The Outline of Science, Vol. 1 (of 4)

J >> J. Arthur Thomson >> The Outline of Science, Vol. 1 (of 4)

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This is an interesting minor chapter in the story of evolution--the
establishment of different kinds of parasites, casual and constant,
temporary and lifelong, external hangers-on and internal unpaying
boarders, those that live in the food-canal and depend on the host's
food and those that inhabit the blood or the tissues and find their food
there. It seems clear that ichneumon grubs and the like which hatch
inside a caterpillar and eat it alive are not so much parasites as
"beasts of prey" working from within.

But there are two sides to this minor chapter: there is the evolution of
the parasite, and there is also the evolution of counteractive measures
on the part of the host. Thus there is the maintenance of a bodyguard of
wandering amoeboid cells, which tackle the microbes invading the body
and often succeed in overpowering and digesting them. Thus, again, there
is the protective capacity the blood has of making antagonistic
substances or "anti-bodies" which counteract poisons, including the
poisons which the intruding parasites often make.

THE EVIDENCES OF EVOLUTION--HOW IT CAME ABOUT

Sec. 1

Progress in Evolution

There has often been slipping back and degeneracy in the course of
evolution, but the big fact is that there has been progress. For
millions of years Life has been slowly creeping upwards, and if we
compare the highest animals--Birds and Mammals--with their predecessors,
we must admit that they are more controlled, more masters of their
fate, with more mentality. Evolution is on the whole _integrative_; that
is to say, it makes against instability and disorder, and towards
harmony and progress. Even in the rise of Birds and Mammals we can
discern that the evolutionary process was making towards a fuller
embodiment or expression of what Man values most--control, freedom,
understanding, and love. The advance of animal life through the ages has
been chequered, but on the whole it has been an advance towards
increasing fullness, freedom, and fitness of life. In the study of this
advance--the central fact of Organic Evolution--there is assuredly much
for Man's instruction and much for his encouragement.

Evidences of Evolution

In all this, it may be said, the fact of evolution has been taken for
granted, but what are the evidences? Perhaps it should be frankly
answered that the idea of evolution, that the present is the child of
the past and the parent of the future, cannot be _proved_ as one may
prove the Law of Gravitation. All that can be done is to show that it is
a key--a way of looking at things--that fits the facts. There is no lock
that it does not open.

But if the facts that the evolution theory vividly interprets be called
the evidences of its validity, there is no lack of them. There is
_historical_ evidence; and what is more eloquent than the general fact
that fishes emerge before amphibians, and these before reptiles, and
these before birds, and so on? There are wonderfully complete fossil
series, e.g. among cuttlefishes, in which we can almost see evolution in
process. The pedigree of horse and elephant and crocodile is in general
very convincing, though it is to be confessed that there are other cases
in regard to which we have no light. Who can tell, for instance, how
Vertebrates arose or from what origin?

There is _embryological_ evidence, for the individual development often
reads like an abbreviated recapitulation of the presumed evolution of
the race. The mammal's visceral clefts are tell-tale evidence of remote
aquatic ancestors, breathing by gills. Something is known in regard to
the historical evolution of antlers in bygone ages; the Red Deer of
to-day recapitulates at least the general outlines of the history. The
individual development of an asymmetrical flat-fish, like a plaice or
sole, which rests and swims on one side, tells us plainly that its
ancestors were symmetrical fishes.

There is what might be called _physiological_ evidence, for many plants
and animals are variable before our eyes, and evolution is going on
around us to-day. This is familiarly seen among domesticated animals and
cultivated plants, but there is abundant flux in Wild Nature. It need
hardly be said that some organisms are very conservative, and that
change need not be expected when a position of stable equilibrium has
been secured.

There is also _anatomical_ evidence of a most convincing quality. In the
fore-limbs of backboned animals, say, the paddle of a turtle, the wing
of a bird, the flipper of a whale, the fore-leg of a horse, and the arm
of a man; the same essential bones and muscles are used to such diverse
results! What could it mean save blood relationship? And as to the two
sets of teeth in whalebone whales, which never even cut the gum, is
there any alternative but to regard them as relics of useful teeth which
ancestral forms possessed? In short, the evolution theory is justified
by the way in which it works.

Sec. 2

Factors in Evolution

If it be said "So much for the _fact_ of evolution, but what of the
_factors_?" the answer is not easy. For not only is the problem the
greatest of all scientific problems, but the inquiry is still very
young. The scientific study of evolution practically dates from the
publication of _The Origin of Species_ in 1859.

