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

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THE INCLINED PLANE OF ANIMAL BEHAVIOUR

Sec. 1

Before passing to a connected story of the gradual emergence of higher
and higher forms of life in the course of the successive ages--the
procession of life, as it may be called--it will be useful to consider
the evolution of animal behaviour.

Evolution of Mind

A human being begins as a microscopic fertilised egg-cell, within which
there is condensed the long result of time--Man's inheritance. The long
period of nine months before birth, with its intimate partnership
between mother and offspring, is passed as it were in sleep, and no one
can make any statement in regard to the mind of the unborn child. Even
after birth the dawn of mind is as slow as it is wonderful. To begin
with, there is in the ovum and early embryo no nervous system at all,
and it develops very gradually from simple beginnings. Yet as mentality
cannot come in from outside, we seem bound to conclude that the
potentiality of it--whatever that means--resides in the individual from
the very first. The particular kind of activity known to us as thinking,
feeling, and willing is the most intimate part of our experience, known
to us directly apart from our senses, and the possibility of that must
be implicit in the germ-cell just as the genius of Newton was implicit
in a very miserable specimen of an infant. Now what is true of the
individual is true also of the race--there is a gradual evolution of
that aspect of the living creature's activity which we call mind. We
cannot put our finger on any point and say: Before this stage there was
no mind. Indeed, many facts suggest the conclusion that wherever there
is life there is some degree of mind--even in the plants. Or it might be
more accurate to put the conclusion in another way, that the activity we
call life has always in some degree an inner or mental aspect.

[Illustration: OKAPI AND GIRAFFE

The Okapi is one of the great zooelogical discoveries. It gives a good
idea of what the Giraffe's ancestors were like. The Okapi was unknown
until discovered in 1900 by Sir Harry Johnston in Central Africa, where
these strange animals have probably lived in dense forests from time
immemorial.]

In another part of this book there is an account of the dawn of mind in
backboned animals; what we aim at here is an outline of what may be
called the inclined plane of animal behaviour.

A very simple animal accumulates a little store of potential energy, and
it proceeds to expend this, like an explosive, by acting on its
environment. It does so in a very characteristic self-preservative
fashion, so that it burns without being consumed and explodes without
being blown to bits. It is characteristic of the organism that it
remains a going concern for a longer or shorter period--its length of
life. Living creatures that expended their energy ineffectively or
self-destructively would be eliminated in the struggle for existence.
When a simple one-celled organism explores a corner of the field seen
under a microscope, behaving to all appearance very like a dog scouring
a field seen through a telescope, it seems permissible to think of
something corresponding to mental endeavour associated with its
activity. This impression is strengthened when an amoeba pursues
another amoeba, overtakes it, engulfs it, loses it, pursues it again,
recaptures it, and so on. What is quite certain is that the behaviour of
the animalcule is not like that of a potassium pill fizzing about in a
basin of water, nor like the lurching movements of a gun that has got
loose and "taken charge" on board ship. Another feature is that the
locomotor activity of an animalcule often shows a distinct
individuality: it may swim, for instance, in a loose spiral.

But there is another side to vital activity besides acting upon the
surrounding world; the living creature is acted on by influences from
without. The organism acts on its environment; that is the one side of
the shield: the environment acts upon the organism; that is the other
side. If we are to see life whole we must recognise these two sides of
what we call living, and it is missing an important part of the history
of animal life if we fail to see that evolution implies becoming more
advantageously sensitive to the environment, making more of its
influences, shutting out profitless stimuli, and opening more gateways
to knowledge. The bird's world is a larger and finer world than an
earthworm's; the world means more to the bird than to the worm.

The Trial and Error Method

Simple creatures act with a certain degree of spontaneity on their
environment, and they likewise react effectively to surrounding stimuli.
Animals come to have definite "answers back," sometimes several,
sometimes only one, as in the case of the Slipper Animalcule, which
reverses its cilia when it comes within the sphere of some disturbing
influence, retreats, and, turning upon itself tentatively, sets off
again in the same general direction as before, but at an angle to the
previous line. If it misses the disturbing influence, well and good; if
it strikes it again, the tactics are repeated until a satisfactory way
out is discovered or the stimulation proves fatal.

