Part 2

EVERY one knows in a general way what is meant by the doctrine of descent--that it is the theory which maintains that the forms of life, animals and plants, which we see on our earth to-day, have not been the same from all time, but have been developed, by a process of transformation, from others of an earlier age, and are in fact descended from ancestors specifically different. According to this doctrine of descent, the whole diversity of animals and plants owes its origin to a transformation process, in the course of which the earliest inhabitants of our earth, extremely simple forms of life, were in part evolved in the course of time into forms of continually increasing complexity of structure and efficiency of function, somewhat in the same way as we can see every day, when any higher animal is developed from a single cell, the egg-cell, not suddenly or directly, but connected with its origin by a long series of ever more complex transformation stages, each of which is the preparation for, and leads on to the succeeding one. The theory of descent is thus a theory of development or evolution. It does not merely, as earlier science did, take for granted and describe existing forms of life, but regards them as having become what they are through a process of evolution, and it seeks to investigate the stages of this process, and to discover the impelling forces that lie behind it. Briefly, the theory of descent is an attempt at a scientific interpretation of the origin and diversity of the animate world.

In these lectures, therefore, we have not merely to show on what grounds we make this postulate of an evolution process, and to marshall the facts which necessitate it; we must also try to penetrate as far as possible towards the causes which bring about such transformations. For this reason we are forced to go beyond the limits of the theory of descent in the narrow sense, and to deal with the general processes of life itself, especially with reproduction and the closely associated problem of heredity. The transformation of species can only be interpreted in one of two ways; either it depends on a peculiar internal force, which is usually only latent in the organism, but from time to time becomes active, and then, to a certain extent, moulds it into new forms; or it depends on the continually operating forces which make up life, and on the way in which these are influenced by changing external conditions. Which of these alternatives is correct we can only undertake to determine when we know the phenomena of life, and as far as possible their causes, so that it is indispensable to make ourselves acquainted with these as far as we can.

When we look at one of the lowest forms of life, such as an Amœba or a single-celled Alga, and reflect that, according to the theory of evolution, the whole realm of creation as we see it now, with Man at its head, has evolved from similar or perhaps even smaller and simpler organisms, it seems at first sight a monstrous assumption, and one which quite contradicts our simplest and most certain observations. For what is more certain than that the animals and plants around us remain the same, as long as we can observe them, not through the lifetime of an individual only, but through centuries, and in the case of many species, for several thousand years?

This being so, it is intelligible enough that the doctrine of evolution, on its first emergence at the end of the eighteenth century, was received with violent opposition, not on the part of the laity only, but by the majority of scientific minds, and instead of being followed up, was at first opposed, then neglected, and finally totally forgotten, to spring up anew in our own day. But even then a host of antagonists ranged themselves against the doctrine, and, not content with loftily ignoring it, made it the subject of the most violent and varied attacks.

This was the state of affairs when, in 1858, Darwin's book on _The Origin of Species_ appeared, and hoisted the flag of evolution afresh. The struggle that ensued may now be regarded as at an end, at least as far as we are concerned--that is, in the domain of science. The doctrine of descent has gained the day, and we can confidently say that the Evolution theory has become a permanent possession of science that can never again be taken away. It forms the foundation of all our theories of the organic world, and all further progress must start from this basis.

In the course of these lectures, we shall find at every step fresh evidence of the truth of this assertion, which may at first seem all too bold. It is not by any means to be supposed that the whole question in regard to the transformation of organisms and the succession of new forms of life has been answered in full, or that we have now been fortunate enough to solve the riddle of life itself. No! whether we ever reach that goal or not, we are a long way from it as yet, and even the much easier problem, how and by what forces the evolution of the living world has proceeded from a given beginning, is far from being finally settled; antagonistic views are still in conflict, and there is no arbitrator whose authoritative word can decide which is right. The _How?_ of evolution is still doubtful, but not the _fact_, and this is the secure foundation on which we stand to-day: The world of life, as we know it, has been evolved, and did not originate all at once.

