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“The law of migration of organisms.” Individuals of the same species which live in one place, he says, compete with one another as to food and reproduction. This competition causes single individuals to overstep the boundary of the original place of abode, that is, to emigrate. Besides these active migrations, there are also often passive migrations,—that is to say, separate individuals are often carried away from the original dwelling-place by currents of air or water, or by other casualties. The changed conditions of life in the new home cause changes in those individuals who have emigrated, and in their descendants; and if the emigrants are separated from the original home by streams, mountains, and other obstacles, they are sufficiently isolated from the rest of their kind who have remained to form a new race. It cannot be denied that such cases do occur in nature. But it is only in exceptional cases that all these favourable conditions coincide. In many cases the individuals who have emigrated or lost themselves will not be able to exist in the new home ; if the climatic and other natural conditions are very different, they will die out; and if they are not different enough to cause the emigrants to die out, the peculiarities which appear in them will not be very important. Further, it will be very fortunate if the isolation is lasting. In most cases the very places to which former individuals have migrated will be reached later on by other individuals; then the isolation is not complete. * Die Darwinische Theorie und das Migrationsgesetz der Organismen, Leipzig 1868. Cf. A. Weismann, Ueber den Einfluss der Isolirung auf die

Artbildung, Leipzig 1872. Haeckel, Nat. Schöpfungsgeschichte, p. 238. Huber, Die Lehre Darwin's, pp. 162,257, 294.

At any rate Wagner's law of migration is only calculated to explain a few single formations of race; and if it is to produce extended effects we must suppose that numberless migrations and numberless fortunate chances have occurred which may very well be imagined and described on paper, but which by no means take place so often in reality. After all that has been said we may conclude, and I will go no farther to-day, that it is arbitrary to assume, as Darwin does, that natural selection can be compared to man's power of artificial selection, and that whatever is proved to have been accomplished by the latter, can without further ado be ascribed also to the former. As a matter of fact, too, we find that among cultivated plants, garden and pot flowers, vegetables and grains of different kinds, and also among domestic animals, countless varieties and races have developed, whereas in the realm of nature, among the wild plants and animals, the varieties are much less numerous, and far more rarely remain fixed and develop into races." This seems to support the theory that although the species possess more or less mutability, this mutability is not unbounded, but is confined to certain limits. * A. Wigand (Der Darwinismus, i. 47) shows at length that “variability in domestic races and variability in nature are such different things, that it is out of the question to draw any analogy from the former, or to con

clude anything from it as to the variations from which the natural species, races, etc., are supposed to proceed.”

XXVI.
THE THEORY OF DESCENT—Continued.

DARWIN has modified the theory of descent in several ways since the year 1859, and many of those who became believers in this theory, in consequence of the impulse which he gave to it, have sought to support it by other hypotheses in addition to the working of natural and sexual selection." But all those who support the theory of descent unanimously assume that the separate species of plants and animals are descended from a few simple forms, that is, that there exists an unlimited mutability of species. All the careful observations which have been made hitherto go to prove that species is more or less variable, but not that its variability is unlimited. The Giessen botanist Hoffmann made experiments for years with garden plants. He gives the following as the result: “1. There are some very strong varieties which may be made permanent. They are connected with the common stock by sports, intermediate forms, or genealogically. But it has not yet been ascertained whether these varieties can be made permanent in such a manner as to prevent a return to the original form. 2. There are other varieties which cannot be made permanent, and this is 1 Huber, Zur Kritik, etc., pp. 4, 23. A. Weismann, Bericht uber die Weiterentwicklung der Descendenz-Theorie im J. 1872. Archiv. fur Anthr. the commonest case. 3. The extent of the varieties is limited, but it differs with different plants.” And Hoffmann gives this as the general result of his observations, “Experience strengthens the belief that transmutation through the formation of varieties is extensive, but not unlimited ; on the contrary, it is contained within clearly defined limits.” An eminent breeder, Settegast, says the same of domestic animals. There are races which are specially variable, and are therefore well suited for breeding, because their form and qualities adapt themselves easily to the objects of the breeder, which vary according to the varying requirements. On the other hand, there are races which resist all change, and which, as Settegast expresses it, receive all the efforts made by the breeder to vary them with wooden obstinacy. Everywhere the breeder finds a limit ; for instance, in the case of pigs, as Settegast asserts, the strongest influence of breeding, climate, etc., has not been even able to transform diverging rows of teeth into parallel ones, or a long lachrymal bone into a short one.” The observations which have been made about artificial breeding may not yet be complete enough to justify a final conclusion; but so far as they extend at present, they tell much more against than for the supposition of an unlimited mutability of species. But if even artificial selection can only produce limited effects, we must expect, according to what I said in my last lecture, that natural selection and similar influences will produce results still more limited.”

vi. 119–144. WOL. II. E

[graphic]

* Untersuchungen, pp. 25, 26. 2 Cf. Huber, Die Lehre Darwins, p. 227.

* Baer (Studien, p. 383; cf. pp. 245,298) thinks that a limited mutability, a development of “the separate species of a genus, or at most of very closely related genera, from a common ancestral form * is likely.

But this holds good not only of the present, but also of the past, so far as we know it. Ancient figures and sculptures on Egyptian and Assyrian monuments show that the domestic races of Africa and Asia entirely resembled the present ones. The Ibis which exists now exactly resembles the Ibis of the time of Pharaoh, as Cuvier and Humboldt have ascertained from the mummies that have been preserved. In the Egyptian pyramids and in the Swiss Lake dwellings, wheat and barley corns have been found, which cannot be distinguished from the present grain. “The barley,” says Humboldt, in his Geography of Plants, “which fed the horses of the Atridae was undoubtedly similar to that which we now gather in, in harvest. All the plants and animals which now exist on the earth do not seem to have changed their characteristic form for many thousands of years.”

In face of these facts Darwin no doubt maintains that a few thousand years is much too short a time to produce great changes; many thousand years would be necessary for this, as according to his theory they only develop very gradually. But to this it has been replied with perfect justice, that the mere length of time will not help us at all. Time in itself has no effect on the changes; these can only be produced by physical processes in the course of time, and if these processes cannot be discovered by observation within a limited time, it is arbitrary to assume that they might take place in a longer period."

* The transmutation hypothesis in its usual form assumes unlimited mutability; by so doing it contradicts the positive facts of our empirical knowledge, and in order to conceal this weakness it vainly has recourse to

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