Chapter XVIII: Part 18

These facts not only tell against an unknown vitalistic principle of evolution, but in general against the idea of the determination of the great paths of evolution by purely internal causes. If there were a principle of evolution the dissimilarity in the duration of life could not be so excessive; if there were a 'senile stage' of species and a natural death of species comparable to the natural death of individuals, it would not have been possible for most of the Nautilidæ to have been restricted to the Silurian epoch, and yet for one species to have continued to live till now; and if there were a 'tendency' of species to vary persistently onwards, and to 'become further and further removed from the primitive type,' as has been maintained, then such ancient and primitive Cephalopod forms like the _Nautilus_-species could not have persisted until now, but must long ago have Wen transmuted into higher forms. The converse, however, is conceivable enough, namely, that the great mass of the species of a group such as the Nautilidæ were crowded out by superior rivals in the struggle for existence, but that certain species were able to survive on specially protected or otherwise favoured areas. We have a fine example of this in the few still living species of the otherwise extinct class of Ganoid fishes. During the Primary and Secondary epochs these Ganoids peopled all the seas, but at the boundary between the Cretaceous and the Tertiary period they retrograded considerably, simultaneously with the great development of bony fishes or Teleosteans, and now they are only represented by a dozen species distributed over the earth, and most of these are purely river forms, while the others at least ascend the rivers during the spawning season to secure the safety of their progeny. For the rivers are sheltered areas as compared with the seas, and large fishes like the Ganoids will be able there to hold their own in the struggle better than they could in the incomparably more abundantly peopled sea.

Thus I can only regard it as playing with ideas to speak of birth, blossoming, standstill, decay, and death of species in any other than a figurative sense. Undoubtedly the life of the species may be compared with that of the individual, and if the comparison be used only to make clear the difference between the causes of the two kinds of phenomena, there can be no objection to it, only we must beware of thinking we have explained anything we do not know by comparing it with something else that is also unknown.

We have already shown that the natural death of multicellular organisms is a phenomenon which first made its appearance with the separation of the organism into somatic or body cells and reproductive or germ cells, and that death is not an inevitable Nemesis of every life, for unicellular organisms do not necessarily die, though they may be killed by violence. These unicellular organisms have thus no natural death, and we have to explain its occurrence among multicellular organisms as an adaptation to the cellular differentiation, which makes the unlimited continuance of the life of the whole organism unnecessary and purposeless, and even prejudicial to the continuance of the species. For the species it is enough if the germ-cells alone retain the potential immortality of the unicellulars, while, on the other hand, the high differentiation of the somatic cells necessarily involves that they should wear themselves away in the performance of their functions, and so become subject to death, or at least that they should undergo such changes that they are no longer capable of functioning properly, so that thus the organism as a whole loses the power of life.

There can be no doubt whatever that death is virtually implied in the very constitution of a multicellular organism, and is thus, so to speak, a foreseen occurrence, the inevitable end of a development which begins with the egg-cell and reaches its highest point with the liberation of the germ-cells, that is, with reproduction, and then enters on a longer or shorter period of decadence, leading to the natural death of the individual.

It is only by straining the analogy that this course of development can be compared with the origin and transformation or extinction of species. Not even the entirely external analogy of the blossoming from a small beginning and the subsequent decay is always correct; for in the fresh-water snails of Steinheim, at any rate, almost the whole of the members of the species underwent a transformation at a particular time, and became a new species, which was after a long time retransformed without any appreciable decrease in the number of individuals being observable. To speak of a 'senile stage' of the species, of a stiffening of its form, of an incapacity for further transformation, is to indulge in a play of fancy quite inadmissible in the domain of natural science.

It is admitted, however, that there is a correct idea at the base of all this, for many species have not passed over into new forms, but have simply died out because they were unable to adapt themselves to changed conditions. This did not happen because they had become incapable of variation, but because they could not produce variations of sufficient magnitude, or variations of the kind required to enable the species to remain an active competitor in the struggle for existence.

It obviously depends upon the coincidence of manifold circumstances, whether an adaptation can be successfully effected or not. Above all, it must be able to keep pace with the changes in the conditions of life, for if these advance at a more rapid rate the organisms will succumb in the midst of the attempt at adaptation. It is probably in this way that the striking disappearance of the Trilobites is to be explained, as Neumayr has pointed out, for the Nautilidæ, a new group of enemies, multiplied so quickly at their expense that they had not time to evolve any effective means of protection. It cannot be maintained for a moment that every species is able to protect itself against extermination by any other; the increased fertility, the increased rapidity of locomotion, the increased intelligence and similar qualities, may all be insufficient, and then extinction follows; not, however, because the species has become 'senile,' but because the variations possible to its organization do not suffice to maintain it in the struggle.

