Part 4 (1/2)
GREEN HYDRA.
A little freshwater polyp, about half an inch long, with a crown of tentacles round the mouth. It is seen giving off a bud, a clear ill.u.s.tration of as.e.xual reproduction. When a tentacle touches some small organism the latter is paralysed and drawn into the mouth.]
[Ill.u.s.tration: Photo: J. J. Ward, F.E.S.
EARTHWORM.
Earthworms began the profitable habit of moving with one end of the body always in front, and from worms to man the great majority of animals have bilateral symmetry.]
[Ill.u.s.tration: DIAGRAM ILl.u.s.tRATING THE BEGINNING OF INDIVIDUAL LIFE.
1. An immature sperm-cell, with 4 chromosomes (nuclear bodies) represented as rods.
2. A mature sperm-cell, with 2 chromosomes.
3. An immature egg-cell, with 4 chromosomes represented as curved bodies.
4. A mature egg-cell, with 2 chromosomes.
5. The spermatozoon fertilises the ovum, introducing 2 chromosomes.
6. The fertilised ovum, with 4 chromosomes, 2 of paternal origin and 2 of maternal origin.
7. The chromosomes lie at the equator, and each is split longitudinally. The centrosome introduced by the spermatozoon has divided into two centrosomes, one at each pole of the nucleus. These play an important part in the division or segmentation of the egg.
8. The fertilised egg has divided into two cells. Each cell has 2 paternal and 2 maternal chromosomes.]
[Ill.u.s.tration: Reproduced from the Smithsonian Report, 1917.
GLa.s.s MODEL OF A SEA-ANEMONE.
A long tubular sea-anemone, with a fine crown of tentacles around the mouth. The suggestion of a flower is very obvious. By means of stinging la.s.soes on the tentacles minute animals on which it feeds are paralysed and captured for food.]
[Ill.u.s.tration: THIS DRAWING SHOWS THE EVOLUTION OF THE BRAIN FROM FISH TO MAN The Cerebrum, the seat of intelligence, increases in proportion to the other parts. In mammals it becomes more and more convoluted. The brain, which lies in one plane in fishes, becomes gradually curved on itself. In birds it is more curved than the drawing shows.]
-- 6.
Great Acquisitions.
In animals like sea-anemones and jellyfishes the general symmetry of the body is radial; that is to say, there is no right or left, and the body might be halved along many planes. It is a kind of symmetry well suited for sedentary or for drifting life. But worms began the profitable habit of moving with one end of the body always in front, and from worms to man the great majority of animals have bilateral symmetry. They have a right and a left side, and there is only one cut that halves the body. This kind of symmetry is suited for a more strenuous life than radial animals show; it is suited for pursuing food, for avoiding enemies, for chasing mates. And with the establishment of bilateral symmetry must be a.s.sociated the establishment of head-brains, the beginning of which is to be found in some simple worm-types.
Among the other great acquisitions gradually evolved we may notice: a well-developed head with sense-organs, the establishment of large internal surfaces such as the digestive and absorptive wall of the food-ca.n.a.l, the origin of quickly contracting striped muscle and of muscular appendages, the formation of blood as a distributing medium throughout the body, from which all the parts take what they need and to which they also contribute.
Another very important acquisition, almost confined (so far as is known) to backboned animals, was the evolution of what are called glands of internal secretion, such as the thyroid and the supra-renal. These manufacture subtle chemical substances which are distributed by the blood throughout the body, and have a manifold influence in regulating and harmonising the vital processes. Some of these chemical messengers are called hormones, which stimulate organs and tissues to greater activity; others are called chalones, which put on a brake. Some regulate growth and others rapidly alter the pressure and composition of the blood. Some of them call into active development certain parts of the body which have been, as it were, waiting for an appropriate trigger-pulling. Thus, at the proper time, the milk-glands of a mammalian mother are awakened from their dormancy. This very interesting outcome of evolution will be dealt with in another portion of this work.
THE INCLINED PLANE OF ANIMAL BEHAVIOUR.
-- 1.
Before pa.s.sing 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 partners.h.i.+p between mother and offspring, is pa.s.sed 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.
[Ill.u.s.tration: OKAPI AND GIRAFFE.
The Okapi is one of the great zoological 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 acc.u.mulates 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 fas.h.i.+on, 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 a.s.sociated 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 pota.s.sium 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 s.h.i.+p. 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 s.h.i.+eld: 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.
[Ill.u.s.tration: 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 a.s.sociative or communicating nerve-cell (A.C.).
5. Other branches of the a.s.sociative 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.]
[Ill.u.s.tration: Photo: British Museum (Natural History).
THE YUCCA MOTH.
The Yucca Moth, emerging from her coc.o.o.n, 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.]
[Ill.u.s.tration: INCLINED PLANE OF ANIMAL BEHAVIOUR.
Diagram ill.u.s.trating 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. a.s.sociative ”learning.” VII. Intelligent behaviour. VIII. Rational conduct (man).