Principles of animal biology /

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Foreword

NCREASING recognition of the cultural value of biological study carries with it both dangers and opportunities. The chief danger is lest the discipline of older and more exact sciences like chemistry and physics may be sacrificed to make place for miscellanies of easily memorisable facts illustrating nothing which can be properly regarded as scientific generalisations. Biological works are still replete with statements referred to as “laws,” whereas they are in reality mnemonics. Because of the complexity of its subject-matter, biological study necessitates the apprehension of a great diversity of data, which have been collected and classified by laborious effort. It may happen in the early growth of any branch of knowledge that those on whom this task falls in time forget the wood for the trees or are even attracted to studying the subject because their preference for trees is greater than their interest in the wood itself. Hence zoological text books in the past have been largely filled with tedious descriptions of the architecture of the body based on dissection of dead specimens. They thus leave in the mind of the reader the impression that zoology is concerned with corpses rather than with animals, as the word implies. If zoology is handled as a cultural subject by those who are interested in its subject-matter in the same way as one is interested in stamp collecting, its cultural value will not be considerable. If it is taught by those who regard it as the experimental study of the properties of living matter, biology will not provide an easy substitute for physics and chemistry. On the contrary, it is well suited to stimulate an interest in vii

Viil FOREWORD these subjects, where such an interest does not previously exist.

An author who is rash enough to add to the number of existing text books might be expected to offer some explanation of the special features of his own contribution. A somewhat different plan is usually adopted in English and American manuals. Most English text books for first-year students of zoology in the University and higher forms of schools are based on the “‘ type system’ introduced by Thomas Henry Huxley. They tend to be packed almost exclusively with the same type of facts as the student encounters in his practical work and can find in his laboratory manual. It has been the tradition in British, though not in American, text books to describe the anatomy of a series of representative animals in great detail without any particular reference to the physiological significance of the parts described, or any recognition of the fact that anatomy was in its early stages an experimental science. In consequence the educational value of biological study as an introduction to scientific method is reduced to-a minimum. |

One illustration will suffice to make my meaning clear. The structural basis of nervous co-ordination is a schematisation known as the reflex arc. This, as indicated at length in Chapter III, is based on elaborate experimental analysis as well as on anatomical data. More than three centuries elapsed between the experiments of Vesalius—who first studied the properties of nerve—and the modern conception of the reflex arc as represented by a familiar diagram to be found in any zoological text book. Those who follow the type system slavishly leave the student with the impression that the reflex are is something which can be actually seen. This is not only regrettable because it is incorrect ; it is regrettable because an examination of the evidence in favour of modern views about the anatomical

FOREWORD bye basis of reflex action provides excellent material for illustrating the method ofan experimental science, and I doubt whether the elementary teaching of chemistry provides better opportunities of doing so. From an educational standpoint I regard it as axiomatic that it is of far greater importance that a student of biology should understand the reasoning which enables us to interpret the path of the nervous impulse in a simple reflex than that he or she should remember technical names for all the branches of the vagus nerve in the rabbit or dogfish. Under the “ type system ” the universities of England in particular have trained two generations of students who, for the most part, have no clear notion of what is implied by the “ nervous impulse ’’ when they have completed their graduate courses in zoology.

American zoologists have broken away from the severely architectural tradition of the Huxleyan type system. The greater influence of biology in American education may be partly duetothis. The presentation of biological information in the form of general ‘‘ topics,’’ as adopted in some American manuals and in one attractive text book recently published in Great Britain, is certainly calculated to enlist the interest of the student. On the other hand, it tends to vagueness and a lack of logical coherence. To make knowledge palatable is important. It is, however, important todomore. Asa training in scientific method a succession of logically disconnected themes dealing with different aspects of behaviour, separate systems of organs, economic applications, etc., is hardly more satisfactory than disconnected descriptions of the architecture of a series of separate animal “types.” I have ventured to add to the number of existing text books, because I believe that it is possible to present the necessary minimum of data included in an introduction to zoology without losing sight of the logical unity of biological science. Each chapter is based on a x FOREWORD question, and the question is one which naturally arises from the last issue previously dealt with.

