Chapter XIX: Part 19

This is due to the fact that the variations of direct origin sooner or later obtain an influence in determining the viability of their possessors, either increasing or diminishing it. It is this, in association with the unlimited multiplication of individuals, which gives a basis to the principle of transformation, which it is the immortal merit of Charles Darwin and Alfred Russel Wallace to have introduced into science: _the principle of selection_. We have seen that this principle may have a much more comprehensive meaning than was attributed to it by either of these two naturalists; that there is not merely a struggle between individuals which brings about their adaptation to their environment, by preserving those which vary in the most favourable way and rejecting those which vary unfavourably, but that there is an analogous struggle between the parts of these individuals, which, as Wilhelm Roux showed, effects the adaptation of the parts to their functions, and that this struggle must be assumed to occur even between the determinants and biophors of the germ-plasm. There is thus a germinal selection, a competition between the smaller and larger particles of the germ-plasm for space and food, and that it is through this struggle that there arise those definitely and purposefully directed variations of the individual, which are transmissible because they have their seat in the immortal germ-plasm, and without which an adaptation of individuals in the sense and to the extent in which we actually observe it would be altogether inconceivable. I have endeavoured to show that the whole evolution of the living world is guided essentially by processes of selection, in as far as adaptations of the parts to one another, and of the whole to the conditions of life, cannot be conceived of as possible except through these, and that all fluctuations of the organism, from the very lowest up to the highest, are forced into particular paths by this principle, by 'the survival of the fittest.' This ends the whole dispute as to whether there are indifferent 'characters' which have no influence on the existence of the species, for even the characters most indifferent for the 'person' would not exist unless the germinal constituents (determinants) which condition them had been victorious in the struggle for existence over others of their kind, and even the 'indifferent' characters, which depend solely upon climatic or other external influences, owe their existence to processes of germinal selection, for those elements of the determinants concerned were victorious which throve best under such influences. But should variations thus produced by external influence increase so far that they become prejudicial to the survival of their bearers, then they are either set aside by personal selection or, if that be no longer possible, they lead to the extinction of the species. Thus the multitude of small individual variations, which probably occur in every species, but which strike us most in Man--the differences in the development of mouth, nose, and eyes, in the hair, in the colour of skin, &c., as far as they are without significance in the struggle for existence--depend upon processes of germinal selection, which permitted the greater development of one group of determinants, or of one kind of biophor in one case, of another in another. The proportionate strength of the elements of the germ-plasm is not readily lost at once, but is handed on to successive generations, and thus even these 'indifferent' characters are transmitted.

It is obvious that, if the principle of selection operates in nature at all, it must do so wherever living units struggle together for the same requirements of life, for space and food, and these units need not be persons, but may represent every category of vital units, from the smallest invisible units up to the largest. For in all these cases the conditions of the selection-process are given: individual variability, nutrition, and multiplication, transmission of the advantage attained, and, on the other hand, limitation of the conditions of existence--especially food and space. The resulting struggle for existence must, in every category of vital units, be most acute between the individual members of each category, as Darwin emphasized in the case of species from the very first, and persistent variations of a species of living units can only be brought about by this kind of struggle. Strictly speaking, therefore, we should distinguish as many kinds of selection-processes as there are categories of living units, and these could not be sharply separated from one another, apart from the fact that we have to infer many of them, and cannot recognize their gradations. Here, as everywhere else, we must break up the continuity of nature into artificial groups, and it seems best to assume and distinguish between four main grades of selective processes corresponding to the most outstanding and significant categories of vital units, namely: Germinal, Histonal, Personal, and Cormal Selection.

