Similarly, then, we may also assume that the changes. in organisms which have been left to themselves have not been completed with quite such imperceptible slowness as Darwin's own view seems to require, but that after every important change in the conditions of existence there has resulted, as it were by starts, a rapid development of some forms and a retrogression of others. We may very well assume also that every such disturbance of the natural equilibrium produces a tendency to variation, and thus gives opportunity for the origin of new forms, which rapidly establish and perfect themselves when the conditions are favourable to them. All the various principles which modern inquirers have introduced into the doctrine of descent in order to complement the principle of natural selection, as, e.g., migration, the isolation of species, &c., are only more or less happily apprehended special aspects of the decisive main principle of the disturbance of equilibrium, which must necessarily produce the stability of species where the conditions long remain identical.

It is easy to see how by this view of the doctrine of transmutation a great many objections which have been raised against it are at once disposed of, while, on the other hand, Darwin's theory is modified in a very essential point.

Darwin's view so far runs quite parallel with Lyell's geology, in that chief importance is laid upon the silent and continuous, though to ordinary observation imperceptible, changes which are continually going on, but the result of which only becomes apparent in long periods of time. Agreeably with this view, Darwin supposed that modifications of species originally arise quite fortuitously, and that the majority of them again disappear, like ordinary malformations, without leaving any sign, while some few of them, which bring some advantage to their possessors in the struggle for existence, maintain and establish themselves through natural selection and heredity.

We must, of course, admit, even on our view, that very

slow modifications of form may occur, especially where they are produced by very slow modifications of the conditions. of existence, as, e.g., in the gradual elevation or depression of whole countries. But even in this case it will appear to us more probable that the organic forms oppose a certain resistance to the change in their life-conditions, which maintains their state unaltered until, when the disturbing influences reach a certain height, a disturbing crisis breaks in. This does not exclude, however, a gradual modification, and we do not wish our view of the attainment of a condition of equilibrium to be so taken as though it were a condition of absolute immutability. On the other hand, the development of new kinds from the purely fortuitous development of new qualities must indeed be doubted, so far at least as the main lever of the change is supposed to be found here.

Let us again remember that we have to deal with long periods of time, and that the general tendency to variation must have been greatest at the beginning of these periods. Then we can easily see that at a given moment of time the whole series of variations has, as it were, been tried, and that what at the beginning of the period has not led to a new kind will be ever less likely to do so, because the forms are ever slowly becoming more definite and disparate. But if we choose to consider the period which we here regard as the period of adaptation for the relations indicated as at least in itself exclusively governed by the law of the persistence of useful accidents, there arise further objections of different kinds.

First let us suppose that the period of adaptation follows upon a disturbance of equilibrium, and for that very reason involves an increased tendency to variation. Why now are we to exclude all immediate causal connexion between the change of the conditions of existence and the change. of forms? Why, we are even now rightly restoring Lamarck to honour, who derived from immediately efficient causes combined with heredity all modifications of

forms, and therefore, e.g., the increase, strengthening, and development of any organ from its increased use. But here many still unknown forces may be in operation, without our being therefore obliged to take refuge in a mystical intervention of the teleological principle. Fechner even brings in psychical influences too, and that without leaving the circle of the mechanical conception of nature, since psychical phenomena are at the same time physical. "The cock," he says, "has spurs, a crest of feathers, a high red comb. The two first are explained on the principle of the struggle for existence. Cocks on which these had been fortuitously formed conquered their adversaries in fight by means of the spurs, and by the crest were better protected against bites, so that they remained masters of the field. But undoubtedly they must have waited long for these fortuitous arrangements to occur; and when we think that such accidents must be supposed in the case of all other animals, in order to explain the existence of all these adaptations, our brain will grow dizzy. I am more inclined to think that when the organisation was more easily variable than it is now, the psychical effort to be a vigorous match for the foe, to protect oneself against his attack, and the fury against him which still sets the spur in activity, ruffles the crest, and swells the comb, was able, if not to produce these parts by a suitable modification of the nutritive processes in existing cocks, yet to implant the disposition thereto in the germs, and so in their descendants; and here I regard, of course, the psychical efforts and conditions only as the inner side of the physical organisation on which these modifications depended, while I hold the whole play of psychical impulses as connected with their physical basis by the general principle of tendency to stability, without attempting a more precise explanation." 69

We leave the value of this idea undetermined, only observing that there is just as little reason for rejecting 69 Fechner, Einige Ideen, &c., S. 71 f.

it unexamined as for accepting it without proofs. Amongst other phenomena, however, which are difficult to explain from mere selection, there is one in particular which is very widely spread that seems to demand a direct and positive causal connexion between the form and the conditions of life. This is mimicry,' an adaptation extremely common, especially in insects, and leading to the most remarkable illusions, of the form and colour of animals to their environment, or even to other organisms.70

On the general principle this illusive imitation of strange forms seems to agree admirably with natural selection, for it is always a protection to the particular animal against its enemies. We may, therefore, easily suppose that individuals which have fortuitously undergone a modification of this protective kind must have lived longer and exercised a greater influence in the propagation of their kind than others. If this be once. admitted, the protective adaptation of form and colour must necessarily have gone further. But here comes the great difficulty of explaining the first variation of a protective kind. An opponent of Darwin, Mr. Bennet," has pointed out that the resemblance of many insects to the ground on which they live, to the colour of dry bark or fallen leaves, or to the bright colours of the flowers on which they commonly settle, comes about through so long a series of illusive traits and markings, that it is the less possible to admit the sudden appearance of such a variation, as the nearest related species often possess an entirely different aspect. Then Mr. Bennet goes on to argue that a fortuitous occurrence of one portion of this new marking would be of no advantage, because the creature would certainly not have deceived its enemies. But until by mere fortuitous variation, which may just as easily occur

70 Comp. Wallace, Contributions to the Theory of Natural Selection.

71 We follow an address of Mr. Bennet to the British Association at

Liverpool, given in the Naturforscher, iv. No. 15, 1871, S. 118 ff, which is said to have "been approved by very competent inquirers."

in one direction as in another, the whole of the colourmarks and changes of form happen to meet so that the illusion is perfect, requires such a combination of coincidences that the probability against it is enormous. We must also assume enormous periods of time in order that a single such coincidence of all these modifications may be expected. In dealing with the questions of cosmogony, indeed, we have deliberately impugned the blind dread of great numbers; but here the case is very different. Mimicry can only be developed during a period of much the same climatic conditions, in the face of the same enemies and the same vegetation; and these periods must, generally speaking, not be made too long.

Darwin explains protective imitation by supposing the creature to have had originally a certain rough similarity with some element of its environment, so that natural selection would only have to develop further this important beginning, partly by more distinctly marking the protective similarity, partly too by adapting the habits of life to the employment of this protection. In fact this explanation seems the only one which is compatible with the exclusive application of the principle of selection. Instead of the fortuitous concourse of a quantity of delicate lines and combinations of colour, we should thus have a rough primitive whole, which at least in some cases could already deceive enemies, and thus give an impulse to the known process of natural selection. But now it must be observed that there are cases to which this kind of explanation cannot possibly be applied. These are all those cases in which the protective form, and especially the colour, deviates very strongly and strikingly from the forms and colours of the nearest related species. But such cases are uncommonly numerous. Bennet mentions a case where a kind of butterfly deviates very far from all its relatives, which are almost pure white, and imitates the brilliant colours of a butterfly of quite a different class. The latter is poisonous to birds, and is therefore avoided

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