The number of fossil species hitherto described is reckoned at about 80,000—certainly only a mere fragment of the wealth of forms of life which have arisen on our earth throughout this long period, and which must have passed away again; for very few species outlive a geological epoch, and even genera appear only for a longer or shorter time, and then disappear for ever. But even of many of the older classes, such, for instance, as the Cystoids among the Echinoderms of the Silurian seas, no living representative remains; and in the same way, the Ichthyosaurs or fish-lizards of the Secondary times have completely disappeared from our modern fauna, and many other animal types, like the class of Brachiopods and the hard-scaled Ganoid fishes, have almost died out and are represented only by a few species in specially sheltered places, such as the great depths of the sea, or in rivers.
Thus an incredible wealth of animal and plant species was potentially contained in these simplest and lowest 'Biophorids' which lay far below the limits of microscopic visibility—an indefinitely greater wealth than has actually arisen, for that is only a small part of what was possible, and of what would have arisen had the changes of life-conditions and life-possibilities followed a different course. The greater the complexity of the structure of an organism is, the more numerous are the parts of it which are capable of variation, and the different directions in which it can adapt itself to new conditions; and it will hardly be disputed that potentially the first Biophorids contained an absolutely inexhaustible wealth of forms of life, and not merely those which have actually been evolved. If this were not so, Man could not still call forth new animal and plant forms, as he is continually doing among our domesticated animals and cultivated plants, just as the chemist is continually 'creating' new combinations in the laboratory which have probably never yet occurred or been formed on the earth. But just as the chemist does not really 'create' these combinations, but only brings the necessary elements and their forces together in such a combination that they must unite to form the desired new body, so the breeder only guides the variational tendencies contained in the germ-plasm, and consciously combines them to procure a new race. And what the breeder does within the narrow limits of human power is being accomplished in free nature, through the conditions which allow only what is fit to survive and reproduce, and thus bring about the wonderful result—as though it were guided by a superior intelligence—the adaptation of species to their environment.
Thus in our time the great riddle has been solved—the riddle of the origin of what is suited to its purpose, without the co-operation of purposive forces. Although we cannot demonstrate and follow out the particular processes of transformation and adaptation in all their phases with mathematical certainty, we can understand the principle, and we see the factors through the co-operation of which the result must be brought about. It has lately become the fashion, at least among the younger school of biologists, to attach small value to natural selection, if not, indeed, to regard it as a superseded formula; mathematical proofs are demanded or, at any rate, desired. I do not believe that we shall ever arrive at giving such proofs, but we shall undoubtedly succeed in clearing up much that now remains obscure, and in essentially modifying and correcting many of the theories we have formed in regard to this question. But what has been already gained must certainly be regarded as an enormous advance on the knowledge of fifty years ago. We now know that the modern world of organisms has been evolved, and we can form an idea, though still only an imperfect one, how and through the co-operation of what factors it could and must have evolved.
When I say must, this refers only to the course of evolution from a given beginning; but as to this beginning itself, the spontaneous generation of the lowest Biophorids from inorganic material, we are far from having understood it as a necessary outcome of its causes. And if we have assumed it as a reasonable postulate, we by no means seek to conceal that this assumption is far from implying an understanding of what the process of biogenesis was. I do not merely mean that we do not know under what external conditions the origin of living matter, even in the smallest quantity, can take place; I mean, especially, that we do not understand how this one substance should suddenly reveal qualities which have never been detected in any other chemical combination whatever—the circulation of matter, metabolism, growth, sensation, will, and movement. But we may confidently say that we shall never be able fully to understand these specific phenomena of life, as indeed how should we, since nothing analogous to them is known to us, and since understanding always presupposes a comparison with something known. Even although we assume that we might succeed in understanding the mere chemistry of life, as is not inconceivable, I mean the perpetuum mobile of dissimilation and assimilation, the so-called 'animal' functions of the living substance would remain uncomprehended: Sensation, Will, Thought. We understand in some measure how the kidneys secrete urine, or the liver bile; we can also—given the sensitiveness to stimulus of the living substance—understand how a sense-impression may be conveyed by the nerves to the brain, carried along certain reflex paths to motor nerves and give rise to movement of the muscles, but how the activity of certain brain-elements can give rise to a thought which cannot be compared with anything material, which is nevertheless able to react upon the material parts of our body, and, as Will, to give rise to movement—that we attempt in vain to understand. Of course the dependence of thinking and willing upon a material substratum is clear enough, and it can be demonstrated with certainty in many directions, and thus materialism is so far justified in drawing parallels between the brain and thought on the one hand, and the kidneys and urine on the other, but this is by no means to say that we have understood how Thought and Will have come to be. In recent times it has often been pointed out that the physical functions of the body increase very gradually with the successive stages of the organization, and from the lowest beginnings ascend slowly to the intelligence of Man, in exact correspondence with the height of organization that has been reached by the species; that they begin so imperceptibly among the lower animal forms that we cannot tell exactly where the beginning is; and it has been rightly concluded from this that the elements of the Psyche do not originate in the histological parts of the nervous system, but are peculiar to all living matter, and it has further been inferred that even inorganic material may contain them, although in an unrecognizable expression, and that their emergence in living matter is, so to speak, only a phenomenon of summation. If we are right in our assumption of a spontaneous generation it can hardly be otherwise, but saying this does not mean that we have understood Spirit, but at most secures us the advantage and the right of looking at this world, as far as we know it, as a unity. This is the standpoint of Monism.