Histonal Selection includes all the processes of selection which take place between the elements of the body (soma), as distinguished from the germ-plasm, of the Metazoa and Metaphyta, not only between the 'tissues' in the stricter sense, but also between the parts of the tissues, that is, the lower vital units of which they are composed, and which Wilhelm Roux, when he published his _Kampf der Teile_ ('Struggle of the Parts'), called 'molecules.' It occurs between all the parts of the tissues down to the lowest vital units, the biophors. We must also reckon under histonal selection the processes of selection which take place between the elements of the simplest organisms, and through which these have gradually attained to greater complexity of structure and increased functional capacity. As long as no special hereditary substance had been differentiated, variations which arose in the simplest organisms through selection-processes of this kind were necessarily transmitted to the descendants, but after this differentiation had taken place this could no longer occur--'acquired' modifications of the soma were no longer transmitted, and the importance of histonal selection was limited to the individual. But this form of selection must be of the greatest importance in regard to the adaptations of the parts which develop from the ovum, especially during the course of development, and it is also indispensable all through life in maintaining the equilibrium of the parts, and their adaptation to the varying degree of function required from them (use and disuse). But its influence does not reach directly beyond the life of the individual, since it can only give rise to 'transient' modifications, that is, to changes which cease with the individual life.

In contrast to this is Germinal Selection, which depends upon the struggle of the parts of the germ-plasm, and thus only occurs in organisms with differentiation of somatoplasm and germ-plasm, especially in all Metazoa and Metaphyta--forming in these the basis of all hereditary variations. But not every individual variation to which germinal selection gives rise persists and spreads gradually over the whole species, for, apart from the cases we have already mentioned, in which indifferent variations favoured by external circumstances gain the victory, this happens only if the variations in question are of use to their bearer, the individual. Any variation whatever may arise in a particular individual purely through germinal selection, but it is only the higher form of selection--Personal Selection--that decides whether the variation is to persist and to spread to many descendants so that it ultimately becomes the common property of the species. Germinal and personal selection are thus continually interacting, so that germinal selection continually presents hereditary variations, and personal selection rejects those that are detrimental and accepts those that are useful. I will not repeat any exposition of the marvellous way in which personal selection reacts upon germinal selection, and prevents it from continuing to offer unfavourable variations, and compels it to give rise to what is favourable in ever-increasing potency. Although it apparently selects only the best-adapted _persons_ for breeding, it really selects the favourable id-combinations of the germ-plasm, that is, those which contain the greatest number of favourably varying determinants. We saw that this depends upon the multiplicity of ids in the germ-plasm, since every primary constituent of the body is represented in the germ-plasm, not once only, but many times, and it is always half of the homologous determinants contained in the germ-plasm of an individual which reach each of its germ-cells, always, moreover, in a different combination. Thus, with the rejection of an individual by personal selection, a particular combination of ids, a particular kind of germ-plasm is in reality removed, and thus prevented from having any further influence upon the evolution of the species. By this means germinal selection itself is ultimately influenced, because only those ids remain unrejected in the germ-plasm whose determinants are varying in directions useful to the species. Thus there comes about what until recently was believed to be impossible: the conditions of life give rise to useful directions of variation, not directly, certainly, but indirectly.

We may distinguish as a fourth grade of selection Cormal Selection, that is, the process of selection which effects the adaptation of animal and plant stocks or corms, and which depends on the struggle of the colonies among themselves. This differs from personal selection only in that it decides, not the fitness of the individual person, but that of the stock as a whole. It is a matter of indifference whether the stocks concerned are stocks in the actual material sense, or only in the metaphorical sense of sharing the common life of a large family separated by division of labour. In both cases, in the polyp-stock as well as in the termite or ant-colony, the collective germ-plasm, with all its different personal forms, is what is rejected or accepted. The distinction between this cormal selection and personal selection is, therefore, no very deep one, because here too it is in the long run the two sexual animals which are selected, not indeed only in reference to their visible features, but also in reference to their invisible characters, those, namely, which determine in their germ-plasm the constitution of their neuter progeny or, in the case of polyps, their asexually reproducing descendants.