In discussing germinal selection I mentioned the view expressed by Emery, that excessive variation in the same direction from intra-germinal causes has not rarely been the cause of the extinction of species. I also mentioned the very similar view of Döderlein, who could not refer at that time to germinal selection, but assumed internal compelling forces, which pressed a variation irresistibly forward in the direction in which it had started, even beyond the bounds of what is useful for the desired end, and which might thus bring about the extinction of the species. I cannot entirely agree with these views, as I have already indicated, because I do not believe that the impulse to variation can ever become irresistible and uncontrollable. If it could, then we should not see, as we do, innumerable cases in which the augmentation or diminution of a part has gone on precisely to the point at which it ceases to be purposeful. Even the degeneration of organs only proceeds as far as is necessary to accomplish a particular end, as we see plainly from the parasitic Crustaceans of different orders. In many of these parasitic forms the swimming legs degenerate, but in the female only, because these attach themselves by suckers or in some other manner to their host, so that they cannot let go again. But the males need their swimming legs to seek out the females. The females too require them in their youth, in order to seek out the fish from which they are to obtain their food-supply, and thus the degeneration of the swimming legs has come to a full stop exactly at the point where they cease to be of use; they develop in early youth and degenerate later, when the animal becomes sessile. In accordance with the law of biogenesis we may say that while the degeneration is complete in the final stages of ontogeny, its retrogression was not continued back to the germ, but only to the young stages. From this it follows that the progress of a variation may at any time have a goal fixed for it, and we have seen that this is possible by means of personal selection, which accumulates the never-failing fluctuations of the variation in the direction of plus or of minus. In the individual id a determinant _X_ may perhaps decrease and possibly also increase without limit, although we have no certain knowledge in regard to the latter point, but as this determinant is contained in all the ids, there are always plus and minus fluctuations by means of which personal selection can operate.

But of course it requires a certain amount of time for this, and in the fact that this time is often not available lies, I think, the reason why excessive differentiations have often led to the extinction of a species, not because the increase of the excessive organ must go on irresistibly, but because changes in the conditions have made the exuberant organ inappropriate, and it could not degenerate quickly enough to save the species from extinction.

Brandes has recently given a beautiful illustration of this by associating the existence of the remarkable sabre-toothed tigers (_Machairodus_) with enormously long canine teeth, which lived in the Diluvial period in South America, with the gigantic Armadillos which lived there at the same time, whose bony armature two yards in height now excites our admiration. He rightly points out that the dentition of _Machairodus neogæus_ is by no means a typically perfect dentition for a beast of prey, like that of the Indian tiger or the lion; as far as incisors and molars are concerned it was much less effective than that of these predatory animals, and the great length of the dagger-like flattened canines, which protruded far beyond the mouth, entirely prevented the bringing together of the teeth of the upper and lower jaw after the fashion of a pair of pincers. He rightly infers from this that this dentition was adapted to a specialized mode of nutrition, and he regards the great mailed Armadillos, such as the three-yards-long heavy Glyptodont of the Pampas, as the victims into which they were wont to thrust their sabre-teeth in the region of the unprotected neck, and thus to master the almost invulnerable creature, which was invincible as far as all other predatory animals were concerned. Thus the remarkable dentition is explained on the one hand, and on the other the amazing extent and hardness of the victim's coat of mail. Thus, too, we can understand why there should have been at that time a whole series of cat-like animals with sabre-like teeth, in which the length and sharpness of the teeth increased with the bodily size, for these predatory animals corresponded to a whole series of Armadillos, whose size was increasing, as was also the strength of their armour.

Of course this interpretation is hypothetical, but it contains much internal probability, so that it may be taken as a good illustration of the reciprocal increase of adaptations between two animal groups. We understand now why, on the one hand, this colossal tortoise-like armour should have developed in a mammal, and, on the other hand, why these enormously long sabre-teeth should have been evolved; we also understand--and this is the point with which we are here chiefly concerned--why these two 'excessive' developments should ultimately lead to the destruction of their possessors. For a long period the Armadillos were able to save themselves from extermination by increasing their bodily size and the strength of their armour, and they thus saved themselves from persecution on the part of beasts of prey with smaller and weaker teeth. But the predatory animals followed suit and lengthened their teeth and increased their bodily size, until ultimately even the strongest armour of the victim afforded no efficient protection, and the mighty Glyptodonts were by degrees utterly exterminated. But then the death-knell of the _Machairodus_ had also sounded, for he was so exactly adapted to this one kind of diet that he could no longer overpower other victims and feed on their flesh; the sabre-teeth prevented him from tearing his prey like other predatory animals, he could probably only suck them.

Even if this is a supposititious case, it serves to show that it was not an internal principle of variation that caused the teeth of these carnivores and the armour of their victims to increase so unlimitedly; it was the necessity of adaptation. They did not perish because armour and teeth increased so excessively, but because neither of these adaptations could be neutralized all at once, and small variations were of no use to them in their final struggle for survival.