The psychical phenomena, which we know from ourselves, and can assume among animals with greater certainty the nearer they stand to us, occupy a domain by themselves, and such a vast and complex one that there can be no question of bringing it within the scope of our present studies, and the same is true of the phyletic development of Man. But we must at least take up a position in regard to these problems, and there can be no question that Man has evolved from animal ancestors, whose nearest relatives were the Anthropoid Apes. Not many years ago bony remains of a human skeleton, or at least of some form very near to modern Man, were found in the Diluvial deposits of Java, and this has been designated Pithecanthropus erectus, and perhaps rightly regarded as a transition form between Apes and Man. It is possible that more may yet be discovered; but even if that is not so, the conclusion that Man had his origin from animal forefathers must be regarded as inevitable and fully established. We do not draw conclusions with our eyes, but with our reasoning powers, and if the whole of the rest of living nature proclaims with one accord from all sides the evolution of the world of organisms, we cannot assume that the process stopped short of Man. But it follows also that the factors which brought about the development of Man from his Simian ancestry must be the same as those which have brought about the whole of evolution: change of external influences in its direct and indirect effects, and, besides this, germinal variational tendencies and their selection. And in this connexion I should like to draw attention to a point which has, perhaps, as yet received too little attention.
Selection only gives rise to what is suited to its end; beyond that it can call forth nothing, as we have already emphasized on several occasions. I need only recall the protective leaf-marking of butterflies, which is never a botanically exact copy of a leaf, with all its lateral veins, but is comparable rather to an impressionist painting, in which it is not the reproduction of every detail that is of importance, but the total impression which it makes at a certain distance. If we apply this to the organs and capacities of Man, we shall only expect to find these developed as far as their development is of value for the preservation of his existence and no further. But this may perhaps seem a contradiction of what observation teaches us, that, for instance, our eyes can see to the infinite distance of the fixed stars, although this can be of no importance in relation to the struggle for existence. But this intensity of the power of vision has obviously not been acquired for the investigation of the starry heavens, but was of the greatest value in securing the existence of many of our animal ancestors, and was not less important for our own. In the same way our finely evolved musical ear might be regarded as a perfecting of the hearing apparatus far beyond the degree necessary to existence, but this is not really the case: our musical ear, too, has been inherited from our animal ancestors, and to them, as to primitive Man, it was a necessity of existence. It was quite necessary for the animals to distinguish the higher and lower notes of a long scale, sharply and certainly, in order to be able to evade an approaching enemy, or to recognize prey from afar. That we are able to make music is, so to speak, only an unintentional accessory power of the hearing organs, which were originally developed only for the preservation of existence, just as the human hand did not become what it is in order to play the piano, but to touch and seize, to make tools, and so on.
