we find, on careful examination of the skin of a dead specimen with the
microscope, that there is no green nor yet any blue “pigment” present in
it. I found, on examining the blue specimen which died after living
three years with me, that there is only black pigment overlaid by a
colourless, semi-transparent layer of skin. In this outer skin in the
ordinary green specimens there is scattered a quantity of excessively
minute yellow particles, which, mixed with the blue, produce the green
appearance. The fact is, that the wonderful “dead” turquoise-blue of the
blue frog is a colour-effect similar to that of the blue sky and the
blue of the human eye. It is produced by a peculiar reflection of the
light from minute colourless particles, without the assistance of any
blue-coloured substance. The distinction of these two modes of producing
blue colour is important.
Certain transparent bodies are so constituted that when a beam of light
is directed so as to pass through them, the red, yellow, green, and
purple rays which exist in colourless sunlight are stopped, and only the
blue rays come through. Such a body is blue copperas, or sulphate of
copper; another is methyl blue, one of the aniline dyes; another is pure
water, which gives only a slight advantage to the blue rays, so that the
light must pass through a thickness of 30 feet or more before the blue
tint is obvious. Thus, part of the blueness of the Côte d’Azur is
accounted for—namely, the blueness of the sea when the sunlight is
strong and is reflected from the white rocks and sand lying 30 feet to
100 feet below the surface of the water.
There are, of course, other self-coloured transparent bodies which allow
only rays of one colour to pass. Thus, blood-red, or hæmoglobin, the
pigment of the blood, allows chiefly red rays to pass through it. Yellow
rays only pass through a solution of saffron or of chromic acid; green
only or chiefly through green copperas (sulphate of iron) or through
leaf-green or chlorophyll. Colour is very generally due in natural
objects to such transparent bodies which absorb or stop all the coloured
rays of light as it passes through them, excepting those of one
tint—or, to be more correct, nearly all except those of one tint.
But the blue of the blue frog and a great deal of the blue in nature is
due to another cause. If you are a smoker, or the friend of a smoker,
watch the fine curling lines of smoke ascending from a cigar when it is
being consumed in bright sunshine. You will see that it has a blue, even
an azure blue, tint as the sunlight falls upon it. But if you let the
smoke get between the sun and your eyes you will notice that the little
curling clouds are no longer blue, but reddish-brown, in appearance. The
smoke is not a transparent blue; looked at as a transparent body, it is
brown! Further, when the smoke has passed into the smoker’s mouth and is
ejected after remaining there for a few seconds, the cloud no longer
looks blue, even when the sunlight falls on it and is reflected from it
to your eye. It is now opaque white or colourless, with, perhaps, a
faint tinge of blue. This change is due—as was shown by the experiments
of the late Professor Tyndall upon a variety of clouds and vapours—to
the cooling of the smoke and the increased size of the floating
particles which coalesce as the temperature falls. The larger particles
reflect white light, and the cloud is no longer blue. A cloud formed by
the finest particles gives the strongest blue to the light reflected
from it, and it is to this property of the finest particles of
water-cloud floating in our atmosphere that the blue colour of the sky
is due.
No doubt the question arises, “Why do clouds of the finest particles
reflect a predominant amount of blue light rather than yellow or green
or red?” That question is answered by mathematicians in accordance with
what is ascertained as to the nature and properties of light, but it
would require a long treatise to put those matters even in outline
before the reader. We may in the meanwhile accept the conclusions of the
physicists, and interest ourselves in seeing how they apply to some of
the concrete facts about colour in Nature.
There are other instances of “blueness” due to the reflection of light
from a cloud of excessively minute particles besides that of the azure
sky and the blue, curling smoke of a wood fire. A familiar instance is
the blueness of translucent bodies, such as the “white” of a boiled
plover’s egg, especially when a bit of it is placed on a dead-black
ground. The bluish appearance of watered London milk is another
instance. These bodies look blue owing to the fine, colourless particles
suspended in them, which act on light in the same way as do the fine
particles of newly-produced smoke. Another very interesting case is the
blue colour of the iris of the eye of man and other animals. It is not
due to any blue pigment, but to a reflection from fine particles in the
translucent, but turbid, tissue of the iris overlying the dark, black
chamber of the eye. White geese and white cats frequently have blue
eyes, the blue being thus produced. The only pigment which occurs in the
human eye is a brown one, which gives a colour varying from amber yellow
to very dark brown, almost black, according to the quantity present.
