which fell on the farther side of a stream; his retriever tried to bring
over both at once, but could not succeed; she then, though never before
known to ruffle a feather, deliberately killed one, brought over the
other, and returned for the dead bird. Colonel Hutchinson relates that
two partridges were shot at once, one being killed, the other wounded;
the latter ran away and was caught by the retriever, who on her return
came across the dead bird: "She stopped, evidently greatly puzzled, and
after one or two trials, finding she could not take it up without
permitting the escape of the winged bird, she considered a moment, then
deliberately murdered it by giving it a severe crunch, and afterward
brought away both together. This was the only known instance of her
ever having wilfully injured any game." Here we have reason, though not
quite perfect, for the retriever might have brought the wounded bird
first and then returned for the dead one, as in the case of the two
wild-ducks. I give the above cases as resting on the evidence of two
independent witnesses and because in both instances the retrievers,
after deliberation, broke through a habit which is inherited by them
(that of not killing the game retrieved), and because they show how
strong their reasoning faculty must have been to overcome a fixed habit.
I will conclude by quoting a remark by the illustrious Humboldt. "The
muleteers in South America say, 'I will not give you the mule whose step
is easiest, but _la mas racional_--the one that reasons best;'" and, as
he adds, "this popular expression, dictated by long experience, combats
the system of animated machines better perhaps than all the arguments of
speculative philosophy." Nevertheless some writers even yet deny that
the higher animals possess a trace of reason; and they endeavour to
explain away, by what appears to be mere verbiage, all such facts as
those above given.
It has, I think, now been shown that man and the higher animals,
especially the Primates, have some few instincts in common. All have the
same senses, intuitions, and sensations--similar passions, affections,
and emotions, even the more complex ones, such as jealousy, suspicion,
emulation, gratitude and magnanimity; they practise deceit and are
revengeful; they are sometimes susceptible to ridicule, and even have a
sense of humour; they feel wonder and curiosity; they possess the same
faculties of imitation, attention, deliberation, choice, memory,
imagination, the association of ideas, and reason, though in very
different degrees.
FOOTNOTES:
[Footnote 57: From Chapter III of "The Descent of Man," 1871. All except
three of the author's foot-notes have been omitted.]
[Footnote 58: "The American Beaver and his Works," 1868.--Author's
note.]
[Footnote 59: All the following statements, given on the authority of
these two naturalists, are taken from Rengger's "Naturgesch. der
Säugethiere von Paraguay," 1830, s. 41-57, and from Brehm's
"Thierleben," B.i. s. 10-87.--Author's note.]
[Footnote 60: A critic, without any grounds ("Quarterly Review," July,
1871, p. 72), disputes the possibility of this act as described by
Brehm, for the sake of discrediting my work. Therefore I tried, and
found that I could readily seize with my own teeth the sharp little
claws of a kitten nearly five weeks old.--Author's note.]
THE IMPORTANCE OF DUST: A SOURCE OF BEAUTY AND ESSENTIAL TO LIFE[61]
ALFRED RUSSEL WALLACE
The majority of persons, if asked what were the uses of dust, would
reply that they did not know it had any, but they were sure it was a
great nuisance. It is true that dust, in our towns and in our houses, is
often not only a nuisance but a serious source of disease: while in many
countries it produces ophthalmia, often resulting in total blindness.
Dust, however, as it is usually perceived by us, is, like dirt, only
matter in the wrong place, and whatever injurious or disagreeable
effects it produces are largely due to our own dealings with nature. So
soon as we dispense with horsepower and adopt purely mechanical means of
traction and conveyance, we can almost wholly abolish disease-bearing
dust from our streets, and ultimately from all our highways; while
another kind of dust, that caused by the imperfect combustion of coal,
may be got rid of with equal facility so soon as we consider pure air,
sunlight, and natural beauty to be of more importance to the population
as a whole than are the prejudices or the vested interests of those who
produce the smoke.