I think that the logical background of a course of biology requires much more attention than that of a course in physics and chemistry. If badly taught, there is a real danger not merely of failure to promote the matter-of-fact attitude to life which biological study should instil, but of reinforcing unscientific habits of thought in a domain where detached and disciplined approach is harder to achieve than in physics and chemistry. At the outset the student should be made to realise that biological science, and in particular animal biology, differs from physics and chemistry only in its subject-matter. Its method, the method of science, is identical. That is to say, it treats of objects in the external world: it expresses the relation between them in the most economical way; and it makes no assumptions which involve ethical value. To interpret the phenomena of living matter in the most economical terms we are driven to ask whether the properties of living matter are not manifestations of the same general principles which have been established in the realm of inanimate nature. This is the ultimate goal of all biological enquiry. But living beings differ among themselves ; and biology has to take into account the relation of one type or species of living matter to another as well as the relation of living matter in general to dead matter. I have endeavoured to weave both objectives into a coherent account by adopting the historical approach, and I have done so in the belief that the student will not attain to a sense of the challenging quality of scientific thought, unless he realises the steps by which our present knowledge has been reached.

In Part I the elementary facts of the anatomy of the frog and man are arranged with reference to a continuous examination of the characteristics which distinguish living from non-living matter. Our studies of the machinery of

FOREWORD xi living matter in Chapter VII will lead us to a physical space-time picture of the process of hereditary transmission. In Part II we then turn to the opportunity of using this knowledge as a basis for the interpretation of the diverse manifestations of living matter. It is customary in zoological text books to describe seriatim the anatomy, development and fossil history of the various groups of the animal kingdom. Such a method is adequate for purposes of reference ; but the student who is attracted to the study of biology for the intellectual fascination of the problems it presents for solution will find in this method of presentation, if he has already outgrown the phase of stamp collecting, little to stimulate his interest or quicken his enthusiasm. By dealing with the historical development of the evolutionary theory as a preliminary to the discussion of the present status of the problem of species, I have made the attempt to incorporate a survey of the animal kingdom in which the facts are presented not in the form of a catalogue, but as footnotes to the logical construction of an important generalisation.

To the best of my knowledge this has not been previously attempted, and it is not my intention to underestimate the difficulty of the beginner confronted with so much new material. In order to minimise the tedium and _ bewilderment experienced by the student, especially one who has not studied Latin or Greek, in encountering for the first time a large number of new technical terms, I have adopted in my own teaching a practice which has proved eminently helpful. Each week during the first half of the year a dozen specimens are placed in the laboratory for identification. Hach is a duplicate of a specimen in the departmental museum, and the student is allowed to refer to the museum collection for phylum, class, order, genus and popular name of the specimen. In this way unfamiliarity with systematic nomenclature is overcome by the expenditure of a xii | FOREWORD minimum of effort before the theoretical survey of the animal kingdom is undertaken (Part II, Chapter XI). Generations of industrious collection and classification of facts elapsed before the problems with which modern biology is concerned could be formulated. Many of these facts have to be assimilated before the problems themselves can be clearly envisaged. I think it is now possible to present these facts in relation to the continuous development of the major problems. Zoology is emerging from a purely descriptive phase into one of experimental enquiry. This does not mean that we can dispense with all the facts that filled the text books of the past generation, but it does mean that the facts have to be presented in rather different perspective. Indeed, I have had the gratification of learning from my students that my lectures had helped them to realise that there really is some point in knowing about the bones of the skull. I confess that I had not reached this conclusion at the time when I could repeat all the foramina of the dog’s skull for the purpose of passing the London University Intermediate Science Examination. I am perfectly aware that I am not the first to write a text book with this end in view, and that there exist several introductory works in which the type system has been relegated to its proper place—the laboratory. Nevertheless, I entertain the hope that my own contribution to the reorientation of elementary biological instruction will be welcomed for the special feature to which I have alluded. This book has been written during my tenure of the chair in Zoology at the University of Cape Town, and in planning what I regard as an adequate introductory course in animal biology, I have been admittedly fortunate, in that Cape Town is the seat of a rich littoral fauna. Sea urchins with ripe eggs are obtainable throughout the academic year, so that at the beginning of his work in the laboratory each student in the first-year course carries out the procedure