In a certain sense we may say that simpler, more lowly organisms are more capable of adaptation than those which are highly differentiated and adapted to specialized conditions in all parts of their bodies, since from the former much that is new may arise in the course of time, while very little and nothing very novel can spring from the latter. From the simplest Protozoan the whole world of unicellular organisms could arise, and also the much more diverse Metazoa; from the lower marine worms there could arise not only many kinds of higher marine worms--the segmented worms or Annelids--but also quite new groups of animals, the Arthropods and the Vertebrates. It is hardly likely that a new class of animals will evolve from our modern birds, because these are already so perfectly adapted to their aerial life that they could hardly adapt themselves to life on land or in the water sufficiently well to be able to hold their own in regard to all the possibilities of life with the rest of the dwellers on land or in water. We do indeed know of birds which have returned entirely to a purely terrestrial life--the ostriches, for instance--and of others which have adapted themselves to a purely aquatic life, such as the penguins, but these are small groups of species, and are hardly likely to increase. On the contrary, we can prove that many have already succumbed in the struggle with man, and we anticipate the extermination of others. But the reason why they are so readily exterminated obviously lies in the fact that they have surrendered the advantage given to them by their bird-nature, by adapting themselves to terrestrial life, and that they are not able to regain it, at least not in the short time that is at their disposal if they are to be saved from extermination. The best example of this is the Dodo (_Didus ineptus_). This remarkable-looking bird, of about the size of a swan, lived in flocks upon the island of Mauritius until about the end of the seventeenth century. It had small wings with short quills which were useless for flight. As it could neither escape by flight nor through the water, and could only move clumsily and awkwardly upon land with its short legs and heavy body, it was hopelessly doomed as soon as a stronger enemy made his appearance. It fell a victim to the sailors who first landed on the island and clubbed it with sticks in huge numbers. Until that event it was without doubt excellently adapted to life on that fertile island, for on a volcanic island in the middle of the ocean there were no large enemies, and it was therefore not dependent on the power of flight for safety, and could pick up abundant food from the ground. But when man suddenly appeared on the scene and began to persecute it, it was not the 'senile rigidity' of its organization that prevented it from making use of its wings again; it was the slowness of variation and consequently of selection, which is common to all species, which impelled it to extinction. The same fate will probably overtake the Kiwi of New Zealand (_Apteryx australis_) in the near future, for though it has so far escaped the arrows of the aborigines, it is not likely in its wingless condition to be able to hold out long against European guns, unless close times and preserved forests are instituted for it, as they have been for our chamois.

Even sadder from the biologist's point of view than such extermination of individual species through the vandalism and greed of our own race is the disturbance of whole societies of animals and plants by man that is going on or has been accomplished on most of the oceanic islands, and we must briefly notice these cases while we are dealing with the decadence of species. I refer to the crowding out of the usually endemic animal and plant population on such islands through cultivation. The first step in this work of 'cultivation' is always the cutting down of the forests which for thousands of years have clothed these islands as with a mantle of green, have regulated their rainfall, secured their fertility, and allowed a medley of indigenous animals, usually peculiar to the spot, to arise. We have already spoken of St. Helena. The original and remarkable fauna and flora of this island had for the most part disappeared 200 years ago, through the cutting down of trees in the forests, and these were later wholly destroyed by the introduction of goats, which devoured all the young trees as they grew. But with the forests most of the indigenous insects and birds were doomed to destruction, so that now there is not an indigenous bird or butterfly to be found there; only a few terrestrial snails and beetles of the original fauna still survive.

But it is not only on islands that a large number of species have been decimated or entirely exterminated by deforestation, by the introduction of plants cultivated by man and of the 'weeds' associated with these, and by the importation of domesticated animals. In Central Europe not only have all the larger beasts of prey, like the bear, the lynx, and the wolf, almost completely disappeared, but the reindeer, the bison, the wild ox (Aurochs), and the elk have been exterminated as wild animals, and in North America the buffalo will soon only exist in preserved herds, if that is not already the case. Here, of course, the direct interference of the all-too-powerful enemy, man, has played the largest part in causing the disappearance of the species referred to, but the process may give us an idea of the way in which a superior animal enemy may be able gradually to exterminate a weaker species where there is no attainable or even conceivable variation which might preserve them from such a fate. Several of the mammals which I have mentioned are not yet entirely exterminated; even the Aurochs perhaps still exists in the pure white herds preserved in some British parks; but there are more instances than that of the Dodo of the utter extermination of a species through human agency within historic times. It may be doubtful whether the sea-otter (_Enhydris marina_) has not been already quite exterminated because of its precious fur, but it is quite certain that the huge sea-cow (_Rhytina stelleri_), which lived in large numbers in the Behring Straits at the end of the eighteenth and the beginning of the nineteenth centuries, was completely exterminated by sailors within a few decades.

