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sexual selection, is à priori improbable. In artificial selection, the thought, the experience, and the judicious treatment of the breeder must be considered ; if in the so-called natural selection we do not fall back on a design carried out and guided by the Creator,—and this is not included in the system, and is by no means the belief of most Darwinians,—everything must be left to chance; for “nature” is not a person who can reflect and act, and when we say that "nature selects," we are only speaking figuratively. The breeder is obliged, and is able, carefully to choose out those animals whose pairing will probably transmit and increase certain peculiarities; he will therefore ascertain that these peculiarities exist in both males and females. In nature it is left to chance to decide whether two individuals who suit in this respect will come together; and as the characteristics which are requisite in order to form races for the most part only appear occasionally, it will be a very fortunate chance if they are found in the individuals who pair. But if these characteristics are to be fixed and increased, not only must a suitable pair come together once, but throughout a series of generations suitable pairs must always come together; if once an unsuitable pairing takes place, the advantage which was gained is in a great measure lost. Therefore a long and unbroken series of lucky chances is necessary. And there is yet another difficulty, if experienced breeders are correct in saying that at any rate among the more highly organized mammals the intercrossing of closely related animals is not advantageous ; that if such animals are continuously intercrossed, that is, if the so-called incestuous breeding takes place, a race degenerates and dies out. In order to accomplish the results, therefore, which are essential to the Darwinian theory, it is not only necessary that throughout a series of generations just the right pairs should come together by chance, but also it is essential that these animals should not, at least as a rule, be related to one another. This again greatly diminishes the probability.

1 " The breeder has an object, an end in view ; the struggle for existence has only a result, no object." Baer, Studien, p. 424.

Further, in artificial selection it is possible and even necessary that the individuals who are to produce a new race should be isolated so that they only pair together; for if a cross takes place, and the new race pairs with the original stock, the progeny easily reverts to the old stock. For instance, if fantailed pigeons pair with common pigeons, the race degenerates. As I have said, this isolation can be produced in artificial breeding, but who is to take care that in the natural state the races are kept pure, as is necessary if the result contemplated by Darwin is to be attained. Is it chance ? It is a difficult task, so difficult that a German Darwinian, M. Wagner of Munich, thinks it necessary to come to his master's rescue with a new hypothesis. He thinks that if the pairs who are to form a new race are not isolated, the formation of the race is very

difficult and almost impossible. Therefore we must · see how such isolation can be brought about in the

natural state, and so Wagner hits on what he calls

1 Cf. Huber, Die Lehre Darwin's, p. 342. 2 Cf. Huber, Op. cit. p. 249. 3 Cf. Baer, Studien, p. 348.

“The law of migration of organisms."1 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. 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.

i Die Darwinische Theorie und das Migrationsgesetz der Organismen, Leipzig 1868. Cf. A. Weismann, Ueber den Einfluss der Isolirung auf die Artbildung, Leipzig 1872. Hæckel, Nat. Schöpfungsgeschichte, p. 238. Huber, Die Lehre Darwin's, pp. 162, 257, 294.

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.

1 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 conclude 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

i Huber, Zur Kritik, etc., pp. 4, 23. A. Weismann, Bericht uber die Weiterentwicklung der Descendenz-Theorie im J. 1872. Archiv. fur Anthr. vi. 119–144.

VOL. II.

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