But though we can thus minimize the dangers and the inconveniences
arising from the grosser forms of dust, we cannot wholly abolish it; and
it is, indeed, fortunate we cannot do so, since it has now been
discovered that it is to the presence of dust we owe much of the beauty,
and perhaps even the very habitability of the earth we live upon. Few
of the fairy tales of science are more marvelous than these recent
discoveries as to the varied effects and important uses of dust in the
economy of nature.
The question why the sky and the deep ocean are both blue did not much
concern the earlier physicists. It was thought to be the natural color
of pure air and water, so pale as not to be visible when small
quantities were seen, and only exhibiting its true tint when we looked
through great depth of atmosphere or of organic water. But this theory
did not explain the familiar facts of the gorgeous tints seen at sunset
and sunrise, not only in the atmosphere and on the clouds near the
horizon, but also in equally resplendent hues when the invisible sun
shines upon Alpine peaks and snowfields. A true theory should explain
all these colors, which comprise almost every tint of the rainbow.
The explanation was found through experiments on the visibility or
non-visibility of air, which were made by the late Professor Tyndall
about the year 1868. Everyone: has seen the floating dust in a sunbeam
when sunshine enters a partially darkened room; but it is not generally
known that if there was absolutely no dust in the air the path of the
sunbeam would be totally black and invisible, while if only very little
dust was present in very minute particles the air would be as blue as
the summer sky.
This was proved by passing a ray of electric light lengthways through a
long glass cylinder filled with air of varying degrees of purity as
regards dust. In the air of an ordinary room, however clean and well
ventilated, the interior of the, cylinder appears brilliantly
illuminated. But if the cylinder is exhausted and then filled with air
which is passed slowly through a fine gauze of intensely heated platinum
wire, so as to burn up all the floating dust particles, which are mainly
organic, the light will pass through the cylinder without illuminating
the interior, which, viewed laterally, will appear as if filled with a
dense black cloud. If, now, more air is passed into the cylinder
through the heated gauze, but so rapidly that the dust particles are not
wholly consumed, a slight blue haze will begin to appear, which will
gradually become a pure blue, equal to that of a summer sky. If more and
more dust particles are allowed to enter, the blue becomes paler, and
gradually changes to the colourless illumination of the ordinary air.
The explanation of these phenomena is that the number of dust particles
in ordinary air is so great that they reflect abundance of light of all
wave-lengths, and thus cause the interior of the vessel containing them
to appear illuminated with white light. The air which is passed slowly
over white-hot platinum has had the dust particles destroyed, thus
showing that they were almost wholly of organic origin, which is also
indicated by their extreme lightness, causing them to float permanently
in the atmosphere. The dust being thus got rid of, and pure air being
entirely transparent, there is nothing in the cylinder to reflect the
light, which is sent through its centre in a beam of parallel rays so
that none of it strikes against the sides; hence the inside of the
cylinder appears absolutely dark. But when the larger dust particles are
wholly or partially burnt, so that only the very smallest fragments
remain, a blue light appears, because these are so minute as to reflect
chiefly the more refrangible rays, which are of shorter
wave-length--those at the blue end of the spectrum--and which are thus
scattered in all directions, while the red and yellow rays pass straight
on as before.
We have seen that the air near the earth's surface is full of rather
coarse particles which reflect all the rays, and which therefore produce
no one colour. But higher up the particles necessarily become smaller
and smaller, since the comparatively rare atmosphere will support only
the very smallest and lightest. These exist throughout a great thickness
of air, perhaps from one mile to ten miles high or, even more, and blue
or violet rays being reflected from the innumerable particles in this
great mass of air, which is nearly uniform in all parts of the world as
regards the presence of minute dust particles, produces the constant and
nearly uniform tint we call sky-blue. A certain amount of white or
yellow light is no doubt reflected from the coarser dust in the lower
atmosphere, and slightly dilutes the blue and renders it not quite so
deep and pure as it otherwise would be. This is shown by the increasing
depth of the sky-colour when seen from the tops of lofty mountains,
while from the still greater heights attained in balloons the sky
appears of a blue-black colour, the blue reflected from the
comparatively small amount of dust particles being seen against the
intense black of stellar space. It is for the same reason that the
"Italian skies" are of so rich a blue, because the Mediterranean Sea on
one side and the snowy Alps on the other do not furnish so large a
quantity of atmospheric dust in the lower strata of air as in less
favorably situated countries, thus leaving the blue reflected by the
more uniformly distributed fine dust of the higher strata undiluted. But
these Mediterranean skies are surpassed by those of the central Pacific
ocean, where, owing to the small area of land, the lower atmosphere is
more free from coarse dust than in any other part of the world.
