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.