When a very little of it is present it gives, in combination with the
blue appearance of the unpigmented iris, a green tint, so that green
eyes owe their colour to the same combination of causes as does the
green skin of the little tree-frogs, or “rainettes.”
No solvent will extract any pigment from the skin of the blue frog—nor
by the finest trituration can one obtain any coloured particles from it;
only fine black granules can be separated. Alcohol removes the yellow
pigment from the skin of a green tree-frog (killed, of course, for the
experiment), and for a minute or two the skin becomes blue when its
yellow pigment is thus removed by immersion in spirit; but it rapidly
becomes a dull greyish-brown in colour, and so remains; the green cannot
be preserved in spirit-specimens. It is not fully explained how such a
uniform “dead” blue is produced by the reflection of light from fine
particles, as that observed in the blue frog’s skin.
It appears that the blue and the green colour in the feathers of birds
is in most, if not all, cases produced in the same way as the blue and
green of the tree-frog’s skin. It would be interesting were it found
possible to produce a full dead-blue colour by experimentally placing a
coat of a translucent but turbid colourless medium on a dead-black
plate. This, however, has not been done as a deliberate experiment. It
is, however, recorded that Goethe was delighted to find what he
considered to be a confirmation of his theory of colour when a friend
showed him an oil-painting of a gentleman in a black coat which when
wetted with a sponge turned bright blue. The picture had been recently
“restored,” and the varnish on the black coat was not “dry.” It was
precipitated by the water from the sponge, mixing with the spirit which
held it in solution. A fine colourless cloud was thus produced overlying
the black paint of the coat, and, as in the case of the cerulean frog, a
dead-blue colour, due to reflection of the light by the fine particles,
was the result. Some friendly physicist might repeat this experiment and
study the matter in detail. The red, orange, and yellow colours of
birds’ feathers are produced by pigments which are either insoluble or
only soluble with great difficulty in fluids of the nature of ether.
There is, however, an exception in the case of the African birds called
Turacous, or Plantain-eaters. These birds have some large quill-feathers
in the wing of a rich crimson colour. This splendid red pigment can be
washed out of the feathers by water which is slightly alkaline, and a
fine blood-red solution is obtained. Why this curious exception exists
we do not know. The extracted colour is found to contain the element
copper as one of its chemical components. Plantain-eaters kept in cages
have sometimes washed all the colour out of their feathers owing to the
water supplied to them for bathing and drinking having become foul and
ammoniacal, and thus capable of dissolving the red pigment.
The cultivation on the Riviera of flowers for sale as “cut flowers” in
Paris, London, and Berlin, in the colder months of the year, is now an
enormous business, bringing many thousands of pounds yearly to the small
gardeners around Hyères, St. Raphael, Nice, and Mentone. Roses, violets,
carnations, “mimosa” of various kinds, anemones, lilies, and narcissus
are sent literally in tons by quick trains several times a week from
these realms of sunshine to the dreary North. The commencement of this
trade was due to the suggestion made some fifty years ago by Alphonse
Karr, the French poet and journalist, who had a beautiful garden of his
own at St. Raphael, and found that he could produce flowers in profusion
through the winter. Two years ago I visited this garden (which now
belongs to a French painter) at the beginning of April, and found it
full of interesting flowers and shrubs, enormous bamboos, palm trees,
some twenty different “mimosas,” eucalyptus of several species, camellia
trees, and rose-bushes in quantity.
The influence of man on the vegetation of a favoured locality like the
Riviera is more striking than in the North. But it is worth remembering
that the most familiar tree in England—the common elm—is not a native,
but introduced from South Europe. Our native elm is the wych-elm, or
mountain elm—a much handsomer tree, in the opinion of many, than the
so-called “common elm.” There are doubts as to whether both the spruce
and the larch were not introduced by man at a very remote time, so that
the Scotch fir would be our only aboriginal pine. The oak, beech, birch,
ash, hawthorn, poplar, and alder are undoubted native English trees. The
holly-oak or evergreen oak, the sycamore, plane-tree, sweet chestnut,
horse chestnut, walnut, and probably the lime or linden tree have been
introduced by migrating men at various periods into our islands. With
the exception of rye and oats none of the plants which we cultivate for
food are derived from our own wild plants, and none of our domesticated
animals have been produced from native wild kinds.