It may be said that the Slipper Animalcule has but one answer to every
question, but there are many Protozoa which have several enregistered
reactions. When there are alternative reactions which are tried one
after another, the animal is pursuing what is called the trial-and-error
method, and a higher note is struck.

There is an endeavour after satisfaction, and a trial of answers. When
the creature profits by experience to the extent of giving the right
answer first, there is the beginning of learning.

[Illustration: DIAGRAM OF A SIMPLE REFLEX ARC IN A BACKBONELESS ANIMAL
LIKE AN EARTHWORM

1. A sensory nerve-cell (S.C.) on the surface receives a stimulus.

2. The stimulus travels along the sensatory nerve-fibre (S.F.)

3. The sensory nerve-fibre branches in the nerve-cord.

4. Its branches come into close contact (SY^{1}) with those of an
associative or communicating nerve-cell (A.C.).

5. Other branches of the associative cell come into close contact
(SY^{2}) with the branches or dendrites of a motor nerve-cell (M.C.).

6. An impulse or command travels along the motor nerve-fibre or axis
cylinder of the motor nerve-cell.

7. The motor nerve-fibre ends on a muscle-fibre (M.F.) near the surface.
This moves and the reflex action is complete.]

[Illustration: _Photo: British Museum_ (_Natural History_).

THE YUCCA MOTH

The Yucca Moth, emerging from her cocoon, flies at night to a Yucca
flower and collects pollen from the stamens, holding a little ball of it
in her mouth-parts. She then visits another flower and lays an egg in
the seed-box. After this she applies the pollen to the tip of the
pistil, thus securing the fertilisation of the flower and the growth of
the ovules in the pod. Yucca flowers in Britain do not produce seeds
because there are no Yucca Moths.]

[Illustration: INCLINED PLANE OF ANIMAL BEHAVIOUR

Diagram illustrating animal behaviour. The main line represents the
general life of the creature. On the upper side are activities implying
initiative; on the lower side actions which are almost automatic.

_Upper Side._--I. Energetic actions. II. Simple tentatives. III.
Trial-and-error methods. IV. Non-intelligent experiments. V.
Experiential "learning." VI. Associative "learning." VII. Intelligent
behaviour. VIII. Rational conduct (man).

_Lower Side._--1. Reactions to environment. 2. Enregistered reactions.
3. Simple reflex actions. 4. Compound reflex actions. 5. Tropisms. 6.
Enregistered rhythms. 7. Simple instincts. 8. Chain instincts. 9.
Instinctive activities influenced by intelligence. 10. Subconscious
cerebration at a high level (man).]

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

VENUS' FLY-TRAP

One of the most remarkable plants in the world, which captures its prey
by means of a trap formed from part of its leaf. It has been induced to
snap at and hold a bristle. If an insect lighting on the leaf touches
one of six very sensitive hairs, which pull the trigger of the movement,
the two halves of the leaf close rapidly and the fringing teeth on the
margin interlock, preventing the insect's escape. Then follows an
exudation of digestive juice.]

[Illustration: _Reproduced by permission from "The Wonders of Instinct"
by J. H. Fabre._

A SPIDER SUNNING HER EGGS

A kind of spider, called Lycosa, lying head downwards at the edge of her
nest, and holding her silken cocoon--the bag containing the eggs--up
towards the sun in her hindmost pair of legs. This extraordinary
proceeding is believed to assist in the hatching.]

Reflex Actions

Among simple multicellular animals, such as sea-anemones, we find the
beginnings of reflex actions, and a considerable part of the behaviour
of the lower animals is reflex. That is to say, there are laid down in
the animal in the course of its development certain pre-arrangements of
nerve-cells and muscle-cells which secure that a fit and proper answer
is given to a frequently recurrent stimulus. An earthworm half out of
its burrow becomes aware of the light tread of a thrush's foot, and
jerks itself back into its hole before anyone can say "reflex action."
What is it that happens?