We venture to maintain that everything in the world of organisms that has permanence and significance depends upon adaptation, and has arisen through a sifting of the variations which presented themselves, that is, through selection. Everything is adaptation, from the smallest and simplest up to the largest and most complex, for if it were not it could not endure, but would perish. The principle which Empedocles announced, in his own peculiar and fantastic way, is the dominating one, and I must insist upon what has so often been objected to as an exaggeration--that everything depends upon adaptation and is governed by processes of selection. From the first beginnings of life, up to its highest point, only what is purposeful has arisen, because the living units at every grade are continually being sifted according to their utility, and the ceaseless struggle for existence is continually producing and favouring the fittest. Upon this depends not only the infinite diversity of the forms of life, but also, and chiefly, the closely associated progress of organization.

It cannot be proved in regard to each individual case, but it can be shown in the main that attaining a higher stage in organization also implies a predominance in the struggle for existence, because it opens up new possibilities of life, adaptations to situations not previously utilizable, sources of food, or places of refuge. Thus a number of the lower vertebrates ascended from the water to the land, and adapted themselves to life on dry land or in the air, first as clumsily moving salamanders, but later as actively leaping frogs; thus, too, other descendants of the fishes gained a sufficient carrying power of limbs to raise the lightened body from the ground, and so attained to the rapid walk of the lizards, the lightning-like leaps of the arboreal agamas, the brief swooping of the flying-dragons, and ultimately the continuous flight which we find in the flying Saurians and the primitive birds of the Jurassic period, and in the birds and bats of our own day.

It is obvious that each of these groups, as it originated, conquered a new domain of life, and in many cases this was such a vast one, and contained so many special possibilities, that numerous subordinate adaptations took place, and the group broke up into many species and genera, even into families and orders. All this did not come about because of some definitely directed principle of evolution of a mysterious nature, which impelled them to vary in this direction and in no other, but solely through the rivalry of all the forms of life and living units, with their enormous and ceaseless multiplication, in the struggle for existence. They were, and they are still, forced to adapt themselves to every new possibility of life attainable to them; they are able to do this because of the power of the lowest vital units of the germ to develop numerous variations; and they are obliged to do it because, of the endless number of descendants from every grade of vital unit, it is only the fittest which survive.

Thus higher types branched off from the lower from time to time, although the parent type did not necessarily disappear; indeed it could not have disappeared as long as the conditions of its life endured; it was only the superfluous members of the parent form that adapted themselves to new conditions, and as, in many cases, these required a higher organization, there arose a semblance of general upward development which simulated a principle of evolution always upwards. But we know that, at many points on this long road, there were stations where individual groups stopped short and dropped back again to lower stages of organization. This kind of retreat was almost invariably caused by a parasitic habit of life, and in many cases this degeneration has gone so far that it is difficult to recognize the relationship of the parasite to the free-living ancestors and nearest relatives. Many parasitic Crustaceans, such as the Rhizocephalids, lack almost all the typical characteristics of the crustacean body, and dispense not only with segmentation, with head and limbs, but also with stomach and intestine. As we have seen, they feed like the lower fungi, by sucking up the juices of their hosts, by means of root-like outgrowths from the place where the mouth used to be. Nevertheless, their relationship to the Cirrhipedes can be proved from their larval stages. There are, however, parasites in the kidneys of cuttlefish--the Dicyemidæ--in regard to which naturalists are even now undecided whether they ought to form a lowly class by themselves between unicellular animals and Metazoa, or whether they have degenerated, by reason of their parasitism, from the flat worms to a simplicity of structure elsewhere unknown. They consist only of a few external cells, which enclose a single large internal cell, possess no organs of any kind, neither mouth nor intestine, neither nervous system nor special reproductive organs. But although degeneration cannot be proved in this case, it can be in hundreds of other cases with absolute certainty, as, for instance, in the Crustacea belonging to the order of Copepods, which are parasitic upon fishes, in which we find all possible stages of degeneration, according to the degree of parasitism, that is, to the greater or less degree of dependence upon the host; for organs degenerate and disappear in exact proportion to the need for them, and they thus show us that degeneration also is under the domination of adaptation.

Thus retrogressive evolution also is based upon the power of the living units to respond to changing influences by variation, and upon the survival of the fittest.