If we look at the sky on a perfectly fine summer's day, we shall find
that the blue colour is the most pure and intense overhead, and when
looking high up in a direction opposite to the sun. Near the horizon it
is always less bright, while in the region immediately around the sun it
is more or less yellow. The reason of this is that near the horizon we
look through a very great thickness of the lower atmosphere, which is
full of the larger dust particles reflecting white light, and this
dilutes the pure blue of the higher atmosphere seen beyond. And in the
vicinity of the sun a good deal of the blue light is reflected back into
space by the finer dust, thus giving a yellowish tinge to that which
reaches us reflected chiefly from the coarse dust of the lower
atmosphere. At sunset and sunrise, however, this last effect is greatly
intensified, owing to the great thickness of the strata of air through
which the light reaches us. The enormous amount of this dust is well
shown by the fact that, then only, we can look full at the sun, even
when the whole sky is free from clouds and there is no apparent mist.
But the sun's rays then reach us after having passed, first, through an
enormous thickness of the higher strata of the air, the minute dust of
which reflects most of the blue rays away from us, leaving the
complementary yellow light to pass on. Then, the somewhat coarser dust
reflects the green rays, leaving a more orange coloured light to pass
on; and finally some of the yellow is reflected, leaving almost pure
red. But owing to the constant presence of air currents, arranging both
the dust and vapour in strata of varying extent and density, and of high
or low clouds, which both absorb and reflect the light in varying
degrees, we see produced all those wondrous combinations of tints and
those gorgeous ever-changing colours, which are a constant source of
admiration and delight to all who have the advantage of an uninterrupted
view to the west, and who are accustomed to watch for these not
unfrequent exhibitions of nature's kaleidoscopic colour-painting. With
every change in the altitude of the sun the display changes its
character; and most of all when it has sunk below the horizon, and,
owing to the more favourable angles, a larger quantity of the coloured
light is reflected toward us. Especially is this the case when there is
a certain amount of cloud. The clouds, so long as the sun is above the
horizon, intercept much of the light and colour; but, when the great
luminary has passed away from our direct vision, his light shines more
directly on the under sides of all the clouds and air strata of
different densities; a new and more brilliant light flushes the western
sky, and a display of gorgeous ever-changing tints occurs which are at
once the delight of the beholder and the despair of the artist. And all
this unsurpassable glory we owe to--dust!
A remarkable confirmation of this theory was given during the two or
three years after the great eruption of Krakatoa, near Java. The
volcanic débris was shot up from the crater many miles high, and the
heavier portion of it fell upon the sea for several hundred miles
around, and was found to be mainly composed of very thin flakes of
volcanic glass. Much of this was of course ground to impalpable dust by
the violence of the discharge, and was carried up to a height of many
miles. Here it was caught by the return currents of air continually
flowing northward and southward above the equatorial zone; and since,
when these currents reach the temperate zone, where the surface rotation
of the earth is less rapid, they continually flow eastward, the fine
dust was thus carried at a great altitude completely around the earth.
Its effects were traced some months after the eruption in the appearance
of brilliant sunset glows of an exceptional character, often flushing
with crimson the whole western half of the visible sky. These glows
continued in diminishing splendour for about three years; they were seen
all over the temperate zone; and it was calculated that, before they
finally disappeared, some of this fine dust must have travelled three
times round the globe.