Certain sensory nerve-cells in the earthworm's skin are stimulated by
vibrations in the earth; the message travels down a sensory nerve-fibre
from each of the stimulated cells and enters the nerve-cord. The sensory
fibres come into vital connection with branches of intermediary,
associative, or communicating cells, which are likewise connected with
motor nerve-cells. To these the message is thus shunted. From the motor
nerve-cells an impulse or command travels by motor nerve-fibres, one
from each cell, to the muscles, which contract. If this took as long to
happen as it takes to describe, even in outline, it would not be of much
use to the earthworm. But the motor answer follows the sensory stimulus
almost instantaneously. The great advantage of establishing or
enregistering these reflex chains is that the answers are practically
ready-made or inborn, not requiring to be learned. It is not necessary
that the brain should be stimulated if there is a brain; nor does the
animal will to act, though in certain cases it may by means of higher
controlling nerve-centres keep the natural reflex response from being
given, as happens, for instance, when we control a cough or a sneeze on
some solemn occasion. The evolutionary method, if we may use the
expression, has been to enregister ready-made responses; and as we
ascend the animal kingdom, we find reflex actions becoming complicated
and often linked together, so that the occurrence of one pulls the
trigger of another, and so on in a chain. The behaviour of the
insectivorous plant called Venus's fly-trap when it shuts on an insect
is like a reflex action in an animal, but plants have no definite
nervous system.

What are Called Tropisms

A somewhat higher level on the inclined plane is illustrated by what are
called "tropisms," obligatory movements which the animal makes,
adjusting its whole body so that physiological equilibrium results in
relation to gravity, pressure, currents, moisture, heat, light,
electricity, and surfaces of contact. A moth is flying past a candle;
the eye next the light is more illumined than the other; a physiological
inequilibrium results, affecting nerve-cells and muscle-cells; the
outcome is that the moth automatically adjusts its flight so that both
eyes become equally illumined; in doing this it often flies into the
candle.

It may seem bad business that the moth should fly into the candle, but
the flame is an utterly artificial item in its environment to which no
one can expect it to be adapted. These tropisms play an important role
in animal behaviour.

Sec. 2

Instinctive Behaviour

On a higher level is instinctive behaviour, which reaches such
remarkable perfection in ants, bees, and wasps. In its typical
expression instinctive behaviour depends on inborn capacities; it does
not require to be learned; it is independent of practice or experience,
though it may be improved by both; it is shared equally by all members
of the species of the same sex (for the female's instincts are often
different from the male's); it refers to particular conditions of life
that are of vital importance, though they may occur only once in a
lifetime. The female Yucca Moth emerges from the cocoon when the Yucca
flower puts forth its bell-like blossoms. She flies to a flower,
collects some pollen from the stamens, kneads it into a pill-like ball,
and stows this away under her chin. She flies to an older Yucca flower
and lays her eggs in some of the ovules within the seed-box, but before
she does so she has to deposit on the stigma the ball of pollen. From
this the pollen-tubes grow down and the pollen-nucleus of a tube
fertilises the egg-cell in an ovule, so that the possible seeds become
real seeds, for it is only a fraction of them that the Yucca Moth has
destroyed by using them as cradles for her eggs. Now it is plain that
the Yucca Moth has no individual experience of Yucca flowers, yet she
secures the continuance of her race by a concatenation of actions which
form part of her instinctive repertory.

From a physiological point of view instinctive behaviour is like a chain
of compound reflex actions, but in some cases, at least, there is reason
to believe that the behaviour is suffused with awareness and backed by
endeavour. This is suggested in exceptional cases where the stereotyped
routine is departed from to meet exceptional conditions. It should also
be noted that just as ants, hive bees, and wasps exhibit in most cases
purely instinctive behaviour, but move on occasion on the main line of
trial and error or of experimental initiative, so among birds and
mammals the intelligent behaviour is sometimes replaced by instinctive
routine. Perhaps there is no instinctive behaviour without a spice of
intelligence, and no intelligent behaviour without an instinctive
element. The old view that instinctive behaviour was originally
intelligent, and that instinct is "lapsed intelligence," is a tempting
one, and is suggested by the way in which habitual intelligent actions
cease in the individual to require intelligent control, but it rests on
the unproved hypothesis that the acquisitions of the individual can be
entailed on the race. It is almost certain that instinct is on a line of
evolution quite different from intelligence, and that it is nearer to
the inborn inspirations of the calculating boy or the musical genius
than to the plodding methods of intelligent learning.

