On Germinal Selection as a Source of Definite Variation

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Weismann August. On Germinal Selection as a Source of Definite Variation

PREFACE

GERMINAL SELECTION

APPENDIX

I. THE REJECTION OF SELECTION

II. CHEMICAL SELECTION

III. VARIATION AND MUTATION

IV. REMARKS ON THE HISTORY OF DEFINITELY DIRECTED VARIATIONS

V. HISTORICAL REMARKS CONCERNING THE ULTIMATE VITAL UNITS

VI. THE INITIAL STAGES OF USEFUL MODIFICATIONS

VII. THE ASSUMPTION OF INTERNAL EVOLUTIONARY FORCES

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Numerous and varied are the objections that have been advanced against the theory of selection since it was first enunciated by Darwin and Wallace—from the unreasoning strictures of Richard Owen and the acute and thoughtful criticisms of Albert Wigand and Nägeli to the opposition of our own day, which contends that selection cannot create but only reject, and which fails to see that precisely through this rejection its creative efficacy is asserted. The champions of this view are for discovering the motive forces of evolution in the laws that govern organisms—as if the norm according to which an event happens were the event itself, as if the rails which determine the direction of a train could supplant the locomotive. Of course, from every form of life there proceeds only a definite, though extremely large, number of tracks, the possible variations, whilst between them lie stretches without tracks, the impossible variations, on which locomotion is impossible. But the actual travelling of a track is not performed by the track, but by the locomotive, and on the other hand, the choice of a track, the decision whether the destination of the train shall be Berlin or Paris, is not made by the locomotive, the cause of the variation, but by the driver of the locomotive, who directs the engine on the right track. In the theory of selection the engine-driver is represented by utility, for with utility rests the decision as to what particular variational track shall be travelled. The cogency, the irresistible cogency, as I take it, of the principle of selection is precisely its capacity of explaining why fit structures always arise, and that certainly is the great problem of life. Not the fact of change, but the manner of the change, whereby all things are maintained capable of life and existence, is the pressing question.

It is, therefore, a very remarkable fact, and one deserving of consideration, that to-day (1895), after science has been in possession of this principle for something over thirty years and during this time has steadily and zealously busied itself with its critical elaboration and with the exact determination of its scope, that now the estimation in which it is held should apparently be on the decrease. It would be easy to enumerate a long list of living writers who assign to it a subordinate part only in evolution, or none at all. One of our youngest biologists speaks without ado of the "pretensions of the refuted Darwinian theory, so called,"5 and one of the oldest and most talented inquirers of our time, a pioneer in the theory of evolution, who, unfortunately, is now gone to his rest, Thomas Huxley, implicitly yet distinctly intimated a doubt regarding the principle of selection when he said: "Even if the Darwinian hypothesis were swept away, evolution would still stand where it is." Therefore, he, too, regarded it as not impossible that this hypothesis should disappear from among the great explanatory principles by which we seek to approach nearer to the secrets of nature.

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The meaning of formative laws here is that definite spots on the surfaces of the wings are linked together in such a manner by inner, invisible bonds, as to represent the same spots or streaks, so that we can predict from the appearance of a point at one spot the appearance of another similar point at another, and so on. It is an undoubted fact that such relations exist, that the markings frequently exhibit a certain symmetry, that—to use the words of the most recent observer on this subject, Bateson8—a meristic representation of equivalent design-elements occurs. But I believe we should be very cautious in deducing laws from these facts, because all the rules traceable in the markings apply only to small groups of forms and are never comprehensive nor decisive for the entire class or even for the single sub-class of diurnal butterflies, in fact, often not so for a whole genus. All this points to special causes operative only within this group.

If internal laws controlled the marking on butterflies' wings, we should expect that some general rule could be established, requiring that the upper and under surfaces of the wings should be alike, or that they should be different, or that the fore wings should be colored the same as or differently from the hind wings, etc. But in reality all possible kinds of combinations occur simultaneously, and no rule holds throughout. Or, it might be supposed that bright colors should occur only on the upper surface or only on the under surface, or on the fore wings or only on the hind wings. But the fact is, they occur indiscriminately, now here, now there, and no one method of appearance is uniform throughout all the species. But the fitness of the various distributions of colors is apparent, and the moment we apply the principle of utility we know why in the diurnal butterflies the upper surface alone is usually variegated and the under surface protectively colored, or why in the nocturnal butterflies the fore wings have the appearance of bark, of old wood, or of a leaf, whilst the hind wings, which are covered while resting, alone are brilliantly colored. On this theory we also understand the exceptions to these rules. We comprehend why Danaids, Heliconids, Euploids, and Acracids, in fact all diurnal butterflies, offensive to the taste and smell, are mostly brightly marked and equally so on both surfaces, whilst all species not thus exempt from persecution have the protective coloring on the under surface and are frequently quite differently colored there from what they are on the upper.

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