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No less important is a lay impression, since it is not for specialists and professional biologists that this book is primarily written. For this we are indebted to Mrs Zaida Macan, who has read through the whole typescript; and to Mr Maurice Illingworth, who has read Chapter 12.
We record all this kindness with gratitude and take the opportunity to thank those who have helped us.
It is interesting to speculate on the contents of an Atlantic Charter drawn up by any species of animal other than man. We may start by comparing the lot of man with that of the rest of the animal kingdom, which is separated by a lower grade of intelligence. People are not often drowned as a result of catastrophic floods, few are blown to destruction by strong winds, and death in a forest or heath fire is a rare calamity; nor do abnormal spells of hot or cold weather claim many victims. The same is true for all large animals. But for innumerable small ones such dangers are great, and the populations of many are seriously depleted at intervals by one or other of these causes. A final calamity, which does not often befall man, though it befalls other animals of all sizes, is death at the hands of some beast of prey.
Medical science has rendered the more advanced sections of the community secure against many of the disease-causing parasites which formerly destroyed them in large numbers; plague (the black death), typhus (gaol-fever), and cholera may be mentioned as diseases that once took a heavy toll of life in the British Isles, but do so no longer. In less advanced parts of the world, disease still brings death to enormous numbers of human beings, and in this respect there is not so much difference between man and the other animals. Indeed on theoretical grounds it can be argued that many other animals are better off than man. Most parasites can survive only in living tissue and therefore it is to their advantage that the host should remain not only alive but unhandicapped in the struggle for existence. Too virulent a strain will be as unsuccessful as the one that is not aggressive enough to gain a foothold in the face of the counter-measures taken by the host. There is continual selection of a strain that can establish itself but will not kill. Selection acts on the host too and specimens lacking resistance to a particular parasite are removed at an early age. The result is a state of tolerance, with the host carrying parasites but not inconvenienced by them. This state is not attained by man for two reasons. First, selection of resistant individuals does not take place because medicine prevents it. Secondly, whereas the total population of most animals is separated into numerous units between which there is little interchange, man travels all over the world and carries strains of parasites from a region where tolerance has developed to one where it has not.
Death from starvation may have two causes: the food of a particular species may fail on account of some climatic irregularity, or may be eaten up by some other animal. We avoid the first of these eventualities by means of a highly organized system of transport; when the harvest fails in one part of the world, food is brought from somewhere else. But the recollection of the Bengal famine will serve as a reminder that this danger is not wholly eliminated. Against animals which eat the same food as himself, man brings to bear all the resources of science, and wages a never-ending war on rabbits, rats, grain-weevils, slugs, caterpillars, and a host of smaller pests. Lower animals cannot attack their competitors on anything like the same scale; until recently it might have been stated that they could not do it at all, but now it is know that certain organisms can produce chemical substances that kill competitors – penicillin produced by the mould Penicillium is the obvious example – and this process is comparable with the steps that man takes to safeguard his food supplies.
Were we writing about philosophy and not natural history, we should of course have to insert a passage about the perils peculiar to civilization – death on the roads from motor vehicles, and other accidents with machines, destruction by high explosives and other weapons of war, and mass annihilation by atomic bombs. However, we are not. The purpose of what has been written is to stress that the life of a small animal – and it is with such that this book is mainly concerned – is a continual struggle of extreme severity. The physical environment, predators, parasites, and competitors all have to be contended with. The response has been steady change and continual modification. Some animals have won and held a place where physical conditions are easiest – but dangers from predators, parasites, and competitors consequently greatest. Others have become adapted to conditions where physical or chemical conditions make life difficult – but where accordingly there will be fewer other organisms to harass them. Fresh water provides some of the most striking examples of the latter.
It might appear at first sight that the gulf between land animals and water animals is great, and that the easiest way into fresh water is from the sea. But, although an animal or plant may pass from marine to freshwater conditions with no alteration of structure, the change confronts it with considerable functional or physiological problems. The concentration of salts is generally much lower in fresh water than in the sea and moreover liable to considerable variation according to rainfall and other factors; from the biological point of view the constancy of the marine environment is one of its most notable features. Further, conditions vary widely from one freshwater locality to another. An animal proceeding up the Hampshire Avon, for example, would find, if it turned aside into one of the tributaries coming from the New Forest, an acid water, poor in dissolved salts, very different from that which would surround it if it followed the main river to a source at the foot of the chalk downs. These chemical conditions have proved a barrier which only a few marine animals have surmounted. Some of the snails, the bivalve molluscs, the freshwater shrimps, and the fishes are the most familiar.
Actually it appears from an examination of the groups to which present-day freshwater plants and animals belong, that it has been easier to invade fresh water from land than from the sea.
The animals of marine origin occupy in fresh water the same sorts of situation that they occupied in the sea, and they have not changed greatly. As a result of isolation they are now quite distinct from their nearest marine relatives, but they present no peculiar freshwater facies. Some of the land-animals, too, have effected the change to fresh water with little alteration; others with no more than some general adaptation such as the conversion of appendages from legs to paddles. But some of the animals from the land, having once established themselves in fresh water, have become considerably modified to live in one particular and difficult part of the underwater world such as a torrent; others have achieved remarkable physiological adaptations, such as the ability to live without oxygen. It is among these specialists that we find the peculiar and characteristic freshwater types.
The main problem confronting an animal taking to the water is how to obtain its oxygen. Often the difficulty is not great, because many land animals live in damp places and have a moist surface. If they are quite small, this surface is all they require for respiratory purposes, and accordingly it does not matter greatly whether air or water is the medium beyond the layer of surface moisture. The problem is not quite as simple as this, but we need go no further for the present.
Other animals, whose land ancestors were probably less dependent on humid conditions, spend their lives in the water, but have developed a variety of methods whereby they can utilize atmospheric oxygen. The familiar water-beetles swim to the surface with the aid of their hind legs (which, with the transfer to water, have been modified into efficient paddles), and take in a bubble