Creation by Evolution/Progress Shown in Evolution
PROGRESS SHOWN IN EVOLUTION
By Julian Sorrell Huxley
Honorary Lecturer in Zoölogy, Kings College, London University
Some seem to suppose that evolution is synonymous with change, even if the change is disorderly and chaotic; but if we look at evolution as it actually exists, whether the long-range evolution of species from species or class from class, or the short-range evolution that occurs in the individual development of each human being and each familiar animal from the egg to the adult stage, we find that one of the characteristics of evolutionary change is its orderliness. Each step in each separate evolutionary line is orderly, its significance can be fully understood only as the result of what has gone before and as the necessary prelude to what is to come after. If we turn from single lines of evolution to the evolution of life as a whole, we can ask a new question. Granted that the separate changes of evolution are orderly, can we discern one sole or main direction, or a few main directions, in the general evolution of life? Finally, if we were to find that evolution followed only one or a few main trends, can we say that these trends or directions are, in any real sense of the word, progressive?
The answer to the first of these two questions is definite enough. In its march through time life does follow certain main directions. This fact can be shown by actually tracing the history of animals through geological time by means of their fossil remains, by deciphering the history of the race from the summary of it presented in the history of the developing individual, and, indirectly, by comparing different animals with one another, a comparison that enables us to deduce with reasonable accuracy their family history and genealogical tree. By combining all the various lines of evidence it is possible to arrive at certain perfectly definite conclusions. In the first place, the size of animals has increased during their evolution. None of the earliest mammals were much bigger than a dog; creatures the size of a horse or a hippopotamus were unknown. The same is true of the reptiles; the giant forms appeared late in their geological history. The earliest forms of life were doubtless microscopic. Even the smallest mammal is over a million million times as large as a medium-sized amoeba.
More important than mere increase in size is increase in efficiency. No early form of life can move fast or can see or hear acutely; none has a heart or a blood-system, brain or nerves, jaws or limbs, or true head. The earliest true vertebrates have no jaws or teeth. The earliest land vertebrates could not support their bodies with their limbs. The earliest members of the horse family had limbs incapable of the speed achieved by the later horses, and the teeth of an early horse are not nearly so efficient for grinding as those of a modern horse. The brains of early mammals were barely more than half the proportional size of those of present-day mammals of the same bulk. The improvement of the efficiency of separate organs is the improvement of the tools of life; the improvement of the efficiency of the brain is the improvement of the way in which these tools can be used.
When we examine the geological history of a single group of animals, such as the mammals or the reptiles, whose past record is available to us in the fossil-bearing rocks, we find that improvement in efficiency is actually made in different directions by a number of separate evolutionary lines. The improvement of each line means improvement of efficiency at one particular job, such as improvement in one way of getting a livelihood or in planting its offspring securely in the world. For instance, the early mammals of the Secondary period (Mesozoic age) were small land animals whose fore and hind limbs were not very different from each other—four or five toes on each foot, teeth of a primitive type like those of a shrew or a hedgehog, and small brains. During the Tertiary period these creatures gave rise to a whole set of types. Some of these types, like the whale and the dolphin, are adapted to life in water; others, like the horse and the deer, are adapted to herbivorous diet and rapid running; the higher carnivores, like the lion and the wolf, are adapted to a flesh diet, which is captured by speed, strength, and skill; the bat is adapted to a life in air; the elephant survives by virtue of its huge size and formidable tusks; the mole lives by its adaptation to a burrowing life below ground; the | sloth, by its adaptation to life in trees; armadilloes survive by their protective armour.
Each of these creatures represents the finished product of a line of improvement in one particular tool or method. On the other hand, each individual improvement has been made at the expense of other possible improvements. The original type was primitive but plastic; it was capable of being altered in many possible ways. But the whale, in becoming an efficient swimmer and diver, has lost any possibility of ever being able to run or to fly; the horse, by gaining its efficient running organ in the shape of a long leg with but one toe, has lost the possibility of acquiring a hand for grasping or a clawed foot for catching prey; the elephant’s bulk precludes agility; the mole, though good at burrowing, is worse off as regards the possibility of climbing trees. Such improvement is best termed specialization, and the complete set of divergent specializations which characterize the evolution of a whole group, such as the mammals, is called the adaptive radiation of the group. Biological specialization moves always in one direction only and is achieved at the expense of improvement in other directions. What is more, specialization in improving the efficiency of a physical tool, such as a limb or eye, is bound sooner or later to reach a limit. The elephant is pretty close to the limit of size which is possible or at least advantageous for a purely terrestrial animal. The speed of wild horses or antelopes is close to the greatest speed that is possible to a four-legged land animal; acuteness of vision must reach a limit owing to the impossibility of obtaining cells in the retina below a certain size; and so forth.