Animal Intelligence

The higher reaches of the inclined plane of behaviour show intelligence
in the strict sense. They include those kinds of behaviour which cannot
be described without the suggestion that the animal makes some sort of
perceptual inference, not only profiting by experience but learning by
ideas. Such intelligent actions show great individual variability; they
are plastic and adjustable in a manner rarely hinted at in connection
with instincts where routine cannot be departed from without the
creature being nonplussed; they are not bound up with particular
circumstances as instinctive actions are, but imply an appreciative
awareness of relations.

When there is an experimenting with general ideas, when there is
_conceptual_ as contrasted with _perceptual_ inference, we speak of
Reason, but there is no evidence of this below the level of man. It is
not, indeed, always that we can credit man with rational conduct, but he
has the possibility of it ever within his reach.

Animal instinct and intelligence will be illustrated in another part of
this work. We are here concerned simply with the general question of the
evolution of behaviour. There is a main line of tentative experimental
behaviour both below and above the level of intelligence, and it has
been part of the tactics of evolution to bring about the hereditary
enregistration of capacities of effective response, the advantages being
that the answers come more rapidly and that the creature is left free,
if it chooses, for higher adventures.

There is no doubt as to the big fact that in the course of evolution
animals have shown an increasing complexity and masterfulness of
behaviour, that they have become at once more controlled and more
definitely free agents, and that the inner aspect of the
behaviour--experimenting, learning, thinking, feeling, and willing--has
come to count for more and more.

Sec. 3

Evolution of Parental Care

Mammals furnish a crowning instance of a trend of evolution which
expresses itself at many levels--the tendency to bring forth the young
at a well-advanced stage and to an increase of parental care associated
with a decrease in the number of offspring. There is a British starfish
called _Luidia_ which has two hundred millions of eggs in a year, and
there are said to be several millions of eggs in conger-eels and some
other fishes. These illustrate the spawning method of solving the
problem of survival. Some animals are naturally prolific, and the number
of eggs which they sow broadcast in the waters allows for enormous
infantile mortality and obviates any necessity for parental care.

But some other creatures, by nature less prolific, have found an
entirely different solution of the problem. They practise parental care
and they secure survival with greatly economised reproduction. This is a
trend of evolution particularly characteristic of the higher animals. So
much so that Herbert Spencer formulated the generalisation that the size
and frequency of the animal family is inverse ratio to the degree of
evolution to which the animal has attained.

Now there are many different methods of parental care which secure the
safety of the young, and one of these is called viviparity. The young
ones are not liberated from the parent until they are relatively well
advanced and more or less able to look after themselves. This gives the
young a good send-off in life, and their chances of death are greatly
reduced. In other words, the animals that have varied in the direction
of economised reproduction may keep their foothold in the struggle for
existence if they have varied at the same time in the direction of
parental care. In other cases it may have worked the other way round.

In the interesting archaic animal called _Peripatus_, which has to face
a modern world too severe for it, one of the methods of meeting the
environing difficulties is the retention of the offspring for many
months within the mother, so that it is born a fully-formed creature.
There are only a few offspring at a time, and, although there are
exceptional cases like the summer green-flies, which are very prolific
though viviparous, the general rule is that viviparity is associated
with a very small family. The case of flowering plants stands by itself,
for although they illustrate a kind of viviparity, the seed being
embryos, an individual plant may have a large number of flowers and
therefore a huge family.

Viviparity naturally finds its best illustrations among terrestrial
animals, where the risks to the young life are many, and it finds its
climax among mammals.

Now it is an interesting fact that the three lowest mammals, the
Duckmole and two Spiny Ant-eaters, lay eggs, i.e. are oviparous; that
the Marsupials, on the next grade, bring forth their young, as it were,
prematurely, and in most cases stow them away in an external pouch;
while all the others--the Placentals--show a more prolonged ante-natal
life and an intimate partnership between the mother and the unborn
young.

Sec. 4

There is another way of looking at the sublime process of evolution. It
has implied a mastery of all the possible haunts of life; it has been a
progressive conquest of the environment.

1. It is highly probable that living organisms found their foothold in
the stimulating conditions of the shore of the sea--the shallow water,
brightly illumined, seaweed-growing shelf fringing the Continents. This
littoral zone was a propitious environment where sea and fresh water,
earth and air all meet, where there is stimulating change, abundant
oxygenation and a copious supply of nutritive material in what the
streams bring down and in the rich seaweed vegetation.