VIII
FRESH-WATER JELLY-FISHES
Most people nowadays know a jelly-fish when they see one—and recognise
that it is eminently a product of the sea—one sees them washed up on
the seashore, soft discs of transparent jelly of the size of
cheese-plates (Fig. 2). They have a mouth in the centre of the disc,
often at the end of a depending trunk, like the clapper of a bell. Some
have tentacles, sometimes yards long, which sting like nettles. They
also have eye-spots, an internal system of canals and muscles which
enable them to swim by causing the edge of the disc or bell to contract
and expand in alternate strokes. There are hundreds of kinds of marine
jelly-fish varying in size from a sixpence to that of a dinner table,
and until twenty-five years ago none were known to live in ponds, lakes,
or rivers. Although they often are carried up estuaries, and may stay
for a time in brackish water, or even in fresh water, none were known
which really lived and bred in fresh water. They were regarded, as are
star-fishes and sea-urchins, as distinctively marine, and debarred by
the delicacy of their watery jelly-like substance from tolerating the
change from sea water to fresh water as a permanent thing. All
fresh-water animals—fishes, shell-fish, cray-fish, worms, and
polyps—are derived from closely similar marine animals, are in fact
sea-things which have suffered a change, and been able to stand it.
[Illustration: FIG. 2.—The common jelly-fish (_Aurelia aurita_)
one-third the natural size; _or_, one of the four arms or fleshy
tentacles surrounding the diamond-shaped mouth; _Tc_, one of the eight
eye-bearing tentacles at the edge of the disc; _GP_, opening of one of
the four sub-genital pouches, which bring sea-water close to the ovaries
and spermaries, which, however, do not open into these pouches; _x_ and
_y_, outline of the sub-genital pouches seen through the jelly.]
These being our preconceptions about jelly-fish, great was the
excitement when, in 1880, hundreds of beautiful little jelly-fish were
suddenly discovered briskly expanding and contracting, rising and
sinking in the water of a large fresh-water tank in the middle of London
(Fig. 3). You never know who or what may turn up in London. A badger, a
green parakeet, a whale, an African pigmy, an Indian scorpion, and a
voice worth ten thousand a year, have all, to my knowledge, been
stumbled upon unexpectedly at different times in the highways of London.
A new jelly-fish was perhaps one of the least expected “casual
visitors.” It was found in the large tank four feet deep in which the
great tropical water-lily—the Victoria regia—and other tropical water
plants are grown in the Botanic Gardens, Regent’s Park. It came up by
hundreds every year for some ten years after its first appearance, dying
down in six weeks or so each season.
[Illustration: FIG. 3.—The fresh-water jelly-fish (_Limnocodium_)
enlarged four times linear measurement, as it is seen dropping through
the water in a glass jar. _PT_, one of the four principal tentacles.
_MR_, the margin of the disc. _Ve_, the delicate muscular frill or
velum.]
All the specimens were males, and the puzzle was to find out how it
reproduced itself. After a few seasons had passed I determined to solve
this problem. I made the guess that perhaps the jelly-fish were budded
off from a fixed weed-like polyp growing in the depths of the tank—as
is the case with many of the marine jelly-fishes. I remember that one
leading member of the council, which still presides over the destinies
of the Botanic Gardens, confided to me in a hushed whisper his belief
that Providence created this new jelly-fish year by year in the tank in
honour of the august patroness of the Botanic Society—Her Royal
Highness the Duchess of Teck. I was obliged to make an end of this
flattering theory when I discovered, after long searching with my
assistant—attached to the rootlets of floating water weeds a minute
three-branched polyp (Fig. 4), from which, as we subsequently were able
to observe, the jelly-fish were pinched off as tiny spheres about
one-sixteenth of an inch in diameter. No females of this jelly-fish were
ever discovered. The polyps lived on from year to year, and budded off
each season a swarm of pretty but futile male jelly-fish. They ripened
and died on attaining a diameter somewhat less than that of a shilling.