Thus specialization and adaptive radiation, though they increase immensely the efficiency of life as a whole and enable it to reach its greatest limits in this or that direction, are yet in a sense double-edged. In opening the door to one kind of improvement they close it to other kinds, and in the long run even turn out to be blind alleys, to which positive limits are set. We can easily recognize the limitations of specialization as a method of evolutionary improvement by considering specialization for a parasitic existence. An internal parasite, such as a tapeworm or a malarial parasite, has no need to find or to digest its own food, to move from place to place, or to detect enemies at a distance. Accordingly we find that most internal parasites have no mouth or digestive system, no means of locomotion, or very much reduced means, no well-developed sense-organs. On the other hand, parasites must be specially adapted, for instance, to resist the action of digestive juices or of protective devices in the blood of their host, and especially to enable them to get transferred from one host to another. We thus find that in respect to reproduction and life-cycle, parasites are usually much more elaborate than tree-living animals; for instance, the dog tapeworm can be transmitted to a dog only by entering the body of an animal like a rabbit, there going through a special cycle of life, and then being eaten again by a dog.
We usually say that parasites are degenerate, because we note their striking loss of organs and faculties, but they are only particular examples of specialization, with, as usual, elaboration and improvement in one direction and loss in others. The whale, in gaining blubber and tail fin, has lost hair and hind limb; the horse, in improving its middle digit to a hoof, has lost the other four digits on each foot.
But besides such one-sided specialization, there are examples of evolutionary improvement which are all-round, or balanced, and do not deprive their possessors of their precious plasticity. For instance, the change from cold-bloodedness to warm-bloodedness in vertebrates was such a change. In becoming warm-blooded, the bird or the mammal lost nothing which their reptilian ancestors possessed; they merely acquired a new and valuable piece of vital machinery, which enables them to be much more independent of the temperature of the outer world than they were before. In the same way, the reproductive methods of reptiles and birds represent a pure gain when compared with those of their fish-like and amphibian ancestors. The evolution of the protective membrane or amnion, which makes a water cushion round the embryo, and the other embryonic membrane or allantois, which enables the embryo to breathe within the egg-shell, made it possible for reptiles to be independent of water for their breeding, and so helped to open up to them vast tracts of the earth’s surface which other vertebrates had until then not been able to conquer. Such changes, involving the improvement of the all-round achievements of the organism without depriving it of valuable possibilities, may properly be called biological progress. They are simply examples of specialization that is not one sided, but balanced.
We may take one further example, which brings out the difference between the two processes. The most primitive members of the group to which we and all other backboned animals belong-forms like Amphioxus, for instance—have no true eye, have probably only a very slight sense of smell (certainly no nasal organ of our type), and no ear. The lower vertebrates, such as the fishes, have very efficient sight and smell but practically no sense of hearing. Both birds and mammals (in general) have acute hearing and much improved sight. Here there is a real biological advance; the efficiency of all three senses has enormously improved, and improved in a balanced way, in passing from Amphioxus to higher vertebrate. But in this same field we may find unbalanced improvement, one-sided specialization. The improvement in the utilization of the sense of sight, which is so obvious in the whole group of monkeys and apes and culminates in man, has been accompanied by a degeneration in the power of smell; the same has been true in many birds, which also rely almost entirely upon sight. On the other hand, the mole relies almost entirely upon touch and hearing, and its eyes have degenerated. Thus in all these forms an unbalanced improvement in one direction has led to a cutting down of faculty in another.
The main improvements of life during its evolution must obviously be improvements of the balanced type, not mere specializations, since it seems certain that no highly specialized animal or plant has ever succeeded in becoming the ancestor of a new group or type, such being the privilege of biologically balanced or generalized creatures.
