IN Pope’s Essay on Man we read ’The proper study of Mankind is Man. ’ Man as regards his physical body alone is a highly complex being whose attributes have engaged the attention of learned men throughout the ages, and it is only by a prolonged study, advancing little by little as the centuries have passed, that our knowledge of the structure and functions of the human body has reached its present, far from complete stage. No wonder the Psalmist exclaimed ’For I am fearfully and wonderfully made.’ Natural as it was, however, for men to be preoccupied with their own bodies if only in order to combat the many ills which assailed them, there have been, from the time of Aristotle onwards, many who have, studied the lives and structures of the animals and plants around them. Thus there has grown up, stage by stage, a body of knowledge which we call Biology—the study of living organisms. Man is but part of the living world, and what we can learn from the study of animals and plants can help us to understand ourselves. Nevertheless, since we know most about ourselves from first-hand observation, it will be helpful at the start if we consider in turn our own activities in order that we may acquire a knowledge of the chief characteristics of all living things.
ACTIVITIES OF LIVING THINGS INCLUDING OURSELVES
Assimilation of Food
We need to eat food daily to maintain our life and activities. The food we eat appears externally quite unlike our bodily substance, though in reality it consists of chemical compounds similar to those in our own bodies.
Much of the food we eat is used inside our bodies to furnish a supply of energy for our work and other activities ; just as an engine consumes fuel by oxidation to provide the energy for the work it does, so food is oxidized for the same purpose within ourselves.
This involves breathing by which oxygen is taken in and the products of oxidation, namely carbon dioxide and water vapour, are given out.
The activities of the body result in the continual breaking down of our bodily substance, producing waste substances which are eliminated from the body in the form of excreta.
Growth and Repair
During the period of youth, food is used also for growth and throughout life to repair broken down tissues.
Man, in common with all living organisms, possesses the power of reproduction, that is, of giving rise to new generations of men and women.
Through the various sense organs, e.g. the eye, the ear, the nose, the skin, we are rendered conscious of the external happenings in our environment and we show this by what we do in consequence of what we see, hear, smell or feel. All living thingsthis power of perception of external stimuli and of response— a function which the biologist calls ’Irritability. ’
It is assumed that those who read this article have at least an elementary knowledge of Physics and Chemistry and are familiar with the fundamental laws which ’govern ’ the Inorganic world of non-living matter, such as the laws of Conservation of Energy and the Indestructibility of Matter. These laws apply equally in the Organic world of living organisms, since the physical and chemical changes which occur within their bodies are essentially the same as those which take place in non-living substances. The burning of one gram of sugar in air or oxygen produces certain definite weights of carbon dioxide and water and results in the conversion of a definite quantity of potential energy stored in the sugar into the same amount of free energy in the form of heat and light. To this reaction the law of Conservation of Energy applies and also that of the Indestructibility of Matter, since the weights of the carbon dioxide and water produced are the same as those of the sugar and oxygen reacting when the sugar is burnt. Conversely the same amount of energy derived from the sunlight is absorbed and stored in potential form when a green plant byconverts the same weights of carbon dioxide and water into sugar and oxygen.
In spite of the above facts, some biologists are inclined to believe that there is a non-material ’vital force ’associated with living matter which is absent from non-living matter and which constitutes the real distinction between them. Biologists who hold this belief belong to the Vitalistic School, while those who oppose it form the Mechanistic School, of thought. The Vitalists support their view by pointing to the principle of Biogenesis, first established by Pasteur as a result of his work on the cause of decay. This states that no living organism ever comes into existence except by derivation from a previously existing living organism and never arises by what used to be called Spontaneous Generation from non-living matter — ’ Omnia viva ex vivo. ’ Possibly one day new light will be shed on the problem, as in the case of Einstein ’s discovery that matter and energy are mutually convertible—a discovery which has profoundly modified the previously conceived ideas held by physicists concerning the nature of matter.