The roots of all the transformations of organisms, then, lie in changes of external conditions. Let us suppose for a moment that these might have remained absolutely alike from the epoch of spontaneous generation onwards, then no variation of any kind and no evolution would have taken place. But as this is inconceivable, since even the mere growth of the first living substance must have exposed the different kinds of biophors composing it to different influences, variation was inevitable, and so also was its result--the evolution of an animate world of organisms.

External influences had a twofold effect at every stage upon every grade of vital unit, namely, that of directly causing variation and that of selecting or eliminating. Not only the biophors, but every stage of their combinations, the histological elements, chlorophyll bodies, muscle-disks, cells, organs, individuals, and colonies, can not only be caused to vary by the external influences to which they are subjected, but can be guided by these along particular paths of variation, so that among the variations which crop up some are better adapted to the conditions than others, and these thrive better, and thus alone form the basis of further evolution. In this way definite tendencies of evolution are produced, which do not move blindly and rigidly onwards like a locomotive which is bound once for all to the railroad, but rather in exact response to the external conditions, like an untrammelled pedestrian who makes his way, over hill and dale, wherever it suits him best.

The ultimate forces operative in bringing about this many-sided evolution are the known--and although we do not recognize it as yet, perhaps the unknown--chemico-physical forces which certainly work only according to laws; and that they are able to accomplish such marvellous results is due to the fact that they are associated in peculiar and often very complex different kinds of combinations, and thus conform to the same sort of regulated arrangements as those which condition the operations of any machine made by man. All complex effects depend upon a co-operation of forces. This is seen, to begin with, in the chemical combinations whose characters depend entirely upon the number and arrangement of the elementary substances of which they consist; the atoms of carbon, hydrogen, and oxygen, which compose sugar, can also combine to form carbonic acid gas and water, or alcohol and carbonic acid gas; and the same thing is true if we ascend from the most complex but still inanimate organic molecules to those chemical combinations which, in a still higher form, condition the phenomena of life, to the lowest living units, the biophors. Not only do these last differ in having life, but they themselves may appear in numerous combinations, and can combine among themselves to form higher units, whose characters and effectiveness will depend upon these combinations. Just as man may adjust various metallic structures, such as wheels, plates, cylinders, and mainsprings in the combination which we call a watch, and which measures time for us, so the biophors of different kinds in the living body may form combinations of a second, third, &c., degree, which perform the different functions essential to life, and by virtue of their specific, definite combination of elementary forces.

But if it be asked, what replaces human intelligence in these purposeful combinations of primary forces, we can only answer that there is here a self-regulation depending upon the characters of the primary vital parts, and this means that these last are caused to vary by external influences and are selected by external influences, that is, are chosen for survival or excluded from it. Thus combinations of living units must always result which are appropriate to the situation at the moment, for no others can survive, although, as we have seen, they must arise. This is our view of the causes of the evolution of the world of organisms; the living substance may be compared to a plastic mass which is poured out over a wide plain, and in its ceaseless flowing adapts itself to every unevenness, flows into every hole, covers every stone or post, leaving an exact model of it, and all this simply by virtue of its constitution, which is at first fluid and then becomes solid, and of the form of the surface over which it flows.

But it is not merely the surface in our analogy which determines the form of the organic world; we must take account not only of the external conditions of existence, but also of the constitution of the flowing mass, the living substance itself, at every stage of its evolution. The combination of living units which forms the organism is different at each stage, and it is upon this that its further evolution depends; this difference determines what its further evolution _may_ be, but the conditions of life determine what it _must_ be in a particular case. Thus, in a certain sense, it was with the first biophors, originating through spontaneous generation, that the whole of the organic world was determined, for their origin involved not only the physical constitution by which the variations of the organism were limited, but also the external conditions, with their changes up till now, to which organisms had to adapt themselves. There can be no doubt that on another planet with other conditions of life other organisms would have arisen, and would have succeeded each other in diverse series. On the planet Mars, for instance, with its entirely different conditions as regards the proportions in weight and volume of the chemical elements and their combinations, living substance, if it could arise at all, would occur in a different chemical composition, and thus be equipped with different characters, and without doubt also with quite different possibilities of further development and transformation. The highly evolved world of organisms which we may suppose to exist upon Mars, chiefly on the ground of the presence of the remarkable straight canals discovered by Schiaparelli, must therefore be thought of as very different from the terrestrial living world.