FOREWORD Xill for artificial insemination, and himself witnesses the cleavage of the sea urchin’s egg up to the gastrula stage. Laboratories remote from the sea can obtain excellent series of slides of segmentation in the sea urchin from the laboratory of Dr. Sigmund (Stuttgart). I have added an appendix which embodies the course in practical work which accompanied these lectures in the University of Cape Town (during 1928). It runs closely parallel with the lectures, and includes many lecture bench demonstrations, a procedure usually adopted in chemical laboratories but rarely in zoology. In this way the student is given some first-hand acquaintance with the experimental principles of the subject without necessitating the extensive changes in laboratory equipment which class work in both dissection and experiment involves. In spite of the difference of approach, I have covered most of the ground which ordinarily comes into an introductory course, the chief omissions being the more polysyllabic aspects of embryology, and descriptive and purely economic zoology. It is reasonable to assume that for details the student has access to a dissecting manual such as that of Marshall and Hurst, or of Gilchrist and von Bonde.

In field-work classes I have always insisted that my own students should make the acquaintance of the common parasites of the types they study. But I regard the life histories of helminthine, insect and protozoan parasites, that fill up so much time in many elementary courses, as out of place. Economic zoology is a specialist subject and should be separated entirely from an introduction to the principles of biology. I do not claim that my own way of teaching is an ideal one, but I do venture to believe that at a time when methods of teaching zoology are undergoing considerable changes it is worth while that those of us who are trying out new methods should place them at the disposal of others. Many, Iam aware, will shake their heads at the introduction of so much new and controversial

X1V FOREWORD material into a book of so elementary a character. I have found in teaching that young people are not interested in a dead subject, but they are attracted to what is—like themselves—in a stage of rapid growth; and in my experience the surest way in which to make dull facts palatable is to make them topical. That perhaps is why newspapers are more popular than academic lectures.

To ensure a sound grasp of the principles of genetics, I believe that it is essential for the student to work out examples based on experimental data. A set of problems for solution in connection with the subject matter of Chapter VII are to be found at the end of the book.

To Mr. G. P. Wells, M.A., of University College, London, I must express my very warm thanks for undertaking to revise the manuscript of this book, and see it through the press.

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Contents

FOREWORD . . ° : : : ° . : : PART I THE NATURE OF LIVING MATTER CHAP.

I. THE ELEMENTS OF ANIMAL BIOLOGY . ; : s II. THE MACHINERY OF RESPONSE . 2 : : : III. THE MACHINERY OF CO-ORDINATION . : 2 : IV. ~ THE SOURCES OF ENERGY OF THE ANIMAL BODY . ; Vv. THE TRANSPORT OF MATERIALS IN THE ANIMAL BODY . VI. THE DEVELOPMENT OF A NEW ORGANISM . : : VII. THE MACHINERY OF INHERITANCE : : : :

PART II THE ORIGINS OF LIVING MATTER VIII. THE EVOLUTIONARY PROBLEM . : : : : IX. THE PRINCIPLE OF BIOGENESIS . : : : ;