Were I to try to give, in advance, even an approximate idea of the confidence with which we can take our stand on this foundation, I should be almost embarrassed by the wealth of facts on which I might draw. It is hardly possible nowadays to open a book on the minute or general structural relations, or on the development of any animal whatever, without finding in it evidences in favour of the Evolution theory, that is to say, facts which can only be understood on the assumption of the evolution of the organic world. This, too, without taking into account at all the continually increasing number of facts Palæontology is bringing to light, placing before our eyes the forms which the Evolution theory postulates as the ancestors of the organic world of to-day: birds with teeth in their bills, reptile-like forms clothed with feathers, and numerous other long-extinct forms of life, which, covered up by the mud of earlier waters, and preserved as 'fossils' in the later sedimentary rocks, tell us plainly how the earlier world of animals and plants was constituted. Later, we shall see that the geographical distribution of plant and animal species of the present day can only be understood in the light of the Evolution theory. But meantime, before we go into details, what may justify my assumption is the fact that the Evolution theory enables us to predict.

Let us take only a few examples. The skeleton of the wrist in all vertebrate animals above Fishes consists of two rows of small bones, on the outer of which are placed the five bones of the palm, corresponding to the five fingers. The outer row is curved, and there is thus a space between the two rows, which, in Amphibians and Reptiles, is filled by a special small bone. This 'os centrale' is absent in many Mammals, notably, for instance, in Man, and the space between the two rows is filled up by an enlargement of one of the other bones. Now if Mammals be descended from the lower vertebrates, as the theory of descent assumes, we should expect to find the 'os centrale' even in Man in young stages, and, after many unsuccessful attempts, Rosenberg has at last been able to demonstrate it at a very early stage of embryonic development.

This prediction, with another to be explained later, is based upon the experience that the development of an individual animal follows, in a general way, the same course as the racial evolution of the species, so that structures of the ancestors of a species, even if they are not found in the fully developed animal, may occur in one of its earlier embryonic stages. Further on, we shall come to know this fact more intimately as a 'biogenetic law,' and it alone would be almost enough to justify the theory of evolution. Thus, for instance, the lowest vertebrates, the Fishes, breathe by means of gills, and these breathing organs are supported by four or more gill-arches, between which spaces, the gill-slits, remain open for the passage of water. Although Reptiles, Birds, and Mammals breathe by lungs, and at no time of their life by gills, yet, in their earliest youth, that is, during their early development in the egg, they possess these gill-arches and gill-slits, which subsequently disappear, or are transformed into other structures.

On the strength of this 'biogenetic law' it could also be predicted that Man, in whom, as is well known, there are twelve pairs of ribs, would, in his earliest youth, possess a thirteenth pair, for the lower Mammals have more numerous ribs, and even our nearest relatives, the anthropoid Apes, the gorilla and chimpanzee, have a thirteenth rib, though a very small one, and the siamang has even a fourteenth. This prediction also has been verified by the examination of young human embryos, in which a small thirteenth rib is present, though it rapidly disappears.

During the seventies I was engaged in investigating the development of the curious marking which adorns the long body of many of our caterpillars. I studied in particular the caterpillars of our Sphingidæ or hawk-moths, and found, by a comparison of the various stages of development from the emergence of the caterpillar from the egg on to its full growth, that there is a definite succession of different kinds of markings following each other, in a whole range of species, in a similar manner. From the standpoint of the Evolution theory, I concluded that the markings of the youngest caterpillars, simple longitudinal stripes, must have been those of the most remote ancestors of our present species, while those of the later stages, oblique stripes, were those of ancestors of a later date.

If this were the case, then all the species of caterpillar which now exhibit oblique stripes in their full-grown stage must have had longitudinal stripes in their youngest stages, and because of this succession of markings in the individual development, I was able to predict that the then unknown young form of the caterpillar of our privet hawk-moth (_Sphinx ligustri_) must have a white line along each side of the back. Ten years later, the English zoologist, Poulton, succeeded in rearing the eggs of _Sphinx ligustri_, and it was then demonstrated that the young caterpillar actually possessed the postulated white lines.

Such predictions undoubtedly give the hypothesis on which they are based, the Evolution theory, a high degree of certainty, and are almost comparable to the prediction of the discovery of the planet Neptune by Leverrier. As is well known, this, the most distant of all the planets, whose period of revolution round the sun is almost 165 of our years, would probably never have been recognized as a planet, had not Adams, an astronomer at the Greenwich Observatory, and afterwards Leverrier, deduced its presence from slight disturbances in the path of Jupiter's moons, and indicated the spot where an unknown planet must be looked for. Immediately all telescopes were directed towards the spot indicated, and Galle, at the Berlin Observatory, found the sought-for planet.

