history of the growth of the tadpole and its change into the young frog
or toad (as the case may be) do not, as a rule, know about the laying of
the eggs. In the early spring (end of March) the full-grown frogs and
toads which have passed the winter buried in holes and cracks in the
ground in a state of torpor wake up and make their way to neighbouring
good-sized ponds. In these the eggs are deposited. The male frogs wait
for the females whom they seize from behind, placing their arms under
hers and round the chest. They hold so firmly that nothing will persuade
them to let go. They often retain their hold for days or even weeks.
Sometimes by mistake they seize a fish and hold on securely to its
head—a fact which has led to the belief among country-folk that the
frog is an enemy of the carp, and tries to blind him by forcing his
hands into the carp’s eyes. At this season a frog will clasp your finger
or the handle of a stick so persistently that you can lift him out of
the water. A large pad of a black colour grows in the breeding-season on
the inside of the first finger of the frog’s hand, and is richly
supplied with nerves. It is this growth which is sensitive and when
touched sets up the cramp-like clasping action of the muscles of the
arms. The eggs are eventually squeezed from the female’s body, and are
fertilised by the spermatic fluid of the male as they pass into the
water. They are, when “laid,” covered with only a thin transparent layer
of albumen (or white of egg), and it is only after a few hours that this
imbibes water and swells up into a ball-like mass around each little
black egg.
Years ago I used to collect the spawning toads and frogs at Baden, near
Vienna, in order to observe (in the laboratory of the celebrated
microscopist, Professor Stricker, the most gifted of his day) the
earliest changes in the little black egg, the size of a rape-seed, which
follow upon fertilisation. Properly placed in a watch-glass full of
water under a low power of the microscope one little egg could be
watched for hours. If it had not been fertilised, nothing occurred. But
if it had been, then there were strange movements of its surface and a
puckering and sinking in along one definite line, coming and going, but
at last becoming well marked like a deep furrow. Without actually
splitting, the little sphere was divided by the cleft into two halves.
Then, at right angles to the first cleft, a second began to form, and so
on, until in the course of hours the sphere became divided on its
surface like a blackberry. The separate pieces thus marked out are the
first “cells,” or units, of living protoplasm of the young tadpole. They
continue to divide and to chemically convert the granular matter with
which they are charged into living material whilst the mass slowly, in
the course of days (taking up water for its increase in actual size),
becomes elongated, and shows the rudiments of head, eyes, ears, spinal
cord, and projecting tail. It is a fascinating task to watch this
gradual development—and a difficult, but necessary, one (which has now
been carried out in the minutest detail by patient students), to harden
with chemical solutions the growing embryos taken at successive stages,
to embed them in wax or paraffin (as Stricker was the first to do), and
to cut them into the finest slices, then to clarify these slices in
balsam-varnish, examine them with the microscope, and record and draw
every “cell,” every constituent unit, as they increase in number and
complication of arrangement. That wonderfully difficult feat has now
been carried out not only in the case of the frog and toad, but in the
case of hundreds of different kinds of animals of all sorts. Thus we
know the history of the growth from the egg in its minutest details in
every kind of animal—the “cell-lineage” of the tissues of the
full-grown animal traced back to the single original egg-cell.
The egg of animals is always originally a single “cell”—that is to say,
a minute corpuscle of slimy consistence, with a dense capsulated kernel
or “nucleus” within it. The kernel or nucleus divides into two, and the
cell itself divides; each of the daughter cells again divides, and so
the process continues, until thousands, and in larger animals millions,
of cells are the result, as the mass of cells takes up nourishment and
increases in volume. When (as is the case in many animals, _e.g._
starfishes, worms, and mammals) there is only a little granular
food-material mixed in with the protoplasm of the egg-cell, that cell is
of small size, only the one two-hundredth of an inch in diameter (see
Fig. 31). But in the frog there is much granular food-material, and the
egg-cell is distended to the size of a rape-seed. When there is still
more, as in the bird and many fishes, the egg-cell does not entirely
divide as it does in smaller eggs on commencing growth after
fertilisation. The protoplasm collects into a disc incompletely
separated from the food-material, and it is the disc only which divides
into two, four, eight, and ever so many more cells. Some of the cells
resulting from the division of the disc form the embryo’s body, and
others spread, as they multiply, all over the rest of the egg-ball from
its edges so as to enclose the granular food-material in a sac, called
the yelk sac. In the frog, on the contrary, the protoplasm does not
separate as a disc: the whole egg-cell or ball divides to form the
embryo-cells, and the food granules are included in the substance of the
dividing cells. “Growth from the egg” is a long story; we must revert
now to the tadpoles and their parents.
