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.