We might with justice regard as lacking in discernment those who, in the face of such experiences, still doubt that the earth revolves round the sun, and we might fairly say the same of any one who, in the face of the known facts, would dispute the truth of the Evolution theory. It is the only basis on which an understanding of these facts is possible, just as the Kant-Laplace theory of the solar system is the only basis on which an adequate interpretation of the facts of the heavens can be arrived at.

To this comparison of the two theories it has been objected that the Evolution theory has far less validity than the other, first, because it can never be mathematically demonstrated, and secondly, because at the best it can only interpret the transformations of the animate world, and not its origin. Both objections are just: the phenomena of life are in their nature much too intricate for mathematics to deal with, except with extreme diffidence; and the question of the origin of life is a problem which will probably have to wait long for solution. So, if it gives pleasure to any one to regard the one theory as having more validity than the other, no one can object; but there is no particular advantage to be gained by doing so. In any case, the Evolution theory shares the disadvantage of not being able to explain everything in its own province with the Kant-Laplace cosmogony, for that, too, must presuppose the first beginning, the rotating nebula.

Although I regard the doctrine of descent as proved, and hold it to be one of the greatest acquisitions of human knowledge, I must repeat that I do not mean to say that everything is clear in regard to the evolution of the living world. On the contrary, I believe that we still stand merely on the threshold of investigation, and that our insight into the mighty process of evolution, which has brought about the endless diversity of life upon our earth, is still very incomplete in relation to what may yet be found out, and that, instead of being vainglorious, our attitude should be one of modesty. We may well rejoice over the great step forward which the dominant recognition of the Evolution theory implies, but we must confess that the beginnings of life are as little clear to us as those of the solar system. But we can do this at least: we can refer the innumerable and wonderful inter-relations of the organic cosmos to their causes--common descent and adaptation--and we can try to discover the ways and means which have co-operated to bring the organic world to the state in which we know it.

When I say that the theory of descent is the most progressive step that has yet been taken in the development of human knowledge, I am bound to give my reasons for this opinion. It is justified, it seems to me, even by this fact alone, that the Evolution idea is not merely a new light on the special region of biological science, zoology and botany, but is of quite general importance. The conception of an evolution of the world of life upon the earth reaches far beyond the bounds of any single science, and influences our whole realm of thought. It means nothing less than the elimination of the miraculous from our knowledge of nature, and the placing of the phenomena of life on the same plane as the other natural processes, that is, as having been brought about by the same forces, and being subject to the same laws. In the domain of the inorganic, no one now doubts that out of nothing nothing can come: energy and matter are from everlasting to everlasting, they can neither be increased or decreased, they can only be transformed--heat into mechanical energy, into light, into electricity, and so on. For us moderns, the lightning is no longer hurled by the Thunderer Zeus on the head of the wicked, but, careless alike of merit or guilt, it strikes where the electric tension finds the easiest and shortest line of discharge. Thus to our mode of thought it now seems clear that no event in the world of the living depends upon caprice, that at no time have organisms been called forth out of nothing by the mighty word of a Creator, but they have been produced at all times by the co-operation of the existing forces of nature, and every species must have arisen just where, and when, and in the form in which it actually did arise, as the necessary outcome of the existing conditions of energy and matter, and of their interactions upon each other. It is this correlation of animate nature with natural forces and natural laws which gives to the doctrine of evolution its most general importance. For it thus supplies the keystone in the arch of our interpretation of nature and gives it unity; for the first time it makes it possible to form a conception of a world-mechanism, in which each stage is the result of the one before it, and the cause of the succeeding one.

How deeply all our earlier opinions are affected by this doctrine will become clear if we fix our attention on a single point, the derivation of the human understanding from that of animal ancestors. What of the reason of Man, of his morals, of his freedom of will? may be asked, as it has been, and still is often asked. What has been regarded as absolutely distinct from the nature of animals is said to differ from their mental activities only in degree, and to have evolved from them. The mind of a Kant, of a Laplace, of a Darwin--or to ascend into the plane of the highest and finest emotional life, the genius of a Raphael or a Mozart--to have any real connexion, however far back, with the lowly psychical life of an animal! That is contrary to all our traditionary, we might say our inborn, ideas, and it is not to be wondered at that the laity, and especially the more cultured among them, should have opposed such a doctrine whose dominating power was unintelligible to them, because they were ignorant of the facts on which it rests. That a man should feel his dignity lowered by the idea of descent from animals is almost comical to the naturalist, for he knows that every one of us, in his first beginning, occupied a much lowlier position than that of our mammalian ancestors--was, in fact, as regards visible structure, on a level with the Amœba, that microscopically minute unicellular animal, which can hardly be said to possess organs, and whose psychical activities are limited to recognizing and engulfing its food. Very gradually at first, and step by step, there develop from this single cell, the ovum, more and more numerous cells; this mass of cells segregates into different groups, which differentiate further and further, until at last they form the perfect man. This occurs in the development of every human being, and we are merely unaccustomed to the thought that it means nothing else than an incredibly rapid ascent of the organism from a very low level of life to the highest.