We may therefore gain from what is going on before our eyes, so to speak, some sort of idea of the way in which the extermination of species may go on even independently of man at the present time, and how it must have gone on also in past ages of the earth's history. Migrations of species have taken place ceaselessly, although very slowly, for every species is endeavouring slowly to extend its range and to take possession of new territories, and thus the fauna and flora of any region must have changed in the course of time, new species must have settled in it from time to time, and the conditions of life must have changed, and in many cases this must have led to the extermination of species, in the same way, though not so quickly, as human interference now brings about their doom.

This is true for plants as for animals. A good example, not indeed of complete extermination, but of very considerable diminution in the numbers of individuals of a plant-species by the advent of a species of mammal, is communicated to us by Chun in regard to Kerguelen Land. A flowering plant, the Kerguelen cabbage (_Pringlea antiscorbutica_), has been greatly reduced in numbers since the thoughtless introduction of rabbits to this uninhabited island (1874). While, in 1840, Captain Ross used this plant in great quantities as a preventative against scurvy in his crew, and even carried away stores to last for months, the Valdivia Expedition in 1898 found rabbits in abundance, but the Kerguelen cabbage had been entirely exterminated at every spot accessible to these prolific and voracious rodents. It was only found growing upon perpendicular cliffs or upon the islands lying out in the fiords.

An avoidance of the threatened destruction of a species by its adaptation to the new circumstances can only be possible when the changes occur very slowly, and will therefore be more likely to be achieved in the case of physical changes in the conditions of life, such as climatic changes, a change in the mutual relations of land and sea, and so on. But it appears that even climatic changes do not evoke any variation and new adaptation as long as the species can avoid the changes by migrating. The often quoted case of Alpine and Arctic plants proves this at any rate, that those species which inhabited the plateaus and highlands of Europe did not all vary to suit the change when a warmer climate prevailed, but that in part at least they followed the climate to which they were already adapted, that is, that they migrated towards the north on the one hand and higher up the Alps on the other. It cannot be denied that many of the insects and plants did adapt themselves at that time to the warmer climate, and became the modern species which now inhabit the plains, for many related species occur on the Alps and in the plains, but apparently many others simply made their escape from a climate which no longer suited their requirements. Thus, as far as I am aware, there is no species of _Primula_ in South or Central Germany which could be derived from the beautiful red _Primula farinosa_ of the Alps, but this species occurs also upon the old glacier-soil at the northern base of the Alps, and in similar soil again in the north of Germany and on the meadows of Holstein. Similar examples might be cited in regard to the Alpine-Arctic butterflies.

It is intelligible enough that we are still very far from being able to give a precise account of the main changes in the plant and animal world during the history of the earth in regard to the special causes which have produced them. Possibly the future will throw more light upon this subject by extending our knowledge of the fossil remains of all countries. But so much at least we can say at present, that there is no reason to refer the dying out of the earlier forms to anything else than the changes in the conditions of life, the struggle for existence, and the limitation of the power of transformation and adaptation due to the organization which the species has already attained; there is no trace of any such thing as a phyletic principle of life in the vitalistic sense, as far as the decadence of species is concerned.

LECTURE XXXVI

SPONTANEOUS GENERATION AND EVOLUTION: CONCLUSION

Spontaneous generation--Experimental tests impossible--Only the lowest and smallest forms of life can be referred to spontaneous generation--Chemical postulates for spontaneous generation--Empedocles modernized--The locality of spontaneous generation--Progress of organization--Direct and indirect influences causing variation--The various modes of selection--Everything depends upon selection--Sinking from heights of organization already attained--Paths of evolution--The forces effecting it--Plasticity of living matter--Predetermination of the animate world--Many-sided adaptation of each group--Aquatic mammals and insects, parasites--Nägeli's variation in a definite direction--Analogy of the traveller--Genealogical trees--The diversity of forms of life is unlimited--The origin of the purposeful apart from purposive forces working towards an end--The limits of knowledge--Limitation of the human intelligence by selection--Human genius--Conclusion.

We have now reached the end of our studies, and they have given us satisfaction, at least in so far that they have brought us certainty in regard to the chief and fundamental question which can be asked in reference to the origin of the modern animate world of organisms. There remains no doubt in our minds that the theory of descent is justified; we know, just as surely as that the earth goes round the sun, that the living world upon our earth was not created all at once and in the state in which we know it, but that it has gradually evolved through what, to our human estimate, seem enormously long periods of time. This conclusion is now firmly established and will never again become doubtful. The assumption, too, that the more lowly organisms formed the beginnings of life, and that an ascent has taken place from the lowest to the higher and highest, has become to our minds a probability verging upon certainty. But there remains one point which we have not yet touched upon--the problem of the origin of these first organisms.

There are only two possibilities: either that they have been borne to our earth from outside, from somewhere else in the universe, or that they have originated upon our earth itself through what is called 'spontaneous generation'--_generatio spontanea_.