Leaving this debatable ground, we find that it is possible from a study of animals and plants to establish a number of Biological ’Laws ’or Principles which demonstrate the fundamental unity of the organic world and which are comparable to those which operate in the inorganic world in that they are of general application. These principles are :—
The interdependence of living organisms.
Organization and division of labour.
Interdependence of Living Organisms
Animals and plants depend on one another for food. All animals obtain their food from plants either directly or indirectly as in the case of carnivores, whichon herbivores.
On the other hand, while plants are not entirely dependent on animals for the raw materials—carbon dioxide, water and mineral salts, from which they manufacture their food—it remains true that animals do provide plants with these materials by respiration and excretion or, after death, by the decay of their bodies through the activity of fungi and bacteria. This is well illustrated by the Carbon Cycle and Nitrogen Cycle.
Insects depend on plants for food and plants in many cases on insects for, while the fruits and of plants are often dispersed by animals.
Organization and Division of Labour
The Cell Theory
The vast majority of animals and plants have bodies composed of numerous units called cells and commence their lives as separate organisms in the form of a single cell which by repeated division builds up the body. The Protozoa and Protophyta consist of single units resembling the cells of the higher plants and animals and capable of performing all the basic functions of living organisms. These are said to be unicellular animals or plants.
The bodies of the multicellular animals and plants show organization and division of labour, I.e. their bodies are composed of organs, each one performing one or other of the functions such as limbs for locomotion, lungs for breathing, and kidneys for excretion. Division of labour is seen also to some extent in some of the unicellular organisms, e.g. Paramecium, different functions being assigned to various parts of the cell. The more highly developed the animal or plant, the greater is the degree of organization and division of labour shown.
Multicellular animals are divided into two main: the Diploblastic animals, which have two cell layers in their body wall —the outer layer is known as the ectoderm and the inner the endoderm ; and the Triploblastic animals with three layers : the ectoderm, endoderm and mesoderm, this latter layer lying between the other two. The endoderm in both classes is always composed of a single layer of cells ; the ectoderm may be one or several cells in thickness, while the mesoderm is always composed of many cells in thickness and usually has a cavity developed within it, dividing the mesoderm into an inner and outer layer. The cavity is known as the coelom, which is well developed in the Earthworm but often restricted to certain parts of the body, e.g. the abdomen in animals such as the Frog. The Triploblastic animals are further divided into the Invertebrates : animals having an ectoderm one cell in thickness and a skeleton, if present, outside the animal, e.g. Insects, Crustacea and Molluscs ; the Vertebrates : animals having an ectoderm more than one cell in thickness and an internal skeleton, e.g. Fish, Amphibia, Reptiles, Birds and Mammals.
In the ’two-layered ’Hydra, division of labour is not so pronounced as in the ’three-layered ’types, e.g. Earthworm. In the former animals the ectoderm cells are concerned with both protection and movement, and the endoderm with digestion and movement. In animals with a mesoderm, a much higher degree of complexity and division of labour is possible. The body cells become differentiated into tissues, I.e. groups of cells of a similar and peculiar structure performing a particular function, such as muscular tissue for performing movements, skeletal tissue for support, and nervous tissue for conduction of nervous impulses. From the different tissues are built up the organs, e.g. to the making of a vertebrate limb go skin, connective, bone, muscle and nervous tissue. In all but the simplest Triploblastic animals, e.g. the liver fluke, the complex body needs internal transport of food, oxygen and waste products to and from the various organs, and this need is met by a transport system which is so regular a feature in animals of varying types and which is known as the blood system.
In primitive Triploblastic animals, e.g. fiatworms, flukes and tapeworms there is no coelom or body cavity, but organs with excretory, reproductive and nervous functions arise in the mesoderm, which consists of cells with large intercellular spaces forming tissue known as the mesenchyme.