There is a tradition that Dr. Edwards, the father of Henri and
grandfather of Alphonse Milne Edwards, directors of the Natural History
Museum of Paris, kept some tadpoles in a sort of cage sunk in the Seine,
so that they could not come to the surface to breathe air nor escape on
to the land, and that they grew to be very big tadpoles, much larger
than the size at which tadpoles usually change into frogs. I tried to
repeat this experiment when I was a boy—without success—and I have
never heard of any one having succeeded with it.[4] It is not cited or
credited at the present day. But some thirty years ago it was discovered
that something of this kind happens in the case of the Mexican
salamander. The English “newts” and the so-called salamanders are
creatures of lizard-like shape, which are closely related to frogs and
toads. They lay eggs in the water, and the young are tadpoles, with
beautiful large plume-like gills on each side of the head. The tadpole
of the common English newt may either lose its gills and leave the water
in the summer, if it was hatched early in the season, or may remain
longer in the gilled condition, and grow to more than two inches in
length, if it was hatched late. In certain lakes in Mexico there is a
tadpole-like creature with gill-plumes, which grows to eight inches or
more in length, and becomes adult and breeds when in that condition. It
is known as the “axolotl,” and was considered to be a distinct kind of
gill-bearing adult tadpole-like animal similar to some few others which
are known (Siren and Necturus). When, however, they were brought to
Europe and kept in a cage with only a small provision of water, some of
these axolotls were found to leave the water, lose their gills, change
their colour and shape in several respects, and become, in fact,
transformed into a terrestrial salamander, of a kind already known in
North America. It was thus established that the axolotl of the Mexican
lake is nothing more nor less than the tadpole of a species of
salamander or newt, which has “given up” the habit of leaving the water,
and actually grows to full size, and lays its eggs without becoming
converted into a gill-less land-dwelling creature! The greatest interest
was excited forty years ago, when the discovery was made that, by
gradually drying up the water in which the axolotl is kept, it can be
induced to resume its transformation, and become changed into a
salamander. Thus, the notion of converting the tadpoles of the common
frog into very big tadpoles by preventing them from leaving the water,
seems not to have been an unreasonable one.
There are some very big kinds of tadpoles, which are the young of toads
of other kinds than our British species. In England we have only two
kinds of frogs—the common frog and the edible frog—and two kinds of
toads, the common toad and the natter-jack or crawling toad
(distinguished by the pale line along the middle of his back). But on
the Continent of Europe there are others besides those which we have.
There is the beautiful little green tree-frog, and there are the
fire-bellied toad, and the obstetric toad (the male of which carries the
eggs after they are laid, coiled in a string around his hind legs); and
then there is the little spur-heeled toad (_Pelobates fuscus_), which
smells like garlic, and is remarkable for having a broad, horny claw on
his heel. This toad is only about two inches and a half long (measured
from snout to vent) when full grown, but its tadpole often exceeds four
inches in length, and in rare cases attains the gigantic size of seven
inches, so that it actually shrinks in size when it ceases to be a
tadpole, and takes on the adult form. Many years ago I found some of
these huge tadpoles in a pond near Antwerp, and thought they must be a
realisation of Dr. Edwards’ experiment. They were enormous, and it was
only on bringing them home that I heard for the first time of the
spur-heeled toad and its gigantic tadpoles (Fig. 44 C).
[Illustration: FIG. 44.—Outline drawings of three European tadpoles of
the actual size of nature. A, a full-sized tadpole of the Common Frog,
_Rana temporaria_, one inch and three-tenths long. B, a fair-sized
tadpole of the Obstetric Toad, _Alytes obstetricans_, common near Paris,
two inches and four-fifths long. C, a tadpole of the Garlic Toad,
_Pelobates fuscus_, common in France, Belgium, and Germany, four inches
and a half long. Specimens of as much as seven inches in length have
been captured.]