The idea that very lowly living organisms might have been concealed within the clefts and crevices of meteorites, and might thus have fallen upon our earth and so have formed the first germs of life, was first formulated by that chemical genius, Justus Liebig. It seems certain that the state of glowing heat in which meteorites are, when they come into our atmosphere, only affects the outer crust of these cosmic fragments, and that living germs, which might be concealed in the depths of their crevices and fissures, might therefore remain alive, but nevertheless it is undoubtedly impossible that any germ should reach us alive in this way, because it could neither endure the excessive cold nor the absolute desiccation to which it would be exposed in cosmic space, which contains absolutely no water. This could not be endured even for a few days, much less for immeasurable periods of time.

But we have to take account, too, of an entirely general reason, which lies in the fact that all life is transient, that it can be annihilated, and is not merely mortal! Everything that is distinctively organic may be destroyed to the extent of becoming inorganic. Not only may the phenomena of life disappear, and the living body as such cease to be, but the organic compounds which form the physical basis of all life are ceaselessly breaking up, and they fall back by stages to the level of the inorganic. It seems to me that we must necessarily conclude from this that the basis of Liebig's idea was incorrect, that is, the assumption that 'organic substances are everlasting and have existed from the first just in the same way as inorganic substances.' This is obviously not the case, for a thing that has an end cannot be everlasting; it must have had a beginning too, and consequently organic combinations are not everlasting, but are transitory; they come and go, they arise wherever the conditions suitable for them occur, and they break up into simpler combinations when these conditions cease to be present. It is only the elements which are eternal, not their combinations, for these are subject to more or less rapid continual change, whether they have arisen outside of organisms or within them.

It seems to me that these considerations destroy the foundations of the hypothesis of the cosmic origin of life on our earth; in any case they leave the hypothesis without great significance; for if we could even admit the possibility of a transference of living organisms from space, the question would only be pushed a little further back by the assumption, and not solved, for the organisms thus brought in must have had their origin on some other planet, since they are, _ex hypothesi_, not everlasting.

Thus we are directed to our earth itself as the place of the origin of the tellurian world of life, and I see no possibility of avoiding the assumption of _spontaneous generation_. It is for me a logical necessity.

Even about the middle of the nineteenth century there was acute discussion in regard to the occurrence of spontaneous generation. In the French Academy especially Pouchet brought forward arguments in favour of it, and Pasteur against it. Pouchet observed that living organisms made their appearance in infusions of hay and other vegetable material in which any possible living germs had presumably been destroyed by prolonged boiling. Living organisms, Algæ, and Infusorians appeared, notwithstanding the fact that the glass bottles in which they were kept were hermetically sealed. But Pasteur showed that the air contains numerous living germs of lowly organisms in its so-called motes, and that, if these were first removed, Pouchet's infusion would not exhibit any signs of life. He caused the air, which was continually passed through the tubes, to stream first along the heated barrel of a gun, and so destroyed these germs, and no organisms were obtained in the infusions. He showed that the air is teeming with germs by an experiment with boiled infusions which were allowed to lie undisturbed for a considerable time in bottles with open necks, one on the roof of the Institute at Paris, the other on the top of the Puy de Dôme in Auvergne, which was at that time still the highest mountain in France. In the Parisian experiment, organisms appeared in the bottles in a very few days, while in those exposed to the pure air at the mountain-top none were seen, even after months had elapsed.

Strangely enough, these and similar experiments were at the time regarded as conclusive proof against the existence of spontaneous generation, though it is obvious enough that the first living being on this earth cannot have sprung from hay, or from any other organic substance, since that would presuppose what we are attempting to explain. After the fiery earth had so far cooled that its outermost layer had hardened to a firm crust, and after water had condensed to a liquid form, there could at first only have been inorganic substances in existence. In order to prove spontaneous generation, therefore, it would be necessary to try to find out from what mingling of inorganic combinations organisms could arise; to prove that spontaneous generation could never have been possible is out of the question.

It would be impossible to prove by experiment that spontaneous generation could _never_ have taken place; because each negative experiment would only prove that life does not arise _under the conditions of the experiment_. But this by no means excludes the possibility that it might arise under other conditions.

Up till now all attempts to discover these conditions have been futile, and I do not believe that they will ever be successful, not because the conditions must be so peculiar in nature that we cannot reproduce them, but above all, because we should not be able to perceive the results of a successful experiment. I shall be able to prove this convincingly without difficulty.

If we ask ourselves the question how the living beings which might have arisen through spontaneous generation must be constituted, and on the other hand, in regard to what kinds of living forms we can maintain with certainty that they _could not_ have arisen thus, it is obvious that we must place on the latter list all organisms which presuppose the existence of others, from which they have been derived. But to this category belong all the organisms which possess a germ-plasm, an idioplasm that we conceive of as composed of primary constituents (_Anlagen_) which have gradually been evolved and accumulated through a long series of ancestors. Thus not only all multicellular animals and plants which reproduce by means of germ-cells, buds, and so forth, but also all unicellular organisms, must be placed in this class. For these last--as we have seen--possess in their nucleus a substance made up of primary constituents, without which the mutilated body is unable to make good its loss, in short, an idioplasm. That this plays the same rôle in unicellular as in multicellular organisms we can infer with the greatest certainty from the process of amphimixis, which runs its course in an analogous way in both cases.