Triploblastic animals are usually segmented, their bodies being made of similar sections called segments which are joined end-to-end in a series from front to back, notable exceptions being the flatworms, roundworms, molluscs and Echinoderms. This is easily seen from the external appearance in the Earthworm and the Cockroach ; in higher types the segmentation is not so apparent but is represented in Vertebrates by the jointed vertebral column, the pairs of spinal nerves, the muscles of the trunk, the ribs, and by the similar structure of the fore and hind limbs. Triploblastic animals with a few exceptions possess at the front end a head in which is situated the mouth. This end habitually moves in a forward direction and hence the main sense organs and the brain are concentrated in the head. The development of the head becomes more marked in more advanced types and the progress in development of the head is called capitalization. Triploblastic animals also show bilateral symmetry, their bodies consisting of similar right and left halves on either side of the long axis of the body. This is not the case in Hydra and other Ccelenterata whose bodies are radially symmetrical, and can be divided into more than two equal halves by planes passing through the long axis.
In plants, too, an increasing development of organization and division of labour can be traced by studying a series commencing with the unicellular forms such as Protococcus, passing through the Thallophyta and Mosses and Ferns to the higher plants known as Conifers and Flowering Plants. These latter plants show division of labour among their organs, e.g.for absorption of water and mineral salts, for respiration and photosynthesis, for supporting the leaves and and for the conduction of water and various solutes, and for reproduction. These organs also are built up of various tissues, but the general degree of organization is not so advanced as in the higher animals. Accompanying an increasing degree of division of labour is found a decreasing degree of the power of Regeneration, and this is reflected in the higher power of regeneration shown by Flowering Plants with their lower degree of organization compared with that displayed by the higher animals in which organization is more pronounced.
The more varied the organs of an animal and the greater the division of labour, the more is the need for the co-ordination of the various parts and the necessity for their working in harmony. The process by which this is effected is called Integration and is effected in animals by means ofand nervous systems. Naturally the more complex the animal the greater is the development of the nervous system with the highly developed nervous system of the Mammals – Nervous systems are not found in plants but are. In the apical meristems of stems and , growth-hormones are produced. These diffuse backwards to the growing regions and regulate the rate of growth. It is the effect of light and of gravity on the distribution of these hormones which causes the characteristic curvatures in response to these stimuli. It is now possible to stimulate the production of roots by by the application of commercial preparations of growth hormones.
In order to survive in any particular environment, animals and plants must show adaptations of structure and habits to the conditions prevailing there. The penalty of maladjustment is extinction through the struggle for existence which marks the lives of all living organisms. Numerous examples of extinct animals and plants are found among the fossilized remains of the fauna and flora of past ages. An outstanding example has been the supplanting of a number of types of the cold-blooded, relatively slow-moving reptiles by the warm-blooded, active mammals with their greater brain development and ability to sucoeed in the struggle for existence. Fossil evidence reveals the existence in the past of huge reptilian animals which have long since become extinct.
When studying any particular type of animal or plant, special note should always be taken of the adaptations shown by it. Of unfailing interest are the various ingenious adaptations to secure cross-found among flowers and to disperse their fruits and by means of animals and wind.
It is common knowledge that men do not expect to gather grapes from thorns or figs from thistles—that is, one expects to find certain characters persisting generation by generation in any particular type of animal or plant. The mechanism by which offspring resemble their parents is known as heredity. The fundamental laws concerned with heredity were discovered by Mendel about 1860, and these are described in a later article. It should be noted in passing that while offspring do resemble their parents, minor variations such as difference in height or colour of hair or fur or of eyes do occur and that no animal or plant is an exact replica of its parents.
CLASSIFICATION OF ANIMALS AND PLANTS
From quite early times Man has found it convenient to catalogue animals and plants. Most of these systems of classification were purely arbitrary, but in the eighteenth century Linnaeus established a system which, though far from perfect, was based on the natural relationships of animals and plants. His greatest contribution was his decision to give a double Latin name to each type. This consisted of a generic name and a specific name. For example, all the Cat tribe were given the name Felis and the various species a distinctive second name commencing with a small letter, e.g. Felis leo— the lion, Felis tigris—the tiger, Felis catus—the wild cat, Felis domestica —the domestic cat.