Among frogs and toads from distant lands are some which bring forth
their young alive, the female retaining the eggs in her body instead of
laying them in water. The black-and-yellow salamander of Europe (which,
like the common toad, has a highly poisonous secretion in the skin)
retains its eggs inside its body until the tadpoles are well advanced in
development, when they pass from her—about seventy in number—into the
water. In the closely allied black Alpine salamander only two, out of
thirty or more eggs produced, develop. These two remain inside their
mother until they have ceased to have gills and have become terrestrial
air-breathing young salamanders like their mother. The Alpine salamander
lives where there are no pools suitable for the tadpoles, and so they
never enter the water, but remain inside the mother’s body. Some
experiments have recently been made with these two species of salamander
by varying the conditions as to moisture in which the young grow to
maturity, and results of considerable interest have been obtained. One
of the most curious arrangements in regard to the young is seen in the
Surinam toad, of which we had living specimens five or six years ago in
the London Zoological Gardens. In this toad the skin of the female’s
back becomes very soft and plastic at the breeding-season. As she lays
the eggs the male takes them one by one and presses them into the soft
skin of her back, into which they sink. The eggs are thus embedded
separately to the number of fifty or sixty, each in a little pit in the
mother’s back. They slowly develop, each in its “pit,” the orifice of
which is closed by a sort of lid. When the young have grown to the
condition of little toads, they push open the lids of the pits and swim
out of their mother’s back. Specimens of these toads, with the eggs and
young, in various stages, embedded in their mother’s back, are to be
seen in most museums of natural history. Toads and frogs catch their
prey by throwing forward the sticky tongue which is attached near the
front of the lower jaw, and so lick up their victim with startling
abruptness. The Cape frog of South Africa (_Xenopus_), like the Surinam
toad (_Pipa_), has no tongue, and is also remarkable for possessing
hard, pointed ends to its toes. It rarely, if ever, leaves the water.
FOOTNOTES
[4] I am told by Mr. Boulenger, of the Natural History Museum, who is
the greatest authority on these animals, that the explanation of this is
that unawares Dr. Edwards made use of the young tadpoles of the
obstetric toad (_Alytes_), which is very common near Paris, though it
does not occur in England. These tadpoles regularly grow to be three
inches and more in length (see Fig. 44 B). Dr. Edwards thought he had
used the tadpoles of the common frog, but had, by accident, got hold of
those of _Alytes_.
XXIII
ABOUT THE STARS
The young astronomer in _Two on a Tower_—that bitter-sweet story in
which our great novelist Hardy tells of the weird fascination with which
the study of the stars appeals to a sensitive nature, exclaims: “The
imaginary picture of the sky as the concavity of a dome whose base
extends from horizon to horizon of our earth, is grand, simply grand,
and I wish I had never got beyond looking at it in that way. But the
actual sky is a horror.” “There is,” he continues, “a size at which
dignity begins; further on there is a size at which grandeur begins;
further on there is a size at which solemnity begins; further on a size
at which awfulness begins; further on a size at which ghastliness
begins. That size faintly approaches the size of the stellar universe.”
“If you are cheerful and wish to remain so,” he concludes, “leave the
study of astronomy alone. Of all the sciences, it alone deserves the
character of the terrible. If, on the other hand, you are restless and
anxious about the future, study astronomy at once—your troubles will be
reduced amazingly. But your study will reduce them in a singular way, by
reducing the importance of everything, so that the science is still
terrible, even as a panacea.” The facts revealed by the study of
astronomy which have this feature of ghastliness and terror relate to
the enormous distances in space at which the stars are placed, and to
their enormous number.