But upon the earth things could not have been very different from what they actually are, even if we allow a good deal to chance and assume that the form of seas and continents might have been quite different, the folding of the surface into mountains and valleys, and the formation of rents and fissures, with the volcanoes that burst from them, need not have turned out exactly as it has done. In that case many species would never have arisen, but others would have taken their place; on the whole, the same types of species-groups would have succeeded each other in the history of the earth. Let us suppose that the Sandwich Islands, like many other submarine volcanoes, had never risen above the surface of the sea, then the endemic species of snails, birds, and plants which now live there could not have arisen, and if the volcanic group of the Galapagos Islands had arisen from the sea not in their actual situation, but forty degrees further south or north, or 1,000 kilometres further west, then it would have received other colonists, and probably fewer of them, and a different company of endemic species would be found there now. But there would be terrestrial snails and land-birds none the less, and on the whole we may say that both the extinct and the living groups of organisms would have arisen even with different formations of land and sea, of heights and depths, of climatic changes, of elevations and depressions of the earth's crust, at least in so far as they are adaptations to the more general conditions of life and not to specialized ones. The great adaptation to swimming in the sea, for instance, must have taken place in any case; swimming worms, swimming polyps (Medusæ), swimming vertebrates, would have arisen; terrestrial animals would have evolved also, on the one hand from an ancestry of worms in the form of jointed animals and land or freshwater worms, and again from an ancestry of fishes. Aerial animals would also undoubtedly have evolved even if the lands had been quite differently formed and bounded, and I know of no reason why the adaptation to flight should not have been attempted in as many different ways as it has actually been by so many different groups--the insects, the reptiles (the flying Saurians of the Jurassic period), the extinct _Archæopteryx_, the birds, and the bats among mammals.

We can trace plainly in every group the attempt not only to spread itself out as far as possible over as much of the surface of the earth as is accessible to it, but also to adapt itself to all possible conditions of life, as far as the capacity for adaptation suffices. This is very obvious from the fact that such varied groups have striven to rise from life on the earth to life in the air, and have succeeded more or less perfectly, and we can see the same thing in all manner of groups. Almost everywhere we find species and groups of species which emancipate themselves from the general conditions of life in their class, and adapt themselves to very different conditions, to which the structure of the class as a whole does not seem in the least suited. Thus the mammals are lung-breathers, and their extremities are obviously adapted for locomotion on the solid earth, yet several groups have returned to aquatic life, as, for instance, the family of otters and the orders of seals and whales. Thus among insects which are adapted for direct air-breathing, certain families and stages of development have returned to aquatic life, and have developed breathing-tubes by means of which they can suck in air from the surface of the water into their tracheal system, or so-called tracheal gills, into which the air from the water diffuses. But the most convincing proof of the organism's power of adaptation is to be found in the fact that the possibility of living parasitically within other animals is taken advantage of in the fullest manner, and by the most diverse groups, and that their bodies exhibit the most marvellous and far-reaching adaptations to the special conditions prevailing within the bodies of other animals. We have already referred to the high degree reached by these adaptive changes, how the parasite may depart entirely from the type of its family or order, so that its relationship is difficult to recognize. Not only have numerous species of flat worms and round worms done this, but we find numerous parasites among the great class of Crustaceans; there are some among spiders, insects, medusoids, and snails, and there are even isolated cases among fishes.