The same principle is thought to explain the exquisite blue colour of
the deep seas and oceans and of many lakes and springs. Absolutely pure
water, like pure air, is colourless, but all seas and lakes, however
clear and translucent, contain abundance of very finely divided matter,
organic or inorganic, which, as in the atmosphere, reflects the blue
rays in such quantity as to overpower the white or coloured
light-reflected from the fewer and more rapidly sinking particles of
larger size. The oceanic dust is derived from many sources. Minute
organisms are constantly dying near the surface, and their skeletons, or
fragments of them, fall slowly to the bottom. The mud brought down by
rivers, though it cannot be traced on the ocean floor more than about
150 miles from land, yet no doubt furnishes many particles of organic
matter which are carried by surface currents to enormous distances and
are ultimately dissolved before they reach the bottom. A more important
source of finely divided matter is to be found in volcanic dust which,
as in the case of Krakatoa, may remain for years in the atmosphere, but
which must ultimately fall upon the surface of the earth and ocean. This
can be traced in all the deep-sea oozes. Finally there is meteoric dust,
which is continually falling to the surface of the earth, but in such
minute quantities and in such a finely-divided state that it can be
detected only in the oozes of the deepest oceans, where both inorganic
and organic débris is almost absent.
The blue of the ocean varies in different parts from a pure blue
somewhat lighter than that of the sky, as seen about the northern tropic
in the Atlantic, to a deep indigo tint, as seen in the north temperate
portions of the same ocean: owing, probably, to differences in the
nature, quantity, and distribution of the solid matter which causes the
colour. The Mediterranean, and the deeper Swiss lakes, are also a blue
of various tints, due also to the presence of suspended matter, which
Professor Tyndall thought might be so fine that it would require ages of
quiet subsidence to reach the bottom. All the evidence goes to show,
therefore, that the exquisite blue tints of sky and ocean, as well as
all the sunset hues of sky and cloud, of mountain peak and Alpine snows,
are due to the finer particles of that very dust which, in its coarser
forms, we find so annoying and even dangerous.
But if this production of colour and beauty were the only useful
function of dust, some persons might be disposed to dispense with it in
order to escape its less agreeable effects. It has, however, been
recently discovered that dust has another part to play in nature; a part
so important that it is doubtful whether we could even live without it.
To the presence of dust in the higher atmosphere we owe the formation of
mists, clouds, and gentle beneficial rains, instead of water spouts and
destructive torrents.
It is barely twenty years ago since the discovery was made, first in
France by Coulier and Mascart, but more thoroughly worked out by Mr.
John Aitken in 1880. He found that if a jet of steam is admitted into
two large glass receivers,--one filled with ordinary air, the other with
air which has been filtered through cotton wool so as to keep back all
particles of solid matter,--the first will be instantly filled with
condensed vapour in the usual cloudy form, while the other vessel will
remain quite transparent. Another experiment was made, more nearly
reproducing what occurs in nature. Some water was placed in the two
vessels prepared as before. When the water had evaporated sufficiently
to saturate the air the vessels were slightly cooled; a dense cloud was
at once formed in the one while the other remained quite clear. These
experiments, and many others, show that the mere cooling of vapour in
air will not condense it into mist clouds or rain, unless _particles of
solid matter_ are present to form _nuclei_ upon which condensation can
begin. The density of the cloud is proportionate to the number of the
particles; hence the fact that the steam issuing from the safety-valve
or the chimney of a locomotive forms a dense white cloud, shows that the
air is really full of dust particles, most of which are microscopic but
none the less serving as centres of condensation for the vapour. Hence,
if there were no dust in the air, escaping steam would remain invisible;
there would be no cloud in the sky; and the vapour in the atmosphere,
constantly accumulating through evaporation from seas and oceans and
from the earth's surface, would have to find some other means of
returning to its source.
One of these modes would be the deposition of dew, which is itself an
illustration of the principle that vapour requires solid or liquid
surfaces to condense upon; dew forms most readily and abundantly on
grass, on account of the numerous centres of condensation this affords.
Dew, however, is now formed only on clear cold nights after warm or
moist days. The air near the surface is warm and contains much vapour,
though below the point of saturation. But the innumerable points and
extensive surfaces of grass radiate heat quickly, and becoming cool,
lower the temperature of the adjacent air, which then reaches
saturation point and condenses the contained atmosphere on the grass.