One may sometimes see on the coast or in some marshland a “pile-driver”
at work. At a quarter of a mile distance you can see the great weight
hoisted up by cranks and chains above the “pile,” which stands upright
but not yet driven very far into the ground. You see the weight let go;
it drops vertically on to the pile, and you watch it rising some two or
three feet on its return journey upwards, when suddenly you hear the
sound of a sharp blow, and only after an effort realise that the sound
was made more than a second ago, and that the workmen have had time to
raise the weight 3 ft. before the sound travelled to you. Sound travels
less than a quarter of a mile in a second. Light also takes time to
travel, but it advances ever so much more quickly than sound, namely,
186,000 miles (and a bit more) in a second. It is, therefore, easy to
calculate the number of miles traversed by light in a minute or in a
year. There are thirty million seconds in a year. The light of the sun
takes eight minutes to reach the earth, so, instead of stating the
number of miles of this distance, we may say that the sun is eight
“light-minutes” distant from the earth (about 89,000,000 miles). This is
an enormous figure. The sun and his planets may be represented
proportionately by a golden ball a foot in diameter, and a number of
little spheres varying in size from that of a dried pea to a boy’s
marble, placed at distances from the golden ball varying from 50 ft. to
200 ft. Such a model is shown in the Museum of Practical Geology in
Jermyn Street, London. Minute and scattered far apart as the planets of
the solar system appear when thus represented, yet the solar system is a
compact little group when we come to consider the distance from it of
the other suns—the “fixed stars,” which exist literally in millions
beyond it. The nearest of these stars (its name is Alpha Centauri) is no
less than three light-years distant from us. A light-year is five and a
half billion (that is, five and a half million million) miles. The
nearest sun to us after our own sun is, therefore, about sixteen billion
miles away, and if its light were suddenly extinguished, we should not
know of its extinction for three years.
How many—we may well ask—how many of these fixed stars—suns like our
own—are there? Roughly speaking, we can see with the naked eye,
reckoning both the northern hemisphere and the southern together (for
the stars seen from the former are other than those seen from the
latter), about 8000. Not many after all, one is inclined to say. But
stop a minute and hear what the telescope reveals. With the best
telescope about one hundred million can be seen, less and less brilliant
and more difficult to see in proportion to their remoteness. And now we
go further even than that. For within the last thirty years the great
science of astronomy has been rejuvenated by the application of
photography to its task. The invention of the “dry” plate, a sensitive
photographic plate which does not spoil by prolonged exposure as the
“wet” plate does, enables the astronomer to keep his telescope fixed by
slow-moving clockwork on to a given region of the sky for four or five
hours or more, and the very faint stars, invisible by the aid of the
most powerful telescope—stars the light from which is so feeble that it
could not affect the plate in a few seconds or minutes, have time by the
continued action of their faint light to print themselves on the plate
and sign, as it were, a definite record of their existence for man to
see and measure, though they are themselves for ever invisible to his
eye. It is not possible to say how many may be recorded in this way by
photography; it depends on length of exposure. But some thousands of
millions of stars can certainly be so recorded. These “unnumbered hosts”
are of various degrees of brightness, and by methods which astronomers
have invented, but cannot be described here, it is actually known how
they differ in size from one another (many are far bigger than our sun),
and with some approach to certainty, how far off they are. Stars of
four, five, ten, and more “light-years” away from us are well known.
Astronomers actually estimate the decreasing abundance in space of stars
as one passes from a sphere or spatial envelope of fifty light-years’
distance to one of 250 light-years. Finally, reasons have been given of
late for considering many of the “photographic” stars to be at a
distance of 32,000 light-years. I will not produce the awful figure in
miles, but the reader can refer back to the number of billion miles in a
light-year! And what is beyond that? No one has seen, nor can any one
guess. We cannot imagine a limit to space; neither can we imagine
unending space dotted with an infinity of suns!
It is a legitimate and, indeed, a necessary inference, from what we know
of these millions of suns—intensely hot, light-giving spheres—that
they, too, like our own sun, are accompanied by much smaller bodies,
planets which circle round them, as our sun’s planets circle round him.
Those planets have cooled down, as have those of the solar system, and
so do not give out light. In any case, they are too small to be seen at
so vast a distance. It is, on the whole, probable that the changes on
some—indeed, many—of these planets have led to the production of
living material similar to, but not necessarily identical with, that on
this earth. It is, on the whole, more likely than not that there are
intelligent beings existing on the planets of thousands of suns
invisible to our eyes: suns revealed only by the print on a photographic
plate of their light, which has taken thousands of years to travel from
the regions of unseen obscurity to us. To have arrived by sober
observation and reasoning at this conception is, indeed, a tremendous
flight of human thought and ingenuity!