If we consider the number of obstacles that have to be overcome in existence within other animals, and how difficult and how much a matter of chance it must be even to reach to such a place as, for instance, the intestine, the liver, the lungs, or even the brain or the blood of another animal, and when, on the other hand, we know how exactly things are now regulated for every parasitic species so that its existence is secured notwithstanding its dependence upon chance, we must undoubtedly form a high estimate of the plasticity of the forms of life and their adaptability. And this impression will only be strengthened when we remember that the majority of internal parasites do not pass directly from one host to another, but do so only through their descendants, and that these descendants, too, must undergo the most far-reaching and often unexpected adaptations in relation to their distribution, their penetration into a new host, and their migrations and change of form within it, if the existence of the species is to be secured.

We are tempted to study these relations more closely; but it is now time to sum up, and we must no longer lose ourselves in wealth of detail. Moreover, the life-history of many parasites, and of the tape-worm in particular, is widely known, and any one can easily fill up the story, of which we have given a mere outline. I simply wish to point out that in parasitic animals there is a vast range of forms of life in which the most precise adaptation to the conditions occurs in almost every organ, and certainly at every stage of life, in the most conspicuous and distinct manner. In the earlier part of these lectures we gained from the study of the diverse protective means by which plants and animals secure their existence the impression that whatever is suited to its end (_Das Zweckmässige_) does not depend upon chance for its origin, but that every adaptation which lies at all within the possibilities of a species will arise if there is any occasion for it. This impression is notably strengthened when we think of the life-history of parasites, and we shall find that our view of adaptations as arising, not through the selection of indefinite variations, but through that of variations in a definite direction, will be confirmed. Adaptations so diverse, and succeeding one another in such an unfailing order as those in the life-history of a tape-worm, a liver-fluke, or a _Sacculina_, cannot possibly depend upon pure chance.

Nevertheless, chance does play a part in adaptations and species-transformations, and that not only in relation to the fundamental processes within the germ-plasm, but also in connexion with the higher stages of the processes of selection, as I have already briefly indicated. After the publication of my hypothesis of germinal selection it was triumphantly pointed out that I had at last been obliged to admit a phyletic evolutionary force, the 'definitely directed' variation of Nägeli and Askenazy. This reproach--if to allow oneself to be convinced be a reproach--is based upon a serious misunderstanding. My 'variation in a definite direction' does not refer to the evolution of the organic world as a whole. I do not suppose, as Nägeli did, that this would have turned out essentially as it has actually done, even although the conditions of life or their succession upon the earth had been totally different; I believe that the organic world, its classes and orders, its families and species, would have differed from those that have actually existed, both in succession and appearance, in proportion as the conditions of life were different. My 'variation in a definite direction' is not predetermined from the beginning, is not, so to speak, exclusive, but is many-sided; each determinant of a germ-plasm may vary in a plus or minus direction, and may continue under certain circumstances in the direction once begun, but its components, the different biophors, may do the same, and so likewise may the groups, larger and smaller, of biophors which form the primordia (_Anlagen_) of the organs within the germ-plasm. Thus an enormously large number of variational tendencies is available for every part of the complete organism, and as soon as a variation would be of advantage it arises--given that it is within the possibilities of the physical constitution of the species. It occurs because its potentialities are already present, but it persists and follows a definite course because this is the one that is favoured. In other words, it is primarily fixed by germinal selection alone, but is then preferred by personal selection above the variants running parallel with it. In my opinion the definite direction of the chance germinal variations is determined only by the advantage which it affords to the species with regard to its capacity for existence. But according to Nägeli the direction of a variation is quite independent of its utility, which may or may not exist. From Nägeli's point of view we could never understand the all-prevailing adaptation, but if the utility of a variant is itself sufficient to raise it to the level of a persistent variational tendency, then we understand it.