Heritable novelties or variations often crop up in living creatures, and
these form the raw material of evolution. These variations are the
outcome of expression of changes in the germ-cells that develop into
organisms. But why should there be changes in the constitution of the
germ-cells? Perhaps because the living material is very complex and
inherently liable to change; perhaps because it is the vehicle of a
multitude of hereditary items among which there are very likely to be
reshufflings or rearrangements; perhaps because the germ-cells have very
changeful surroundings (the blood, the body-cavity fluid, the
sea-water); perhaps because deeply saturating outside influences, such
as change of climate and habitat, penetrate through the body to its
germ-cells and provoke them to vary. But we must be patient with the
wearisome reiteration of "perhaps." Moreover, every many-celled organism
reproduced in the usual way, arises from an egg-cell fertilised by a
sperm-cell, and the changes involved in and preparatory to this
fertilisation may make new permutations and combinations of the living
items and hereditary qualities not only possible but necessary. It is
something like shuffling a pack of cards, but the cards are living. As
to the changes wrought on the body during its lifetime by peculiarities
in nurture, habits, and surroundings, these dents or modifications are
often very important for the individual, but it does not follow that
they are directly important for the race, since it is not certain that
they are transmissible.

Given a crop of variations or new departures or mutations, whatever the
inborn novelties may be called, we have then to inquire how these are
sifted. The sifting, which means the elimination of the relatively less
fit variations and the selection of the relatively more fit, effected in
many different ways in the course of the struggle for existence. The
organism plays its new card in the game of life, and the consequences
may determine survival. The relatively less fit to given conditions
will tend to be eliminated, while the relatively more fit will tend to
survive. If the variations are hereditary and reappear, perhaps
increased in amount, generation after generation, and if the process of
sifting continue consistently, the result will be the evolution of the
species. The sifting process may be helped by various forms of
"isolation" which lessen the range of free intercrossing between members
of a species, e.g. by geographical barriers. Interbreeding of similar
forms tends to make a stable stock; out-breeding among dissimilars tends
to promote variability. But for an outline like this it is enough to
suggest the general method of organic evolution: Throughout the ages
organisms have been making tentatives--new departures of varying
magnitude--and these tentatives have been tested. The method is that of
testing all things and holding fast that which is good.

BIBLIOGRAPHY

(The following short list may be useful to readers who desire to have
further books recommended to them.)

CLODD, _Story of Creation: A Plain Account of Evolution._
DARWIN, _Origin of Species, Descent of Man._
DEPERET, _Transformation of the Animal World_ (Internat. Sci. Series).
GEDDES AND THOMSON, _Evolution_ (Home University Library).
GOODRICH, _Evolution_ (The People's Books).
HEADLEY, _Life and Evolution._
HUTCHINSON, H. NEVILLE, _Extinct Monsters_ (1892).
LULL, _Organic Evolution._
MCCABE, _A B C of Evolution._
METCALF, _Outline of the Theory of Organic Evolution._
OSBORN, H. F., _The Evolution of Life_ (1921).
THOMSON, _Darwinism and Human Life._
WALLACE, _Darwinism._

III

ADAPTATIONS TO ENVIRONMENT

ADAPTATIONS TO ENVIRONMENT

We saw in a previous chapter how the process of evolution led to a
mastery of all the haunts of life. But it is necessary to return to
these haunts or homes of animals in some detail, so as to understand the
peculiar circumstances of each, and to see how in the course of ages of
struggle all sorts of self-preserving and race-continuing adaptations or
fitnesses have been wrought out and firmly established. Living creatures
have spread over all the earth and in the waters under the earth; some
of them have conquered the underground world and others the air. It is
possible, however, as has been indicated, to distinguish six great
haunts of life, each tenanted by a distinctive fauna, namely, the shore
of the sea, the open sea, the depths of the sea, the freshwaters, the
dry land, and the air. In the deep sea there are no plants at all; in
the air the only plants are floating bacteria, though there is a sense
in which a tree is very aerial, and the orchid perched on its branches
still more so; in the other four haunts there is a flora as well as a
fauna--the two working into one another's hands in interesting and often
subtle inter-relations--the subject of a separate study.