Thus, even though we did not know what Ehrenberg demonstrated in the third decade of last century, that Infusorians in an encapsuled state can be blown about everywhere, and can even be carried across the ocean in the dust of the trade-winds, to re-awaken to life wherever they fall into fresh water, we should still not have remained at the standpoint of Leuwenhoek, who regarded Infusorians as having arisen through spontaneous generation. They cannot arise in this way, nor can they have done so at any time, because they contain a substance made up of primary constituents, which can only be of historic origin, and cannot therefore have arisen suddenly after the manner of a chemical combination.

The same is true of all the unicellular organisms, even of those which are much more simple in structure than the Infusorians, whose differentiation into cortical and medullary substances, oral and anal openings, complex arrangements of cilia and much else, betokens a high degree of differentiation in the cell. But even the Amœba is only apparently simple, for otherwise it could not send out processes and retract them again, creep in a particular direction, encyst itself, and so on, for all this presupposes a differentiation of its particles in different directions, and a definite arrangement of them; and there is in addition the marvellous dividing-apparatus of the nucleus which is not wanting even in the Amœba. All this again points to a historic evolution, a gradual acquiring and an orderly arrangement of differentiations, and such an organism cannot have arisen suddenly like a crystal or a chemical combination.

Thus we are driven back to the lowest known organisms, and the question now before us is whether these smallest living organisms, which are only visible under the highest powers of the microscope, may be referred to spontaneous generation. But here too the answer is, No; for although there is no nucleus to be found, and no substance which we can affirm with any certainty to be composed of primary constituents or idioplasm, we do find distinct traces of a previous history, and not the absolutely simple structure of homogeneous living particles, unarranged in any orderly way, which is all that could be derived from spontaneous generation. It has been shown quite recently that the typhus bacillus possesses an extremely delicate much-branched tuft of flagella, which gives it a tremulous motion, and in the cholera bacillus cortical and medullary substances can be distinguished. Thus even here there is differentiation according to the principle of division of labour, and how numerous must be the minute vital particles of which a substance consists when it can form such fine threads as the flagella just mentioned! Nägeli, who elaborated an analogous train of thought in regard to spontaneous generation, calculated the number of these smallest vital particles (his 'micellæ') which must be contained in a 'moneron' of 0.6 mm. diameter, if we take its dry substance at 10 per cent., and he arrived at the amazing figure of 100 billions of vital particles. Even if we suppose the diameter of such an organism to be 0.0006 mm., it would still be composed, according to this calculation, of a million of these vital particles.

We have reached, in the course of these lectures, the conviction that minute living units form the basis of all organisms, namely, our 'life-bearers' or 'biophors.' These must be present in countless multitudes, and in a great number of varieties in the different forms of life, but all agree in this, that they are simple, that is, they are not composed in their turn of living particles, but only of molecules, whose chemical constitution, combination, and arrangement are such as to give rise to the phenomena of life. But they may vary, and on this power depends the possibility of their differentiation, which has taken place in more and more diverse ways in the course of phylogeny. They, too, arise in the existing organism, like all vital units, only by multiplication of the biophors already present, but they do not necessarily presuppose a historic origin; it is conceivable of them, at least as far as their first and simplest forms are concerned, that they may have arisen some time or other through spontaneous generation. In regard to them alone is the possibility of origin through purely chemico-physical causes, without the co-operation of life already existing, admissible. It is only in regard to them that spontaneous generation is not inconceivable.

We must, therefore, assume that, at some time or other in the history of the earth, the conditions necessary to the development of these invisible little living particles must have existed, and that the whole subsequent development of the organic world must have depended upon an aggregation of these biophors into larger complexes, and upon their differentiation within these complexes.

We shall never be able, then, directly to observe spontaneous generation, for the simple reason that the smallest and lowest living particles which could arise through it, the Biophoridæ, are so extremely far below the limits of visibility, that there is no hope of our ever being able to perceive them, even if we should succeed in producing them by spontaneous generation.