Still less is it to be wondered at that the Evolution doctrine met with violent opposition on the part of the representatives of religion, for it stood in open contradiction to that remarkable and venerable cosmogony, the Mosaic story of Creation, which people had been accustomed to regard, not as what it is--a conception of nature at an early stage of human culture--but as an inalienable part of our own religion. But investigation shows us that the doctrine of evolution is true, and it is only a weak religion which is incapable of adapting itself to the truth, retaining what is essential, and letting go what is unessential and subject to change with the development of the human mind. Even the heliocentric hypothesis was in its day declared false by the Church, and Galilei was forced to retract; but the earth continued to revolve round the sun, and nowadays any one who doubted it would be considered mentally weak or warped. So in all likelihood the time is not far distant when the champions of religion will abandon their profitless struggle against the new truth, and will see that the recognition of a law-governed evolution of the organic world is no more prejudicial to true religion than is the revolution of the earth round the sun.

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Having given this very general orientation of the Evolution problem, which is to engage our attention in detail, I shall approach the problem itself by the historical method, for I do not wish to bring the views of present-day science quite suddenly and directly into prominence. I would rather seek first to illustrate how earlier generations have tried to solve the question of the origin of the living world. We shall see that few attempts at solution were made until quite recently, that is, until the end of the eighteenth and the beginning of the nineteenth century. Only then there appeared a few gifted naturalists with evolutionist ideas, but these ideas did not penetrate far; and it was not till after the middle of the nineteenth century that they found a new champion, who was to make them common property and a permanent possession of science. It was the teaching of Charles Darwin that brought about this thorough awakening, and laid the foundations of our present interpretations, and his work will therefore engross our attention for a number of lectures. Only after we have made ourselves acquainted with his teaching shall we try to test its foundations, and to see how far this splendid structure stands on a secure basis of fact, and how deeply its power of interpretation penetrates towards the roots of phenomena. We shall examine the forces by which organisms are dominated, and the phenomena produced, and thereby test Darwin's principles of interpretation, in part rejecting them, in part accepting them, though in a much extended form, and thus try to give the whole theoretic structure a more secure foundation. I hope to be able to show that we have made some real progress since Darwin's day, that deductions have been drawn from his theory which even he did not dream of, which have thrown fresh light on a vast range of phenomena, and, finally, that through the more extended use of his own principles, the Evolution theory has gained a completeness, and an intrinsic harmony which it previously lacked.

This at least is my own opinion, but I cannot ignore the fact that it is by no means shared by all living naturalists. The obvious gaps and insufficiencies of the Darwinian theory have in the last few decennia prompted all sorts of attempts at improving it. Some of these were lost sight of almost as soon as they were suggested, but others have held their own, and can still claim numerous supporters. It would only tend to bewilder if I gave an account of those of the former class, but those which still hold their own must be noticed in these lectures, though it is by no means my intention to expound the confused mass of opinions which has gathered round the doctrine of evolution, but rather to give a presentation of the theory as it has gradually grown up in my own mind in the course of the last four decades. Even this will not be the last of which science will take knowledge, but it will, I hope, at least be one which can be further built upon.

Let us, then, begin at once with that earliest forerunner of the modern theory of descent, the gifted Greek philosopher Empedocles, who, equally important as a leader of the state of Agrigentum, and as a thinker in purely theoretical regions of thought, advanced very notable views regarding the origin of organisms. We must, however, be prepared to hear something that is hardly a theory in the modern scientific acceptation of that term; and we must not be repelled by the unbridled poetical fancy of the speculative philosopher; we have to recognize that there is a sound kernel contained in his amusing pictures--a thought which we meet with later, in much more concrete form, in the Darwinian theory, and which, if I mistake not, we shall keep firm hold of in all time to come.