I. THE SHORE OF THE SEA

The Seaweed Area

By the shore of the sea the zoologist means much more than the narrow
zone between tide-marks; he means the whole of the relatively shallow,
well-illumined, seaweed-growing shelf around the continents and
continental islands. Technically, this is called the littoral area, and
it is divisible into zones, each with its characteristic population. It
may be noted that the green seaweeds are highest up on the shore; the
brown ones come next; the beautiful red ones are lowest. All of them
have got green chlorophyll, which enables them to utilise the sun's rays
in photosynthesis (i.e. building up carbon compounds from air, water,
and salts), but in the brown and red seaweeds the green pigment is
masked by others. It is maintained by some botanists that these other
pigments enable their possessors to make more of the scantier light in
the deeper waters. However this may be, we must always think of the
shore-haunt as the seaweed-growing area. Directly and indirectly the
life of the shore animals is closely wrapped up with the seaweeds, which
afford food and foothold, and temper the force of the waves. The minute
fragments broken off from seaweeds and from the sea-grass (a flowering
plant called Zostera) form a sort of nutritive sea-dust which is swept
slowly down the slope from the shore, to form a very useful deposit in
the quietness of deepish water. It is often found in the stomachs of
marine animals living a long way offshore.

Conditions of Shore Life

The littoral area as defined is not a large haunt of life; it occupies
only about 9 million square miles, a small fraction of the 197,000,000
of the whole earth's surface. But it is a very long haunt, some 150,000
miles, winding in and out by bay and fiord, estuary and creek. Where
deep water comes close to cliffs there may be no shore at all; in other
places the relatively shallow water, with seaweeds growing over the
bottom, may extend outwards for miles. The nature of the shore varies
greatly according to the nature of the rocks, according to what the
streams bring down from inland, and according to the jetsam that is
brought in by the tides. The shore is a changeful place; there is, in
the upper reaches, a striking difference between "tide in" and "tide
out"; there are vicissitudes due to storms, to freshwater floods, to
wind-blown sand, and to slow changes of level, up and down. The shore is
a very crowded haunt, for it is comparatively narrow, and every niche
among the rocks may be precious.

[Illustration: AN EIGHT-ARMED CUTTLEFISH OR OCTOPUS ATTACKING A SMALL
CRAB

These molluscs are particularly fond of crustaceans, which they crunch
with their parrot's beak-like jaws. Their salivary juice has a
paralysing effect on their prey. To one side, below the eye, may be seen
the funnel through which water is very forcibly ejected in the process
of locomotion.]

[Illustration: A COMMON STARFISH, WHICH HAS LOST THREE ARMS AND IS
REGROWING THEM

The lowest arm is being regrown double.

(_After Professor W. C. McIntosh._)]

[Illustration: A PHOTOGRAPH SHOWING A STARFISH (_Asterias Forreri_)
WHICH HAS CAPTURED A LARGE FISH

The suctorial tube-feet are seen gripping the fish firmly. (After an
observation on the Californian coast.)]

[Illustration: _Photo: J. J. Ward, F.E.S._

THE PAPER NAUTILUS (ARGONAUTA), AN ANIMAL OF THE OPEN SEA

The delicate shell is made by the female only, and is used as a shelter
for the eggs and young ones. It is secreted by two of the arms, not by
the mantle as other mollusc shells are. It is a single-chambered shell,
very different from that of the Pearly Nautilus.]

Keen Struggle for Existence

It follows that the shore must be the scene of a keen struggle for
existence--which includes all the answers-back that living creatures
make to environing difficulties and limitations. There is struggle for
food, accentuated by the fact that small items tend to be swept away by
the outgoing tide or to sink down the slope to deep water. Apart from
direct competition, e.g. between hungry hermit-crabs, it often involves
hard work to get a meal. This is true even of apparently sluggish
creatures. Thus the Crumb-of-Bread Sponge, or any other seashore sponge,
has to lash large quantities of water through the intricate canal system
of its body before it can get a sufficient supply of the microscopic
organisms and organic particles on which it feeds. An index of the
intensity of the struggle for food is afforded by the nutritive chains
which bind animals together. The shore is almost noisy with the
conjugation of the verb to eat in its many tenses. One pound of rock-cod
requires for its formation ten pounds of whelk; one pound of whelk
requires ten pounds of sea-worms; and one pound of worms requires ten
pounds of sea-dust. Such is the circulation of matter, ever passing from
one embodiment or incarnation to another.