I do not propose to discuss the chemical problem raised by the possible occurrence of spontaneous generation. We have already seen that dead protoplasm, in addition to water, salts, phosphorus, sulphur, and some other elements, chiefly and invariably contains albumen; an albuminoid substance must, therefore, have arisen from inorganic combinations. No one will maintain that this is impossible, for we continually see albuminoid substances produced in plants from inorganic substances, compounds of carbon and nitrogen; but under what conditions this would be possible in free nature, that is, outside of organisms, cannot as yet be determined. Possibly we may some time succeed in procuring albumen from inorganic substances in the laboratory, and if that happens the theory of spontaneous generation will rest upon a firmer basis, but it will not have been experimentally proved even then. For while dead albumen is certainly nearly allied to living matter, it is precisely _life_ that it lacks, and as yet we do not know what kinds of chemical difference prevail between the dead proteid and the living; indeed we must honestly confess that it is a mere assumption when we take for granted that there are only chemico-physical differences between the two. It cannot be proved, in the meantime, that there is not another unknown power in the living protoplasm, a 'vitalistic principle,' a 'life-force,' on the activity of which these specific phenomena of life, and particularly the continually repeated alternation of disruption and reconstruction of the living substance, dissimilation and assimilation, growth and multiplication, depend. It is just as difficult to prove the converse, that it is impossible that chemico-physical forces alone should have called forth life in a chemical substance of very special composition. Although no one has ever succeeded, in spite of many attempts, in thinking out a combination of chemical substances which--as this wonderful living substance does--on the one hand undergoes combustion with oxygen and, on the other hand, renews itself again with 'nutritive' material, yet we cannot infer from this the impossibility of a purely chemico-physical basis of life, but must rather hold fast to it until it is shown that it is not sufficient to explain the facts, thus following the fundamental rule that natural science must not assume unknown forces until the known ones are _proved_ insufficient. If we were to do otherwise we should have to renounce all hope of ever penetrating deeper into the phenomena. And we have no need to do this, for in a general way we can quite well believe that an organic substance of exactly proportioned composition exists, in which the fundamental phenomena of all life--combustion with simultaneous renewal--must take place under certain conditions by virtue of its composition.

How, and under what external conditions, such a substance first arose upon the earth, from and of what materials it was formed, cannot be answered with any certainty in the meantime. Who knows whether the fantastic ideas of Empedocles in an altered form would not be justified here? I mean that, at the time of the first origin of life, the conditions necessary for many kinds of complex chemical combinations may have been present simultaneously on the earth, and that, out of a manifold variety of such substances, only those survived which possessed that marvellous composition which conditioned their continual combustion, but also their ceaseless reconstruction by multiplication. According to Empedocles, there arose from chaos only parts of animals--heads without bodies, arms without trunks, eyes without faces, and so on--and these whirled about in wild confusion and flew together as chance directed them. But those only survived which had united rightly with others so as to form a whole, capable of life. Translated into the language of our time, that would mean what I have just said--that, of a large number of organic combinations which arose, only a few, perhaps one, would possess the marvellously adjusted composition which resulted in life, and with it self-maintenance and multiplication; and that would be the first instance of selection!

But let us leave these imaginings, and wait to see whether the chemists will not possibly be able to furnish us with a starting-point for a more concrete picture of the first origin of life. In the meantime, we must confess that we find ourselves confronted with deep darkness.

The question as to the 'Where' of spontaneous generation must also be left without any definite answer. Some have supposed that life began in the depths of the sea, others on the shore, and others in the air. But who is to divine this, when we cannot even name theoretically the conditions and the materials out of which albuminoid-like substances might be built up in the laboratory? Nägeli's hypothesis still seems to me to have the greatest probability. According to his theory, the first living particles originated not in a free mass of water, but in the reticulated superficial layer of a fine porous substance (clay or sand), where the molecular forces of solid, fluid, and gaseous bodies were able to co-operate.

Only so much is certain, that wherever life may first have arisen upon this earth, it can have done so only in the form of the very simple and very minute vital units, which even now we only infer to be parts of the living body, but which must first have arisen as independent organisms, the 'Biophoridæ.' As these, according to our theory, possessed the character of life, they must have possessed above all the capacity of assimilating in the sense in which the plants assimilate, that is, of renewing their bodily substance continually from inorganic substances, of growing, and of reproducing. They need not on that account have possessed the chemical constitution of chlorophyll, although the capacity of assimilation in green plants depends upon this substance, for we know colourless fungi, which, notwithstanding the absence of chlorophyll, are able to build up the substance of their body from compounds of carbon and nitrogen.

The first advance to a higher stage of life must have been brought about by multiplication, since accumulations of Biophoridæ, unintegrated but connected masses, would be formed.

In this way the threshold of microscopical visibility would gradually be reached and crossed, but--to argue from the modern Baccilli--long before that time a differentiation of the biophors on the principle of division of labour would have taken place within a colony of Biophoridæ. This first step towards higher organization must probably have taken enormous periods of time, for before any differentiation could occur and bring any advantage the unintegrated aggregates of Biophors must first have become orderly, and have formed themselves into a stable association with definite form and definite structure, somewhat analogous to the spherical cell-colonies of _Magosphæra_ or _Pandorina_. Only then was the further step made of a differentiation of the individual biophors forming the colony, and this is comparable to the species of _Volvox_ among the lower Algæ. The gradual ascent of these colonies of biophors must, then, be referred to the principles to which we attribute the ascent of the higher forms of life to ever-higher and ever-new differentiations; the principles of division of labour and selection.