Years ago (1883) I compared the species to a wanderer who has before him a vast immeasurable land, through which he is at liberty to choose whatever path he prefers, and in which he may sojourn wherever and for as long as he pleases. But although he may go or stay entirely of his own free will, yet at all times his going or staying will be determined--it must be so and cannot be otherwise--by two factors: first, by the paths available at each place--the variations which crop up--and secondly, by the prospects each of these available paths open up to him. He is striving after a restful place of abode which shall afford him comfortable subsistence, his former home having been spoilt for him by increasing expensiveness or too great competition. Even the direction of his first journey will not depend upon chance, since of the many paths available he will, and must, choose that which leads to a habitable and not too crowded spot. If this has been reached--that is to say, if the species has adapted itself to the new conditions--the colonist sets up his abode there, and remains as long as a comfortable existence and a competence are secure; but if these fail him, if grain becomes scarce, or if prices rise, or if a dangerous epidemic breaks out, then he makes up his mind to wander anew, and once more he will choose, among the many available paths, that which offers him the prospect of the speediest and most certain exit from the threatened region, and leads him to another where he may live without risk. There, too, he will remain as long as he is comfortable and not exposed to want or danger, for the species as a whole only becomes transformed when it must. And so it will go on _ad infinitum_; the traveller will, when he is scared away from one dwelling-place, be able to continue his journey in many directions, but he will always select the one path which offers him the best prospects of a comfortable settlement, and will follow it only to the nearest suitable place of abode, and never further. The transformation of a species only goes on until it has again completely adapted itself. In this way he will in the course of years have traversed a large number of different places which, taken together, may lie in a strange and unintelligible course, but this course has nevertheless not arisen through a mere whim, but through the twofold necessity of starting from a given spot--that in which he had previously lived--the constitution of the species, and secondly of choosing the most promising among the many available paths.

But chance does play a part in determining the route of the traveller, for on it depends the nature of the conditions in the surroundings of his previous dwelling-place, when he is forced to make another move; for these conditions change, colonies are extended or depopulated, a town previously cheap becomes dear, competition increases or decreases, disease breaks out or disappears; in short, the chances of a pleasureable sojourn in a particular place may alter and determine the wanderer who is on the point of leaving his place of abode to take a different direction from that which he would probably have chosen, say, ten years earlier.

The analogy might be carried further, as, for instance, to illustrate the possibility of a splitting up of the species; we may suppose that instead of one wanderer there is a pair, who found a family at their first halting-place. Children and grandchildren grow up in numbers and food becomes scarce. One part of the descendants still finds enough to live upon, but the rest set out to look for a new habitation. In this case, too, many paths, sidewards or backwards, stand open to the wanderers, but only those paths will be actually and successfully followed by any company of them which will lead to a habitable place where settlement is possible. If some of the descendants follow paths with no such prospect they will soon turn back or will succumb to the perils of the journey.

It seems to me that the contrast between this and Nägeli's view of the transmutation of species is obvious enough. According to him the wanderer is not free to choose his path, but goes on and on along a definite railway-line that only diverges here and there, and it cannot be foreseen whether the track leads to paradisaic dwellings or to barren wastes--the travellers must just make the best of what they find. They carry a marvellous travelling outfit with them--a sort of _Tischlein, deck' dich_--the Lamarckian principle, but the magic power of this is very doubtful, and it will hardly suffice to guard them against the heat of the deserts, the frost of the Arctic regions, or the malaria of the marshes into which their locomotive blindly carries them.

According to my view, the traveler--that is, the species--has always a large choice of paths, and is able, even while he is on the way, to discern whether he has chosen a right or a wrong one; moreover, in most cases, one or, it may be, a number of the paths lead to the desired dwelling-place. But it also undoubtedly happens that, after long wandering and when many regions have been traversed, a company may finally arrive at a place which is quite habitable and inviting at first sight, but which is surrounded on several sides by the sea or by a rushing stream. As long as the soil remains fertile and the climate healthy all goes well, but when matters change in this respect, and perhaps the only way back lies through marshes and desert land and is therefore impassable, then the colony will gradually die out--that is the death of the species.

