« 이전계속 »
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.3 i 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 Atridæ 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."
1 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
And further, if species only originate by the reproduction and increase of small varieties, and as these varieties occur in every possible manner, we should expect to find in the present animal and vegetable world a mixed diversity of forms without rule or order. But in reality the species of plants and animals differ plainly and sharply from one another. There are no doubt species which display many varieties and transitional forms; but they are the exception, and in these single cases the classification of species may perhaps be faulty. But generally speaking we do not find the quantity of transitional forms that Darwin's theory would lead us to expect. Darwin endeavours to meet this fact by assuming that as the new varieties are formed with extreme slowness, the intermediate and common forms which were destroyed in the struggle for existence, and were removed by natural selection, would be found as a rule in the fossils, and not existing now. But, as a matter of fact, we find only sharply defined species among the fossils, and not the countless transitional forms which must have existed according to Darwin's theory. One of the most eminent palæontologists, Professor Göppert of Breslau, ends a paper, 3 “On the Darwinian theory of transmutation in its bearing on fossil plants," with these words, “ This theory finds no support among the fossil flora, nor among the fossil fauna ; this has in my opinion been proved in the most convincing manner by Reuss.” Darwin has no doubt pointed out that some æons, just as the hypothesis of autogeny has recourse to the primæval age, and “the conditions which once were different.” 1 Baer, Studien, p. 291.
? Pfaff, Grundriss, p. 283. 8 Neues Jahrbuch, etc., von Leonhard und Geinitz, 1865, p. 297.
transitional forms have been found, but these are exceptions which occur comparatively seldom, and here again his principal argument is a purely negative one. It is this; our knowledge of the extinct organisms is said to be very imperfect, and it will always remain imperfect, because many organisms have been entirely destroyed by geological processes. But we can hardly suppose that by some accident only the pure species have been preserved, or have been found up till now, and that the transitional forms have been destroyed or are still unknown. At any rate in this respect also the facts as known at present tell much more against than for the Darwinian theory. It is of course possible that the further progress of research will put many things in a more favourable light; but such possibility is no proof.
I have already said that many scientific inquirers believe the fertility and sterility of the crosses to be a sure test of the species and the variety; this no doubt is not exactly decisive, but at any rate the facts as regards this also, so far as they are at present known, are rather against than for Darwin. I must admit that it is doubtful whether this holds good in the case of some animals. Hæckel, for instance, asserts that permanently fertile hybrids have been bred since 1850 from hares and rabbits in France; and that he himself possesses specimens of this new species, which he calls “Lepus Darwinii.” 3 But of this intercrossing of hares
1 Baer, Studien, p. 375.
* Pfaff, Grundriss, p. 397. A. Wigand, Der Darwinismus, i. 295 : “Where the materials for comparison exist, we are told the time has been too short for change (see above, p. 67); when we grant long periods of time, we are told that the materials for comparison do not exist ; a capital seesaw.”
3 Naturliche Schopfungsgeschichte, p. 131.
and rabbits, which is a regular industry in France, Quatrefages says:1 “ The descendants of these hybrid breeds, Leporidæ, do no doubt reproduce themselves; but it has been observed that after a few generations they degenerate, and again become pure rabbits if they are crossed with one another, and if the race is not renewed from time to time by intercrossing with hares.” Therefore, and this fact is very awkward for the Darwinian theory, Hæckel's Lepus Darwinii does not constitute a new species or distinct race, standing between hares and rabbits; it is an intermediate form which can be produced by crossing, but which does not last, and eventually goes back to the original form. This seems to support the theory that the species are real and lasting, and that the varieties and intermediate forms which arise in the progress of time do not become distinct species, but disappear again. Again Darwin's assertion, that the sterility of hybrids and the temporary character of hybrid races is only a difficulty at first, and that it may be overcome in time, just as in acclimatizing plants and animals success is often only reached after repeated efforts, is simply an assumption and an appeal to possibilities. It may be so; one of the good results of the Darwinian theory is that it stimulates us to continual careful experiments and observations, which, whatever may be their final result, will advance our knowledge of nature. Therefore Darwin has the merit of having reopened a question which was believed to be decided, and of having made it a subject for scientific observation. But he and his followers have no right to
i Quatrefages, Ch. Darwin, etc., p. 253.