These differentiated colonies of biophors have brought us nearer to the lowest known organisms, among which there are some whose existence we can only infer from their pathological effects, since we have not been able to make them visible. The bacillus of measles has never yet been seen, but we cannot doubt its existence, and we must assume that there are bacilli of such exceeding smallness that we shall never be able to see them, even with the most improved methods of staining and the strongest lenses.

These non-nucleated Monera lead on to the stage of nucleus-formation, and this at once implies the cell. As, on our view, the nucleus is primarily a storehouse of 'primary constituents' (_Anlagen_), its origin must have begun at the moment at which the differentiation of the cell-body reached such a degree of differentiation of its parts that a mechanical division into two halves was no longer possible, and that the two products of division, if they were each to develop to a new and intact whole, required a reserve of primordia (_Anlagen_) to give rise to the missing parts. As this higher differentiation would bring about a superiority over the lower forms of life, in that they would make possible the utilization of new conditions of life, but on the other hand could only survive if the differentiation of a reserve of primary constituents, that is, a nucleus, were introduced at the same time, the development of the nucleus can be ranged under the principle of utility to which we traced back the evolution of all higher and more differentiated forms of life. But it would scarcely be profitable to try to follow out in detail the first steps in the progress of organization under the control of selective processes, since we know far too little about the life of the simplest organisms to be able to judge how far their differentiations are of use in improving their capacity for life.

That would be a bold undertaking even in regard to unicellular organisms, and it is only in the case of multicellular organisms that we can speak with greater certainty and really recognize the changing of the external conditions, in the most general and comprehensive sense, as the fundamental cause of the lasting variations of organic forms. We can here distinguish with certainty between the direct and the indirect effect of external influences, and we see how these sources of variation interact upon each other. The lowest and deepest root of variation is without doubt the direct effect of changed conditions. Without this the indirect effect would have had no lever with which to work, for the primitive beginnings of variation would be absent, and an accumulation of these through personal selection could not take place. It is a primitive character of living substance to be variable, that is, to be able to respond to some extent to changed external conditions, and to vary in accordance with them, or--as we might also say--to be able to exist in many very similar but not identical combinations of substances, and we must imagine that even the first biophors which arose through spontaneous generation were different according to the conditions under which, and the substances from which, they originated. And from each of these slightly different beginnings there must, in the course of multiplication by fission, have been produced a whole genealogical tree of divergent variations of the primitive Biophoridæ, since it is inconceivable that all the descendants would remain constantly under the same conditions of life under which they originated. For every persistent change in the conditions of existence, and especially of nutrition, must have involved a variation in the constitution of the organism, whose vital processes, and especially the repair of its body, depended on these conditions.

But the external influences to which the descendants of a particular form of life were subject never remained permanently the same. Not only did the surface of the earth and its climatic conditions change in the course of time with the cooling of the earth, but mountains arose and were levelled again, old land-surfaces sank out of sight or emerged again, and so on; all that, of course, played its part in the transformation of the forms of life, but did so to any considerable extent only at a later stage, when there were already highly differentiated organisms. These unknown primitive beginnings of life must have been forced to diverge into different variations through the different conditions of the same place in which they lived.

Let us think of the simplest microscopic Monera on the mud of the sea-coast, equipped with the faculty of plant-like assimilation, and we shall see that their unlimited multiplication would cause differences in nutrition, for those lying uppermost would be in a stronger light than those below, and would, therefore, be better nourished, and, consequently, would transmit the variations thus induced to their progeny which arose by fission. Thus it is conceivable that even the more or less favourable position as regards light would bring about the origin of two different races from the same parent form, and as it is conceivable in the case of light, so is it also in regard to all the influences which cause variation in the organism.

We have already seen that variations in the lowest (non-nucleated) forms of life caused by the direct influence of the vital processes may be directly transmitted to the descendants, but that in all those whose bodies have already differentiated into a germ- or idioplasmic-substance, in contrast to a somatic substance in the more restricted sense, this hereditary transmission is only possible in the case of the variations of the germ-plasm, and _hereditary_ variations of the species can only arise by the circuitous route of influencing the germ-plasm. The body (soma) can be caused to change by external influences, by the use or disuse of an organ, but variations of this kind are not transmitted; they do not become a lasting possession of the species, but cease with the individual; they are transient changes.

Thus it was only through those external influences--including those from the soma of the organism itself--which affected the germ-substance, either as a whole or in certain of its primary constituents, that hereditarily transmissible variations of the organism arose, and we have already discussed in detail how particular variational tendencies may arise through the struggle of the parts within the germ-plasm, which may give an advantage to certain groups of primary constituents. And these tendencies are of themselves sufficient to cause the specific type to vary further and further in given directions.

Nevertheless, the infinite diversity of the forms of life could never have been brought about in this way alone, if there had not been another--the _indirect_--effect of the changeful external influences.