There were many most interesting points made out as to their structure,
mode of feeding, and growth. You could keep them in a tall glass jar
supported over a small gas-jet (they lived best at a temperature of 80°
Fahr.), and they would swim up by a series of strokes to the top of the
water, and then drop like little parachutes through the eighteen inches
of depth to the bottom—taking in water-fleas and such food on the
way—and immediately would start upwards again. I used to take them
alive in my pocket corked up in a test-tube to show to friends.
[Illustration: FIG. 4.—Four of the minute club-shaped polyps adhering
to a root-fibre of a water-plant. The rounded end becomes nipped off and
swims away, free, as a young jelly-fish.]
After they had disappeared from the tank in Regent’s Park (owing to some
unhappy cleaning of the tank) they suddenly, in 1903, appeared—it seems
incredible—at Sheffield! Then they briefly showed up in 1905 at Munich,
and at Lyons had been captured in 1901—always in a tepid water-lily
tank! We never could make out where they came from originally. Of
course, the polyp must have been brought into the tank with some bundle
of water plants from a tropical lake or river, but we never had any
indication as to when or which.
Since the days of the fresh-water jelly-fish of Regent’s Park, which was
called (a name, but why should it not have a name?) _Limnocodium
Sowerbii_—a jelly-fish of about the same size (Fig. 5) but very
different in shape and tentacles—was discovered in the great African
fresh-water lake Tanganyika—in enormous numbers, and was named
_Limnocnida Tanganyikæ_. Only five years ago the same jelly-fish was
discovered in the Victoria Nyanza, and a little earlier in backwaters of
the Niger. It is a curious and significant fact bearing upon the history
of these three areas of fresh water connected with the three greatest
African rivers—the Congo, the Nile, and the Niger—that the same little
jelly-fish is found in all of them.
[Illustration: FIG. 5.—The African fresh-water jelly-fish
(_Limnocnida_) found in Tanganyika, Victoria Nyanza, and the Niger.]
And now we have just been reminded of _Limnocodium_, the Regents Park
jelly-fish, from a remote and unexpected source. A thousand miles up
the Yang-tse-Kiang River, in China, in the province of Hupi, the
Japanese captain of a river steamer, plying there and belonging to a
Japanese company, captured ten jelly-fish in the muddy waters of the
river. He brought them home, preserved, I suppose, in alcohol or
formalin, and they have been described by Dr. Oka, a Japanese zoologist
of Tokio, in a publication bearing the Latin title _Annotationes
Zoologicæ niponenses_, issued in December 1907. European sea captains
have not rarely been ardent naturalists, but I think the Japanese is the
first captain of a river steamboat who has discovered a new animal on
his beat. I have not heard of Mississippi steamboat captains amusing
themselves in this way—other rivers, other tastes.
Dr. Oka describes the jelly-fish thus brought to him as a _Limnocodium_,
differing in a few details from that of Regent’s Park, so that he
distinguishes this Chinese species as _Limnocodium Kawaii_, naming it
after the naturalist captain, who must have a rare taste for picking up
strange and new things, and a rare goodwill in bringing them home with
him. So here is another fresh-water jelly-fish, for it is not the same
as the Regent’s Park one, though closely like it. Possibly _Limnocodium_
is an Asiatic genus, and the original Sowerby’s _Limnocodium_ will be
found in another Chinese river. But it may prove to be South American,
as is the water-lily Victoria regia.
A very small fresh-water jelly-fish was found some twelve years ago—in
1897—in the Delaware River at Philadelphia, United States, and was
lately described by the well-known naturalist, Mr. Potts. It was budded
off from a very minute polyp resembling that found in the Regent’s Park,
but the jelly-fish was totally different from _Limnocodium_. Only four
or five specimens of this jelly-fish have ever been seen, and the
Philadelphian naturalists ought certainly to look it up again.
An account of the Philadelphian jelly-fish and of other fresh-water
jelly-fishes, with illustrative plates, will be found in the _Quarterly
Journal of Microscopical Science_, 1906. Mr. Charles Boulenger has, in
the same Journal, 1908, described yet another fresh-water jelly-fish
from the Fayoum Lake in Egypt.