It is the courage, the audacity—one may almost call it the superhuman
calmness—of astronomers, in the face of this truly overwhelming
immensity—that not only redeems their study from the oppressive and
terrifying character with which it at first assails the human spirit,
but gives to their proceedings and discoveries, so far as the ordinary
man can follow them, an unequalled fascination. The daring, the
patience, the accuracy, and the supreme intellectual gifts of the great
astronomers rightly fill other men with pride in the fact that there are
human minds capable of revealing things of such stupendous vastness and
of indicating their order and relation to one another. It is a splendid
fact, and one which must give hope and courage to all men, that the
astronomer’s mind does not totter—it is equal to his task. Astronomers
are, in fact, triumphant: they are very far indeed from suffering from
the depression which Mr. Hardy’s young star-gazer experienced.
Among the many conclusions of astronomers as to the movements of the
“heavenly bodies” none is more strange and mysterious in its suggestion
than that recently arrived at to the effect that in all this vast array
of millions of stars, the limits of which we can neither discover nor
imagine, there are two huge streams moving in opposite directions, and
in one or other all the stars are involved. Whence do they start? Where
are they going? There is no answer. Another conclusion, which is arrived
at quite simply by the examination with the spectroscope of the light
coming from the star named Vega by astronomers, is that our sun and its
attendant planets are moving towards that star. It is true that it is
many billions of miles away from us, but we are rushing towards it
somewhat rapidly according to mundane notions—namely, at the rate of
nineteen miles a second! That, I think, is a fact likely to make the
sentimental young astronomer as miserable as any of the records of
immensity. In fact, the only comfort to be got in view of this fact is
in the enormous distances which separate us from other stars, and the
length of time which must elapse before any serious consequence can
ensue from this alarming career. And there is further the probability
that the general result of attractions and repulsions in the vast
roadway of space will, when the time comes, take us safely past Vega,
just as a motor-car passes safely through the traffic and obstructing
“refuges” and lamp-standards of the London streets as you recline in it,
abandoned to the natural forces described as “chauffeurs.”
The spectroscope has done no less than photography to reanimate the
study of astronomy. The fact is that, with these two helping means of
observation, it has become possible for the ordinary man to witness and
appreciate some of the discoveries of astronomers, though the true and
accurate handling of all that is revealed concerning the stars is
essentially a matter of measurement, and therefore only to be dealt with
strictly by mathematicians. The desire to obtain ever more and more
accurate measurement of the movement and the size of the heavenly bodies
is the mainspring of all astronomical discovery, and, indeed, the
attempt to gain more and more detailed measurement of the factors at
work is the motive—more or less immediate—of all accurate
investigation of nature. Recently the astronomers of the Royal
Observatory at Greenwich have photographed the new comet (the third of
1907) in a way in which no comet has ever been photographed before. On
many consecutive nights for several weeks they were at work
photographing it on the dry plate, at intervals of two or three hours,
and the pictures obtained (which I have seen at the rooms of the Royal
Astronomical Society) show the most wonderful changes of form of its
tail, so that they look more like the record of the changes of some
living creature than those of a heavenly body. Already, in October 1909,
Halley’s comet, which has been anxiously awaited, has been seen, though
it is not expected to be bright and visible to all until May 1910.
Comets are among the exceptional delights of the astronomer—that is to
say, big comets, for two or three small comets visible only by a
telescope or by photography turn up every year. Some comets are expected
visitors, others make their appearance quite casually, some because they
apparently have no regular period, some because that period is as yet
undiscovered. Edmund Halley was the first to discover the law of
movement of a comet and to predict the return in 1758 of that seen in
1682. He did not live to witness the verification of his prediction.
This comet, now called Halley’s comet, was, he conjectured, the same
which had appeared in 1531 and in 1607. His prediction of its return
proved to be a year out (owing to perturbations caused by Neptune and
Uranus, two planets undiscovered in his day), but it appeared in 1759,
and went round once again and reappeared in 1835, and now is eagerly
expected by astronomers to appear in full brilliancy in 1910. Its period
is about seventy-five or seventy-six years.
XXIV
COMETS