But let us now leave our parable and inquire what paths the organic world has actually taken in its transformations, in what succession the individual forms of life have evolved from one another; in short, how the actual genealogical tree of this earth's animate population is really constructed in detail. To this I can only reply that we have many well-grounded suppositions, but only real certainty in regard to isolated cases. Thus the genealogical tree of the horse has been traced far back, and a great deal is known of the phylogeny of several Gastropods and Cephalopods, but in regard to the genealogical tree of organisms as a whole we can only make guesses, many of which are probable, but are never quite certain. The palæontological records which the earth's crust has preserved for us for all the ages are much too incomplete to admit of any certainty. Many naturalists, notably Ernst Haeckel, have done good service in this direction, for from what we know of palæontology, embryology, and morphology, they have constructed genealogical trees of the different groups of organisms, which are intended to show us the actual succession of animal and plant forms. But, interesting as these attempts are, they cannot for the most part be anything more than guesswork, and I need not, therefore, state or discuss them here in any detail, since they can afford us no aid in regard to the problem of the origin of species with which these lectures are concerned. In regard to the animal world at least--and the case of plants is probably very similar--the record of fossil forms fails us at an early stage. Thus the oldest and deepest strata in which fossils can be demonstrated, the Cambrian formation, already contains Crustaceans, animals at a relatively high stage of organization, which must have been preceded by a very long series of ancestors of which no trace has been preserved. The whole basal portion of the animal genealogical tree, from the lowest forms of life at least up to these primitive Crustaceans, the Trilobites, lies buried in the deepest sedimentary rocks raised from the sea-floor, the crystalline schists, in which it is unrecognizable. Enormous pressure and, probably also, high temperature have destroyed the solid parts as far as there were any, and the soft parts have only left an occasional impression even in the higher strata.

Thus enormous periods of time must have elapsed from the beginning of life to the laying down of that deepest 'Palæozoic' formation, the Cambrian, for not only does the whole chain which leads from the Biophoridæ to the origin of the first unicellulars fall within this period, as well as the evolution of these unicellulars themselves into their different classes, and their integration into the first multicellulars, but also the evolution of these last into all the main branches of the animal kingdom as it is now, into Sponges, Starfishes, and their allies, Molluscs, Brachiopods, and Crustaceans, for all these branches appear even in the Cambrian formation, and we may conclude that the worms also, most of which are soft and not likely to be preserved, were abundantly present at that time, since jointed animals like the Crustaceans can only have arisen from worms. Moreover, we have every reason for the assumption that Cœlenterates also, that is to say polyps and medusoids, lived in the Cambrian seas, because their near relatives with a solid skeleton, the corals, are represented in the formation next above, the Silurian. The same is true of the fishes, of which the first undoubtedly recognizable remains, the spines of sharks, have been found in the Silurian. These two presuppose a long preparatory history, and thus we come to the conclusion already stated, that all the branches of the animal kingdom were already in existence when the earth's crust shut up within itself the first records available for us of the ancestors of our modern world of organisms.

Of course at that time the higher branches had only been represented by their lower classes, and this is true especially of vertebrates, so that, from the laying down of the Cambrian strata to the modern world of organisms, a very considerable increase of complexity in structure and an infinite diversifying of new groups must have taken place. Amphibians do not appear to have been present in Cambrian times; reptiles are represented in the Carboniferous strata, but only appear in abundance in Secondary times; birds appear first in the Jurassic, but in a very different guise (_Archæopteryx_) from the modern forms, covered indeed with feathers, but still possessing a reptilian tail; later they occur as toothed birds in the Cretaceous, and in Tertiary times they have their present form. The development of mammals must have run almost parallel with that of birds, that is, from the beginning of Secondary times onwards, and their highest and last member appears, as far as is known to research, only in post-Glacial times, in the Diluvial deposits.

To the types which have arisen since the Cambrian period belongs the class of Insects with its twelve orders and its enormous wealth of known species, now reckoned at 200,000. They are demonstrable first in the Devonian, and then in the Carboniferous period, in forms, just as our theory requires, with _biting_ mouth-organs; it is not until the Cretaceous strata that insects with purely suctorial mouth-organs--bees and butterflies--occur, as it was also at that time that the flowers, which have evolved in mutual adaptation with insects, first appeared.

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.