Must this, then, be true also of the human mind? Can it, too, only be developed as far as its development is of advantage to Man's power of survival? I believe that this is certainly the case in a general way; the intellectual powers which are the common property of the human race will never rise beyond these limits, but this is not to say that certain individuals may not be more highly endowed. The possibility of a higher development of certain mental powers or of their combinations—whether it be intelligence, will, feeling, inventive power, or a talent for mathematics, music or painting—may be inferred with certainty from our own principles; for not only may the variational tendencies of individual groups of determinants in the germ-plasm be continued for a series of generations without becoming injurious, that is to say, without being put a stop to by personal selection, but sexual intermingling always opens up the possibility that some predominantly developed intellectual tendencies (Anlagen) may combine in one way or another, and so give rise to individuals of great mental superiority, in whatever direction. In this way, it seems to me, the geniuses of humanity have arisen—a Plato, a Shakespeare, a Goethe, a Beethoven. But they do not last; they do not transmit their greatness; if they leave descendants at all, these never inherit the whole greatness of their father, and we can easily understand this, since the greatness does not depend upon a single character, but upon a particular combination of many high mental qualities (Anlagen). Geniuses, therefore, probably never raise the average of the race through their descendants; they raise the intellectual average only through their own performances, by increasing the knowledge and power handed on by tradition from generation to generation. But the raising of the average of mental capacity, which has undoubtedly taken place to a considerable degree from the Australasian aborigines to the civilized peoples of antiquity and of our own day, can only depend on the struggle for existence between individuals and races.
But if the human mind has been raised to its present level through the same slow process of selection by means of which all evolution has been directed and raised to the height necessary for the 'desired end,' we must see in this a definite indication that even the greatest mind among us can never see beyond the conditions which limit our capacity for existence, and that now and for all time we cannot hope to understand what is supernatural. We can recognize the stars in the heavens, it is true, and after thousands of years of work we have succeeded in determining their distance, their size, and gravity, as well as their movements and the materials of which they are composed, but we have been able to do all this with a thinking power created for the conditions of human existence upon the earth, that is to say, developed by them, just as we do not only grasp with our hands, but may also play the piano with them. But all that involves a higher thinking power that would enable us to recognize the pseudo-ideas of everlastingness and infinity, the limits of causality, in short, all that we do not know but regard as at best a riddle, will always remain sealed to us, because our intelligence did not, and does not, require this power to maintain our capacity for existence.
I say this in particular to those who imagine they have summed up the whole situation when they admit that much is still lacking to complete knowledge, say, to a true understanding of the powers of Nature or of the Psyche, but who do not feel that in spite of all our very considerably increased knowledge we stand before the world as a whole as before a great riddle. But I say it also to those who fear that the doctrine of evolution will be the overthrow of their faith. Let them not forget that truth can only be harmful, and may even be destructive, when we have only half grasped it, or when we try to evade it. If we follow it unafraid, we shall come now and in the future to the conclusion that a limit is set to our knowledge by our own minds, and that beyond this limit begins the region of faith, and this each must fashion for himself as suits his nature. In regard to ultimate things Goethe has given us the true formula, when the 'Nature-spirit' calls to Faust, 'Du gleichst dem Geist, den Du begreifst, nicht mir!' For all time Man must repeat this to himself, but the need for an ethical view of the world, a religion, will remain, though even this must change in its expression according to the advance of our knowledge of the world.
But we must not conclude these lectures in a spirit of mere resignation. Although we must content ourselves without being able to penetrate the arcana of this wonderful world, we must remain conscious, at the same time, that these unfathomable depths exist, and that we may 'still verehren was unerforschlich ist' (Goethe). But the other half of the world, I mean the part which is accessible to us, discloses to us such an inexhaustible wealth of phenomena, and such a deep and unfailing enjoyment in its beauty and the harmonious interaction of the innumerable wheels of its marvellous mechanism, that the investigation of it is quite worthy to fill our lives. And we need have no fear that there will ever be any lack of new questions and new problems to solve. Even if Mankind could continue for centuries quietly working on in the manifold and restless manner that has, for the first time in the history of human thought, characterized the century just gone, each new solution would raise new questions above and below, in the immeasurable space of the firmament, as in the world of microscopical or ultramicroscopical minuteness, new insight would be gained, new satisfaction won, and our enthusiasm over the marvel of this world-mechanism, so extraordinarily complex yet so beautifully simple in its operation, will never be extinguished, but will always flame up anew to warm and illumine our lives.