals began to
flourish, these huge plated forms dwindled away, and we lose sight
of them; and when we find any of their relations again it is only as
our present frogs and newts, salamanders and cæcilians, which have
taken up their refuge in lakes, ponds, ditches, underground waters,
or damp mud. And, curiously enough, those forms of to-day which are
most like the huge _Labyrinthodonts_,[61] as they are called, of the
old coal-forests, are the feeble cæcilians, with their horny scales
and their numerous ribs, although they have now fallen the lowest of
all amphibians, and, with their sightless eyes and ringed and legless
bodies, have taken to burrowing in the ground like worms.
Not so the frogs, which, like the bony fishes, began their career in
later times, and have known how to fit themselves into many nooks and
corners in life. In almost all countries of the globe they hop merrily
about the ponds and ditches, never wandering far from the water, into
which they jump and dive whenever danger threatens. It is true they
are eaten by thousands, both as tadpoles and frogs, by birds, snakes,
water-rats, and fish, and even by each other, but they multiply fast
enough to keep up the supply, and find plenty of insects both in and
out of the ponds. Nor have they kept entirely to a watery life, for
their near relations, the toads, which have toothless mouths and toes
less webbed, have ventured much farther on to the land, protected
partly, no doubt, by the disagreeable acrid juice which they can throw
out from a gland behind the eye whenever they are attacked.