IX
THE STORY OF THE COMMON EEL
Though the Scotch Highlanders are said to have a profound objection to
eating eels on account of the resemblance of these fish to snakes (not a
very good reason, since the quality and not the shape of what one eats
is the important thing), yet eels have been a very popular delicacy in
England in past days. Eel-pie Island, at Richmond, is known to most
Londoners, and eel-pie shops were familiar in London less than a century
ago. A good Thames eel is still appreciated by the few people who
nowadays take some small amount of intelligent interest in what they
eat. Abroad, eels are still popular. Eel-traps are still worked in the
rivers. In such districts as the flat country, on the shores of the
Adriatic, near Venice, millions of young eels are annually “shepherded”
in lagoons and reservoirs, and reared to marketable size. The inland
eel-fisheries of Denmark and Germany are carefully regulated and
encouraged by the Government in those States.
The fact is that railways, ice-storage, and steam-trawling have, in
conjunction, revolutionised our habits in regard to the use of fish as a
daily article of diet. Fresh-water fish are now almost unknown as a
regular source of food in the British Islands. The splendid fish of the
North Sea, the Channel, and the Atlantic coast have pushed them out of
the market. Thirty-eight years ago, when I was a student in Leipzig and
Vienna, “baked carp” was the only fish to be had in the dining-rooms we
frequented. Once a week there were fresh haddock, for those who fancied
them, in the celebrated Auerbach’s Keller. Now the railway and packing
in ice have brought North Sea fish to the centre of Europe, and created
a taste for that excellent food. Even on the Mediterranean at Nice, I
lately saw North Sea turbot, soles, and haddock lying on the
marble-slabs in the fish market side by side with the handsome but small
bass, mullet, gurnards, and sea-bream of the local fishery, and the
carp, pike, trout, and eels of the fresh waters of the South of France.
Nevertheless the eel—the common fresh-water eel—is still valued on the
Continent, as is proved by the fact that the German Imperial Government
has recently sent an important official of the Fisheries Department to
Gloucester in order to make extensive purchases of the “elvers,” or
young eels which come up the river Severn in millions at this season.
The purpose of the German fisheries officials is to place many hundred
thousands of these young eels in German rivers which are not so well
supplied by natural immigration as is the Severn, and by so doing to
increase the supply of well-grown eels hereafter in the river fisheries
of North Germany.
This interesting practical attempt to increase the supply of eels in
Germany will be further appreciated when I relate what has been
discovered within the last twenty years as to the reproduction,
migrations, and habits of the common fresh-water eel. It has been known,
time out of mind, that in the early months of every year millions of
young eels a little over two inches in length, called “elvers” in
English and “civelles” in French, come up the estuaries of the rivers of
Europe in a dense body. They are so closely packed together as the
narrower parts of the stream are reached, that thousands may be taken
out of the water by merely dipping a bucket into the ranks of the
procession. I obtained a few thousand of these “elvers” lately from the
Severn and placed them on exhibition in the central court of the Natural
History Museum in London. The Anglo-Saxon name “eel-fare” is given to
this annual march or “swim” of the young eels from the sea to the fresh
waters.
Though riverside folk have never doubted that the elvers are young eels
which have been hatched from spawn deposited by parent eels in the sea,
and are “running up” to feed and grow to maturity in the rivers and
streams inland, yet country folk away from the big rivers have queer
notions as to the origin and breeding of eels. They catch large, plump
eels a couple of feet long in stagnant ponds hundreds of miles from the
sea, far from rivers, and more than a thousand feet above the sea-level.
They have no notion that those eels originally “ran up” as little eels
from the sea, nor that many of them make their way across wet grass and
by rain-filled ditches back to the rivers and to the sea when they are
seven years old. But that is now known to be the fact. Just as there are
fish, like the salmon, which “run down” to the sea to feed and grow big
and “run up” to breed in the small pools and rivulets far from the
river’s mouth, so there are other fishes, of which the eel is one, which
run up to feed and grow and run down to breed—that is to say, to
deposit and fertilise their eggs in the depths of the ocean.