[Illustration: THE HOATZIN INHABITS BRITISH GUIANA

The newly hatched bird has claws on its thumb and first finger and so is
enabled to climb on the branches of trees with great dexterity until
such time as the wings are strong enough to sustain it in flight.]

[Illustration: _Photograph, from the British Museum (Natural History),
of a drawing by Mr. E. Wilson._

PERIPATUS

A widely distributed old-fashioned type of animal, somewhat like a
permanent caterpillar. It has affinities both with worms and with
insects. It has a velvety skin, minute diamond-like eyes, and short
stump-like legs. A defenceless, weaponless animal, it comes out at
night, and is said to capture small insects by squirting jets of slime
from its mouth.]

[Illustration: _Photo: W. S. Berridge, F.Z.S._

ROCK KANGAROO CARRYING ITS YOUNG IN A POUCH

The young are born so helpless that they cannot even suck. The mother
places them in the external pouch, and fitting their mouths on the teats
injects the milk. After a time the young ones go out and in as they
please.]

It is not an easy haunt of life, but none the worse for that, and it is
tenanted to-day by representatives of practically every class of animals
from infusorians to seashore birds and mammals.

The Cradle of the Open Sea

2. The open-sea or pelagic haunt includes all the brightly illumined
surface waters beyond the shallow water of the shore area.

It is perhaps the easiest of all the haunts of life, for there is no
crowding, there is considerable uniformity, and an abundance of food for
animals is afforded by the inexhaustible floating "sea-meadows" of
microscopic Algae. These are reincarnated in minute animals like the
open-sea crustaceans, which again are utilised by fishes, these in turn
making life possible for higher forms like carnivorous turtles and
toothed whales. It is quite possible that the open sea was the original
cradle of life and perhaps Professor Church is right in picturing a long
period of pelagic life before there was any sufficiently shallow water
to allow the floating plants to anchor. It is rather in favour of this
view that many shore animals such as crabs and starfishes, spend their
youthful stages in the relatively safe cradle of the open sea, and only
return to the more strenuous conditions of their birthplace after they
have gained considerable strength of body. It is probably safe to say
that the honour of being the original cradle of life lies between the
shore of the sea and the open sea.

The Great Deeps

3. A third haunt of life is the floor of the Deep Sea, the abyssal area,
which occupies more than a half of the surface of the globe. It is a
region of extreme cold--an eternal winter; of utter darkness--an eternal
night--relieved only by the fitful gleams of "phosphorescent" animals;
of enormous pressure--2-1/2 tons on the square inch at a depth of 2,500
fathoms; of profound calm, unbroken silence, immense monotony. And as
there are no plants in the great abysses, the animals must live on one
another, and, in the long run, on the rain of moribund animalcules which
sink from the surface through the miles of water. It seems a very
unpromising haunt of life, but it is abundantly tenanted, and it gives
us a glimpse of the insurgent nature of the living creature that the
difficulties of the Deep Sea should have been so effectively conquered.
It is probable that the colonising of the great abysses took place in
relatively recent times, for the fauna does not include many very
antique types. It is practically certain that the colonisation was due
to littoral animals which followed the food-debris, millennium after
millennium, further and further down the long slope from the shore.

The Freshwaters

4. A fourth haunt of life is that of the freshwaters, including river
and lake, pond and pool, swamp and marsh. It may have been colonised by
gradual migration up estuaries and rivers, or by more direct passage
from the seashore into the brackish swamp. Or it may have been in some
cases that partially landlocked corners of ancient seas became gradually
turned into freshwater basins. The animal population of the freshwaters
is very representative, and is diversely adapted to meet the
characteristic contingencies--the risk of being dried up, the risk of
being frozen hard in winter, and the risk of being left high and dry
after floods or of being swept down to the sea.

Conquest of the Dry Land

5. The terrestrial haunt has been invaded age after age by contingents
from the sea or from the freshwaters. We must recognise the worm
invasion, which led eventually to the making of the fertile soil, the
invasion due to air-breathing Arthropods, which led eventually to the
important linkage between flowers and their insect visitors, and the
invasion due to air-breathing Amphibians, which led eventually to the
higher terrestrial animals and to the development of intelligence and
family affection. Besides these three great invasions, there were minor
ones such as that leading to land-snails, for there has been a
widespread and persistent tendency among aquatic animals to try to
possess the dry land.

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