The most important progressive steps in the evolutionary ascent of animal life perhaps deserve mention. Starting from the single-celled type, life made its first great advance through the aggregation of many single cells into a colony; this advance was followed by division of labour for different functions among different kinds of cells, which gave new possibilities of size and balanced specialization of function. Next came the organization of the community of cells into a two-layered creature with a mouth at one end, a stage preserved to-day in sea-anemones and their relatives. Then came the intercalation of a third layer, and the development of a centralized (though primitive) nervous system and primitive kidneys. Then the development of a blood system, a posterior opening to the digestive tube, better locomotor organs, and elaborated sense organs in a region which might properly be called a head. Leaving all but the vertebrates out of consideration for lack of space, we would next come to the enlargement of the brain, the development of a strong internal skeleton, and then to that of paired limbs. These improvements are followed by partial emancipation from the water, as in the amphibians, then total emancipation, as in the reptiles. Still later we find the attainment of the condition of constant temperature, called warm-bloodedness; the improvement of the nourishment and care of the young, both before and after birth; and the rapid improvement of memory, associative power, and animal intelligence. Finally, in man, comes the new step in brain power which we call reason—the power of generalizing, and consequently of giving names to things, and so speech, which has brought in its train the other enormously important progressive development, the possession by the human species of experience and tradition that is cumulative from generation to generation. At each of these levels some types of living beings have specialized and remain fixed to this day in their one-sided efficiency, or else they have been extinguished by more progressive types; others have remained generalized, and some of these have given birth to the progressive types which constitute the next upward step.
After this brief survey, it remains to ask whether the balanced advance we have been discussing can properly be called progress, in the usual sense of that term, or whether we have not been misleading ourselves by using a term which implies real improvement in what to us is valuable, when we should have really called it mere directive change.
When we come to consider the main steps in biological advance that are enumerated above, we find that it is possible to sum them up under a few heads. There has been on the whole a considerable increase in size; there has been improvement in the organs adapted to carrying out each type of function taken separately-organs of digestion, of locomotion, of protection, of support, of sense-perception, of reproduction; there has been improvement in the relation between these organs-that is, in the way in which the different parts of the body and their functions are correlated and coördinated; there has been improvement in the control exercised by the brain over the body as a whole, and in the quality and extent of the information received about the outside world by which this control is achieved; there has been improvement in the self-regulating capacity of the body, as is witnessed by constant temperature or constant chemical composition of blood in higher forms; there has been a decreasing reproductive waste, an increasing care for young; there has been an increase in mutual aid between individuals; there has been an increase of emotional power and of purposive action. If we now examine this list further, we find that every one of the improvements enumerated may be thought of as conferring upon the individual or the race increased power of control over environment, increased internal harmony and self-regulative capacity and consequently increased independence as regards the outer world, increased knowledge, and increased intensity and harmony of mental life.
Whether the list is considered in its first state or in its second, there are very few who will not admit that these biological improvements, which have made for survival and success in evolution, are not also improvements when judged by our human standards of value. We, too, strive for control over nature and for greater independence of outer conditions; we value harmony; we prize knowledge and all the products (when balanced) of increased intensity of emotion and will. It is therefore justifiable, since progress is a word which implies progress toward something which we men find of value, to speak of the observed movement of life that we have so far called biological advance as real biological progress.
It may be argued that this is mere reasoning in a circle; that of course, as we are ourselves products of the evolutionary process, we shall find its movement coincide with our ideas of good. This is in reality not so at all. It is not all kinds of evolutionary movement which we find good in this way, but only the one kind that we have defined as balanced advance. There are many other kinds of evolutionary process. There is, for instance, extinction. Whole groups of animals and plants, some of them of remarkable vigour, size, beauty of adaptation, have wholly disappeared from the face of the earth. The trilobites, the ammonites, the wonderful dinosaur group of reptiles—these are but three examples; and even to-day the process is being very rapidly continued by man, who is, often needlessly, exterminating entire species—strange creatures like the great auk, lovely ones like the passenger pigeon or the sea-otter. Extinction is in itself not a good but an evil. It can only be a good on balance, if and when it is necessary that one group or type should perish that another more advanced group should flourish.
Then, as we have seen, specialization, though sometimes we should call it good, is never an unmixed or a balanced good. Not only that, but in some of the examples considered, such as parasitism, the balance is the other way, and what seems good to us is outweighed by what seems clearly evil. If tapeworms could reason and formulate their opinions about the universe, they would have to admit that the general trend of evolution was very different in its direction from that to which they owed their being, and that on the whole the two were opposed. They would, presumably, have to adopt that philosophy or belief which characterized the Manicheans of the early Christian era. Even the products of specialization that to us is clearly on balance good, though limited—such products, for instance, as the birds—would (if they were able to think it all out) think in rather different terms. Their specialization has led to flight, to intense activity, to colour and song unrivalled among the mammals. Well might they pity earthbound, drab-coloured, hairy creatures, and maintain that activity and the conquest of the air were the highest achievements of evolving life. But they would be wrong. It is simply a matter of hard fact, which takes no account of actual human wishes or hypothetical bird wishes, that for some reason (probably their sacrifice of fore-limb for a tool of flight) the birds' brain-development has been restricted, whereas the mammals, evolving along the lemur-monkey-ape line, were able to develop brain power, which finally culminated in man, and which has made man now the dominant, most successful organism and has enabled him actually to beat the bird at its own game, producing machines more swift and tireless in the air than the birds themselves, and music compared to which even the lark's and the nightingale's songs are but naïve.