Besides struggle for food there is struggle for foothold and for fresh
air, struggle against the scouring tide and against the pounding
breakers. The risk of dislodgment is often great and the fracture of
limbs is a common accident. Of kinds of armour--the sea-urchin's
hedgehog-like test, the crab's shard, the limpet's shell--there is great
variety, surpassed only by that of weapons--the sea-anemone's
stinging-cells, the sea-urchin's snapping-blades, the hermit-crab's
forceps, the grappling tentacles and parrot's-beak jaws of the octopus.

Shifts for a Living

We get another glimpse of the intensity of the seashore struggle for
existence in the frequency of "shifts for a living," adaptations of
structure or of behaviour which meet frequently recurrent vicissitudes.
The starfish is often in the dilemma of losing a limb or its life; by a
reflex action it jettisons the captured arm and escapes. And what is
lost is gradually regrown. The crab gets its leg broken past all
mending; it casts off the leg across a weak breakage plane near the
base, and within a preformed bandage which prevents bleeding a new leg
is formed in miniature. Such is the adaptive device--more reflex than
reflective--which is called self-mutilation or autotomy.

In another part of this book there is a discussion of camouflaging and
protective resemblance; how abundantly these are illustrated on the
shore! But there are other "shifts for a living." Some of the
sand-hoppers and their relatives illustrate the puzzling phenomenon of
"feigning death," becoming suddenly so motionless that they escape the
eyes of their enemies. Cuttlefishes, by discharging sepia from their
ink-bags, are able to throw dust in the eyes of their enemies. Some
undisguised shore-animals, e.g. crabs, are adepts in a hide-and-seek
game; some fishes, like the butterfish or gunnel, escape between stones
where there seemed no opening and are almost uncatchable in their
slipperiness. Subtlest of all, perhaps, is the habit some hermit-crabs
have of entering into mutually beneficial partnership (commensalism)
with sea-anemones, which mask their bearers and also serve as mounted
batteries, getting transport as their reward and likewise crumbs from
the frequently spread table. But enough has been said to show that the
shore-haunt exhibits an extraordinary variety of shifts for a living.

Parental Care on the Shore

According to Darwin, the struggle for existence, as a big fact in the
economy of Animate Nature, includes not only competition but all the
endeavours which secure the welfare of the offspring, and give them a
good send-off in life. So it is without a jolt that we pass from
struggle for food and foothold to parental care. The marine leech called
Pontobdella, an interesting greenish warty creature fond of fixing
itself to skate, places its egg-cocoons in the empty shell of a bivalve
mollusc, and guards them for weeks, removing any mud that might injure
their development. We have seen a British starfish with its fully-formed
young ones creeping about on its body, though the usual mode of
development for shore starfishes is that the young ones pass through a
free-swimming larval period in the open water. The father sea-spider
carries about the eggs attached to two of his limbs; the father
sea-horse puts his mate's eggs into his breast pocket and carries them
there in safety until they are hatched; the father stickleback of the
shore-pools makes a seaweed nest and guards the eggs which his wives are
induced to lay there; the father lumpsucker mounts guard over the bunch
of pinkish eggs which his mate has laid in a nook of a rocky shore-pool,
and drives off intruders with zest. He also aerates the developing eggs
by frequent paddling with his pectoral fins and tail, as the Scots name
Cock-paidle probably suggests. It is interesting that the salient
examples of parental care in the shore-haunt are mostly on the male
parent's side. But there is maternal virtue as well.

[Illustration: TEN-ARMED CUTTLEFISH OR SQUID IN THE ACT OF CAPTURING A
FISH

The arms bear numerous prehensile suckers, which grip the prey. In the
mouth there are strong jaws shaped like a parrot's beak. The
cuttlefishes are molluscs and may be regarded as the highest of the
backboneless or Invertebrate animals. Many occur near shore, others in
the open sea, and others in the great depths.]

[Illustration: GREENLAND WHALE

Showing the double blowhole or nostrils on the top of the head and the
whalebone plates hanging down from the roof of the mouth.]

[Illustration: MINUTE TRANSPARENT EARLY STAGE OF A SEA-CUCUMBER

It swims in the open sea by means of girdles of microscopic cilia shown
in the figure. After a period of free swimming and a remarkable
metamorphosis, the animal settles down on the floor of the sea in
relatively shallow water.]

[Illustration: _Photo: British Museum (Natural History)_

AN INTRICATE COLONY OF OPEN-SEA ANIMALS (_Physophora Hydrostatica_)
RELATED TO THE PORTUGUESE MAN-OF-WAR

There is great division of labor in the colony. At the top are floating
and swimming "persons"; the long ones below are offensive "persons"
bearing batteries of stinging cells; in the middle zone there are
nutritive, reproductive, and other "persons." The color of the colony is
a fine translucent blue. Swimmers and bathers are often badly stung by
this strange animal and its relatives.]