CHAPTER I THE ELEMENTS OF ANIMAL BIOLOGY HE objects with which we are familiar in the world

around us may be classified in two main categories, those that we speak of as living matter, and those that we speak of as non-living matter. A piece of chalk or a motor bicycle are examples of non-living matter. A frog or a University Professor are examples of what is called living matter. Men and women are not such things as dreams are made of. They are things you can see and touch and handle just as truly as you see and handle and touch 4 piece of chalk. Biology is the attempt to apply the matter of fact method that is called science to the study of those objects in the external world which are denoted by the term living matter. Biology is sometimes defined as the study of Life; but it is well to remember that science is not the study of abstract nouns. This characteristic of dealing with facts rather than words differentiates the scientist from the metaphysician, and distinguishes the disciplined, precise and definite character of scientific study from vague, unprofitable and undisciplined discussion. Scientific method has the peculiarity that it preserves the utmost economy ; it dispenses with any assumptions that are unnecessary to the building up of those rules or “laws” which enable its votaries to prophesy correctly about the future, and thus achieve a consummation to which mankind has throughout history aspired with conspicuous lack of success when using other methods. Because economy of thought (William

4, THE ELEMENTS OF ANIMAL BIOLOGY [PT. I he of Occam’s principle : Entia non multiplicanda preter necessitatem) is rigorously maintained in any enquiry which is truly scientific, the ultimate goal of biological enquiry is to find generalisations common to the realms of living matter and non-living matter. So at the threshold of our studies we have to enquire into the nature of this conventional distinction. When in the laboratory we see a sheep’s heart beating three weeks after the owner is legally dead, we are forced to realise that the adjectives living and dead are used in newspapers and police court evidence in a somewhat loose and arbitrary sense wholly inadequate to a comprehensive treatment of the nature of life.

If we take a piece of chalk as an example of dead matter, it requires no elaborate experiment to show that, provided it is not subjected to great heat or mechanical pressure, it does not display any discernible changes in shape or colour or texture over long periods of time, nor does it increase in bulk or give rise to new pieces of chalk, if left to itself. An animal is different in all these respects. It is constantly changing in shape, position, colour, texture, etc.; we may say that it has greater reactivity. It is influenced by innumerable events to which the piece of chalk remains comparatively indifferent—light and darkness, warmth, gentle pressure, drought, mild electrical stimuli ; we may say that it has greater receptivity. Finally the living animal grows and produces other animals like itself; it has the property of reproduction. |

I. REACTIVITY

As the reader reaches this unusual word he may yawn. His face changes in shape. On completing this response, he (or she) may blush. The face changes in colour. As these words are being written the muscles of the writer’s arm are changing in shape, and the fingers are changing in position.

CH. I] REACTIVITY 5

Reactivity is the most characteristic feature of the kind of living matter which we call animals. Some artificial machines show considerable capacity for effecting changes and very powerful activity which a piece of chalk does not display. But, in general, animals show a greater variety of response than the machines which we do not call living. It is a matter of degree rather than kind.

When we study the reactions of an animal we find that they are for the most part confined to some definite portion of it. They are localised. Different portions or organs carry out different types of reaction—movement, emission of fluids, light production, etc. Below is a list of some characteristic reactions of animals and the organs which carry out these reactions. Such organs as the muscles and glands which carry out characteristic changes of behaviour are called collectively Effector organs. These organs will be studied in detail later, when the terms will be fully explained :

TYPE OF RESPONSE EFFECTOR ORGAN Movement (a) limb, etc. Striped 7 (6) gut (peristalsis) bladder (micturition) MES (c) heart beat Cardiac | Fluid Motion Roof of frog’s mouth Ciliated Epithelium Gills of clam, etc.

Secretion Flow of saliva Sweating Glands

Venoms, etc.