Hence, if the atmosphere at the earth's surface became super-saturated
with aqueous vapour, dew would be continuously deposited, especially on
every form of vegetation, the result being that everything, including
our clothing, would be constantly dripping wet. If there were absolutely
no particles of solid matter in the upper atmosphere, all the moisture
would be returned to the earth in the form of dense mists, and frequent
and copious dews, which in forests would form torrents of rain by the
rapid condensation on the leaves. But if we suppose that solid particles
were occasionally carried higher up through violent winds or tornadoes,
then on those occasions the super-saturated atmosphere would condense
rapidly upon them, and while falling would gather almost all the
moisture in the atmosphere in that locality, resulting in masses or
sheets of water, which would be so ruinously destructive by the mere
weight and impetus of their fall that it is doubtful whether they would
not render the earth almost wholly uninhabitable.
The chief mode of discharging the atmospheric vapour in the absence of
dust would, however, be by contact with the higher slopes of all
mountain ranges. Atmospheric vapour, being lighter than air, would
accumulate in enormous quantities in the upper strata of the atmosphere,
which would be always super-saturated and ready to condense upon any
solid or liquid surfaces. But the quantity of land comprised in the
upper half of all the mountains of the world is a very small fraction of
the total surface of the globe, and this would lead to very disastrous
results. The air in contact with the higher mountain slopes would
rapidly discharge its water, which would run down the mountain sides in
torrents. This condensation on every side of the mountains would leave a
partial vacuum which would set up currents from every direction to
restore the equilibrium, thus bringing in more super-saturated air to
suffer condensation and add its supply of water, again increasing the
in-draught of more air. The result would be that winds would be
constantly blowing toward every mountain range from all directions,
keeping up the condensation and discharging, day and night and from one
year's end to another, an amount of water equal to that which falls
during the heaviest tropical rains. All of the rain that now falls over
the whole surface of the earth and ocean, with the exception of a few
desert areas, would then fall only on rather high mountains or steep
isolated hills, tearing down their sides in huge torrents, cutting deep
ravines, and rendering all growth of vegetation impossible. The
mountains would therefore be so devastated as to be uninhabitable, and
would be equally incapable of supporting either vegetable or animal
life.
But this constant condensation on the mountains would probably check the
deposit on the lowlands in the form of dew, because the continual
up-draught toward the higher slopes would withdraw almost the whole of
the vapour as it arose from the oceans, and other water-surfaces, and
thus leave the lower strata over the plains almost or quite dry. And if
this were the case there would be no vegetation, and therefore no animal
life, on the plains and lowlands, which would thus be all arid deserts
cut through by the great rivers formed by the meeting together of the
innumerable torrents from the mountains.
Now, although it may not be possible to determine with perfect accuracy
what would happen under the supposed condition of the atmosphere, it is
certain that the total absence of dust would so fundamentally change the
meteorology of our globe as, not improbably, to render it uninhabitable
by man, and equally unsuitable for the larger portion of its existing
animal and vegetable life.
Let us now briefly summarise what we owe to the universality of dust,
and especially to that most finely divided portion of it which is
constantly present in the atmosphere up to the height of many miles.
First of all it gives us the pure blue of the sky, one of the most
exquisitely beautiful colours in nature. It gives us also the glories
of the sunset and the sunrise, and all those brilliant hues seen in high
mountain regions. Half the beauty of the world would vanish with the
absence of dust. But, what is far more important than the colour of sky
and beauty of sunset, dust gives us also diffused daylight, or skylight,
that most equable, and soothing, and useful, of all illuminating
agencies. Without dust the sky would appear absolutely black, and the
stars would be visible even at noonday. The sky itself would therefore
give us no light. We should have bright glaring sunlight or intensely
dark shadows, with hardly any half-tones. From this cause alone the
world would be so totally different from what it is that all vegetable
and animal life would probably have developed into very different forms,
and even our own organisation would have been modified in order that we
might enjoy life in a world of such harsh and violent contrasts.