[Illustration: A SCENE IN THE GREAT DEPTHS

Showing a deep-sea fish of large gape, two feather-stars on the end of
long stalks, a "sea-spider" (or Pycnogon) walking on lanky legs on the
treacherous ooze, likewise a brittle-star, and some deep-sea corals.]

The fauna of the shore is remarkably _representative_--from unicellular
Protozoa to birds like the oyster-catcher and mammals like the seals.
Almost all the great groups of animals have apparently served an
apprenticeship in the shore-haunt, and since lessons learned for
millions of years sink in and become organically enregistered, it is
justifiable to look to the shore as a great school in which were gained
racial qualities of endurance, patience, and alertness.

II. THE OPEN SEA

In great contrast to the narrow, crowded, difficult conditions of the
shore-haunt (littoral area) are the spacious, bountiful, and relatively
easygoing conditions of the open sea (pelagic area), which means the
well-lighted surface waters quite away from land. Many small organisms
have their maximum abundance at about fifty fathoms, so that the word
"surface" is to be taken generously. The light becomes very dim at 250
fathoms, and the open sea, as a zoological haunt, stops with the light.
It is hardly necessary to say that the pelagic plants are more abundant
near the surface, and that below a certain depth the population consists
almost exclusively of animals. Not a few of the animals sink and rise in
the water periodically; there are some that come near the surface by
day, and others that come near the surface by night. Of great interest
is the habit of the extremely delicate Ctenophores or
"sea-gooseberries," which the splash of a wave would tear into shreds.
Whenever there is any hint of a storm they sink beyond its reach, and
the ocean's surface must have remained flat as a mirror for many hours
before they can be lured upwards from the calm of their deep retreat.

The Floating Sea-meadows

To understand the vital economy of the open sea, we must recognise the
incalculable abundance of minute unicellular plants, for they form the
fundamental food-supply. Along with these must also be included numerous
microscopic animals which have got possession of chlorophyll, or have
entered into internal partnership with unicellular Algae (symbiosis).
These green or greenish plants and animals are the _producers_, using
the energy of the sunlight to help them in building up carbon compounds
out of air, water, and salts. The animals which feed on the producers,
or on other animals, are the _consumers_. Between the two come those
open-sea bacteria that convert nitrogenous material, e.g. from dead
plants or animals that other bacteria have rotted, into forms, e.g.
nitrates, which plants can re-utilise. The importance of these
_middlemen_ is great in keeping "the circulation of matter" agoing.

[Illustration: 1. SEA-HORSE IN SARGASSO WEED. In its frond-like tags of
skin and in its colouring this kind of sea-horse is well concealed among
the floating seaweed of the so-called Sargasso Sea.

2. THE LARGE MARINE LAMPREYS (_PETROMYZON MARINUS_), WHICH MAY BE AS
LONG AS ONE'S ARM, SPAWN IN FRESH WATER. Stones and pebbles, gripped in
the suctorial mouth, are removed from a selected spot and piled around
the circumference, so that the eggs, which are laid within the circle,
are not easily washed away.

3. THE DEEP-SEA FISH _CHIASMODON NIGER_ IS FAMOUS FOR ITS VORACITY. It
sometimes manages to swallow a fish larger than itself, which causes an
extraordinary protrusion of the stomach.

4. DEEP-SEA FISHES. Two of them--_Melanocetus murrayi_ and _Melanocetus
indicus_--are related to the Angler of British coasts, but adapted to
life in the great abysses. They are very dark in colour, and delicately
built; they possess well-developed luminous organs. The third form is
called Chauliodus, a predatory animal with large gape and formidable
teeth.]

[Illustration: FLINTY SKELETON OF VENUS FLOWER BASKET (EUPLECTELLA), A
JAPANESE DEEP-SEA SPONGE]

[Illustration: EGG DEPOSITORY OF _Semotilus Atromaculatus_

In the building of this egg depository, the male fish takes stones from
the bottom of the stream, gripping them in his mouth, and heaps them up
into the dam. In the egg depository he arranges the stones so that when
the eggs are deposited in the interstices they are thoroughly protected,
and cannot be washed down-stream.

1, dam of stones; 2, egg depository; 3, hillock of sand. The arrow shows
the direction of the stream. Upper fish, male; lower, female.]

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