Colour Change

Chameleon’s skin, etc. Pigmentary Effectors Retinal pigment cells of (Chromatophores) Man Bioluminescence — Firefly Photogenic Organs

Glow-worm

6 THE ELEMENTS OF ANIMAL BIOLOGY [PEE i. RECEPTIVITY

When an animal responds in any of the characteristic ways which have been enumerated above, it can generally be found, if sufficient trouble is taken, that the response is related to some definite happening in its surroundings, to some physical event in the external world. The reader perhaps yawns once more as he reads this: the yawn (if it occurs) is definitely related to the words he is reading ; careful study would show that in similar circumstances he (or she) would not yawn, if the written matter announced a legacy of £5,000 to himself or herself. As an illustration of the dependence of an animal’s behaviour on its immediate surroundings one may take the power of colour change in the frog. Like the chameleon the frog exhibits changes of hue; but they are not so rapid. Indeed they require periods of several hours rather than minutes to reach completion; though they are just as striking. Colour response in the frog is influenced by three principal external influences—light, moisture and temperature. If the field of vision is occupied by a light-scattering surface (“‘ white background ’’) the animal will be paler than if the field of vision is occupied by a light-absorbing surface (‘‘ black background ’’); if kept dry the animal will be paler than if it is kept in water; and at 22°C. it will be paler than at 5° C., other factors being constant. So the optimum conditions for pallor are white background, dryness and warmth ; while for darkening of the skin, black background, moisture and cold are best. |

The chameleon like the frog responds to light and temperature, and it will become completely pale after suitable electrical or mechanical stimulation. A toy shocking-coil serves well for the former purpose. If we apply these stimuli to the different regions of the body we find that the same stimulus does not always call forth the same

CH. 1] RECEPTIVITY 7 colour changes. Thus if the skin of the surface of the body, roof of the mouth, and excretory orifice (cloaca) are respectively stimulated by a series of electric shocks, or by hard rubbing with a glass rod, the results obtained may be tabulated thus :—

CoLouR CHANGE IN THE CHAMELEON

Stimulus Area Response Electrical Skin of surface Local pallor

Roof of mouth Generalised pallor

Cloaca Generalised pallor Mechanical Skin of surface No effect

Roof of mouth No effect

Cloaca Generalised pallor

In the case of the chameleon light acts on any portion of the skin, making dark (not pale) the area on which the light falls. This is not the case with the frog or with fishes, many of which respond in a very striking manner to light and darkness. Blinded or blindfolded fish do not respond like fish in which the eye is perfectly intact and accessible to the light. This is true also of the frog, but because colour response in the common frog is influenced powerfully by temperature and moisture, the part played by the eyes can be better illustrated in one of its near cousins, the South African clawed toad (Xenopus levis) which lives in water and is much less susceptible in this respect to the influence of temperature. When the skin colour of clawed toads is compared under different conditions of illumination, the contrast of normal and eyeless or blindfolded individuals is as indicated in the following table :—

CoLOUR CHANGE IN XENOPUS LZVIS White background Black background Normal animal ... Very pale Very dark Eyeless animal ... Intermediate Intermediate We see from this that just as different portions of the body, the effector organs (muscles, glands, the “ pigment cells ”’

8 THE ELEMENTS OF ANIMAL BIOLOGY [PT. I which are responsible for colour changes like the above, etc.), are specially concerned with carrying out responses, so special parts of the body (receptor organs) are specially affected by the various happenings in the animal’s surroundings which call forth response of one kind or another. The term receptor organ replaces the older one sense organ in contemporary biological nomenclature. It is greatly to be preferred, for it involves no departure from our initial resolve to treat the animal as a physical object of the external world. The term sense organ involves a habit of speech which is foreign to a physical science, and there is no reason to perpetuate it in the early stages of biological study. For the same reason the term purpose should never be used in biology. An organ fulfils a certain réle in the economy of the organism. Whether it discharges a purpose we cannot know by any methods whose validity has been vindicated so far. Thus the eye is the main receptor organ through which light influences the behaviour of an animal. In our own case the ear is the organ by which sound vibrations exercise an effect upon us. The ear is a complex organ, and is also the structure by which gravity operates on our behaviour. Some important receptor organs which will be dealt with in a later chapter are given in the list following :—