No, the only reason why we find that the direction of biological progress coincides so closely with much of our own ideas of progress and value is that man happens to be in the main stream of biological progress, not in an eddy or backwater.
The fact that there exists (among other processes of evolution) one which can rightly be styled progress, seems to me of fundamental importance to our thought. It does not in any way prove the existence of a supernatural purpose in evolution. It is merely one among several kinds of evolutionary results, no one of which is any less explicable by natural causes than any other. Paley and his school maintained with great vigor that the existence of adaptive structures closely fitted to the function they were to perform―the webbed foot of a duck, the eye of man, the stream-line form of a fish—were proof of a supernatural designer. Darwin at one stroke swept this argument away by means of his theory of natural selection. This was early recognized by all who accepted Darwinism, but it has not been so readily recognized that precisely the same arguments hold as regards biological progress: granted the existence of Variation and Natural Selection, then biological progress as well as adaptation (which is the product of specialization) must come about.[1]
But the fact of biological progress does show that our ideals and efforts, our whole scheme of values, are not merely isolated flames burning in the darkness of a universe which is neutral or hostile to the effects of its working. It shows, at least as regards the course of events for the several thousand million years during which life has existed on this planet, that the cosmic forces have worked in such a way as to produce a movement that has been not only the most successful movement in evolution but that also chimes in with our sense of values and our Idea of the direction In which we ourselves desire to move.
As a result of the working of the forces of Evolution, man has now become the trustee of the evolutionary process—in other words, of this world's future. It is sometimes asserted that pre-human evolution is purposeful. This, however, is a mistake. As Darwin once and for all pointed out, the adaptation of structure to function, of an animal's means to the requirements of its life, not only require no other purpose at all than the purposes of the contending animals but can be explained only as the result of the automatic working of natural forces, such as heredity, variation, and natural selection; and the same statement is true if we consider the general movement and progress of life's evolution as opposed to the particular evolution of this or that species. These natural forces are in the same category as those which cause chemical combinations, make the chick grow in the egg, or cause our digestive juices to be secreted; but purpose is something that can be defined only psychologically—meaning that a definite end is consciously held in view.
This of course is not to say that there may not be purpose in the background, behind all those apparently blind forces. But we have no means whatever of knowing whether that is so or not; and it Is the first duty of anyone who wishes to think scientifically to suspend judgment on such questions in the absence of evidence.
What is definitely true is that the forces which we can actually detect operating in the evolution of plants and lower animals are automatic and non-conscious; whereas those operating on the human level, as we can again obviously verify for ourselves, are in part conscious and include ideals of truth, beauty, and morality. We may even say that the forces of evolution conspire to act as “a power, not ourselves, which makes for righteousness.”
Our business on this planet, then, is not to worry our heads about the possible forces which may exist behind those which we know, but to strive to mould the known material forces of dead and living matter in accord with the spiritual ideals of value which we possess.
REFERENCES
- Dendy, A. Outlines of Evolutionary Biology. London, 1923.
- Lull, R. S. Organic Evolution. New York, 1917.
- Huxley, J. S. Essays of a Biologist. London, 1923.
- Osborn, H. F. Origin and Evolution of Life. New York and London, 1918.
“Is it not the most sublime, the most stimulating conception that has ever entered human thought, this conception of progress, this new idea absolutely unknown in ancient times, a progress of which we are a part, and in which we are ourselves consciously playing a role of supreme importance?”—Robert Millikan.
"I do not know what I may appear to the world, but to myself I seem to have been only like a boy playing on the seashore and diverting myself now and then finding a smoother pebble or a prettier shell than ordinary, while the great ocean of truth lay all undiscovered before me.”—Sir Isaac Newton.
- ↑ See essay on Progress in J. H. Huxley’s Essays of a Biologist.