TYPE OF STIMULUS RECEPTOR ORGAN Light Retina Sound Cochlea of Ear Gravity (mechanical Labyrinth of Ear displacement) Chemical (a) Gaseous Olfactory Membrane (b) Dissolved Gustatory Membrane Deep Mechanical Pressure End Organs in Muscles Temperature End Organs in Skin Generalised Superficial Pressure Tactile Bulbs in Skin

The power of receptivity is not peculiar to animals in any absolute sense. We can make machines which will respond

CH t| REPRODUCTION 9 to any type of stimulus. <A device which responds to light by movement is seen in the windows of any large optician. Animals, however, excel in receptivity even the most complicated of the machines which man has so far devised.

ur. REPRODUCTION

Reproduction is not a property possessed by all living matter ; nor even by all the units of living matter which we call individuals, as met with in nature. The worker bee does not reproduce its kind, but the worker bee could not have existed if there had been no other bees. Every piece of living matter we know has been manufactured by some other piece of living matter very much like itself. The Ford car, like the frog, has a movable body ; but we do not see little Ford cars disporting on the grass before the garage. Most artificial machines are made to-day by the activity of other artificial machines ; but the machines which co-operate to make a Ford car are not themselves Ford cars or very much like Ford cars. With animals it is a universal rule that like begets like. 'This now commonplace statement has only been known to be true of all animals for about a hundred and fifty years. The history of our knowledge of the reproductive process in animals will be set forth avers’

Animals produce other animals in two different ways. One way is called asexual, and the other, which requires the co-operation of two animals, respectively called the male and female, is called sexual reproduction. The first is a much simpler method, less unlike processes which we can imitate artificially. It is the only method of reproduction in large numbers of minute organisms which are simpler in their organisation than any of the things we ordinarily call animals or plants, as the germs of disease and putrefaction

1 Chapter TX.

10 THE ELEMENTS OF ANIMAL BIOLOGY [pT. I

(bacteria). Asexual reproduction does not occur in the more complex animals like frogs or ourselves.

Asexual Reproduction. As an illustration of the latter, therefore, we shall have to study one of the small organisms that can usually be found in mud. If samples of mud from several ponds are added to a little pond water in which a few bread crumbs have been allowed to soak, after standing

FIG. 1—PARAMECIUM DIVIDING for about a day or two the mud at the bottom when examined under the microscope is found to contain little rapidly moving creatures, denoted by the technical name, Paramecium. Each is of a cigar shape, and if you can observe one when it is not moving too quickly you will see that its movements are due to fine flickering processes called cilia (because of their resemblance to eyelashes— cH. I] REPRODUCTION 11 the Latin word). These cover the whole body. Somewhat similar organisms (Opalina, Balantidium, etc.) can always be found living parasitically in the muddy contents of the hindermost portion of the bowel of the frog. When such organisms as these have gone on feeding for a certain time, they become constricted about the middle, and gradually divide into two, just as a drop of a fluid will divide into separate drops when it has reached a certain size. (Fig. 1.) Each half becomes a new individual and swims off on its own. Thus every Paramecium arises from another Paramecium. Like begets like is a rule that applies equally to Paramecium, the common frog and ourselves. But in all those organisms which are properly called animals in every-day language each sort or species has two kinds of individual, males and females; and reproduction involves the co-operation of a male and a female. Both are equally essential to the production of a new being, though the precise part which the male plays in the process was not clearly understood till about fifty years ago.

Sexual Reproduction. 'The female frog differs from the male in its somewhat larger size. At the breeding season, in Spring, frogs will be found in ponds and streams in pairs, a male tightly clasping a female about the body by its forelimbs, with its ventral surface in contact with her back. Thus they remain, till the eggs are laid. These are shed in great numbers embedded in masses of clear jelly, which may be seen floating on the surface of the water. Each egg is almost completely spherical, black in colour towards the pole which floats uppermost, but lighter beneath. As the eggs are laid the male squirts a viscid stream, the seminal fluid, over the surface of the egg.