On the other hand, the most marvellous displays of instinct are to be found not only in plants, but also in those lowest organisms of the simplest bodily form which are partly unicellular, and in respect of conscious intelligence stand far below the higher plants—to which, indeed, any kind of deliberative faculty is commonly denied. Even in the case of those minute microscopic organisms that baffle our attempts to classify them either as animals or vegetables, we are still compelled to admire an instinctive, purposive behaviour, which goes far beyond a mere reflex responsive to a stimulus from without; all doubt, therefore, concerning the actual existence of an instinct must be at an end, and the attempt to deduce it as a consequence of conscious deliberation be given up as hopeless. I will here adduce an instance as extraordinary as any we yet know of, showing, as it does, that many different purposes, which in the case of the higher animals require a complicated system of organs of motion, can be attained with incredibly simple means.
Arcella vulgaris is a minute morsel of protoplasm, which lives in a concave-convex, brown, finely reticulated shell, through a circular opening in the concave side of which it can project itself by throwing out pseudopodia. If we look through the microscope at a drop of water containing living arcellæ, we may happen to see one of them lying on its back at the bottom of the drop, and making fruitless efforts for two or three minutes to lay hold of some fixed point by means of a pseudopodium. After this there will appear suddenly from two to five, but sometimes more, dark points in the protoplasm at a small distance from the circumference, and, as a rule, at regular distances from one another. These rapidly develop themselves into well-defined spherical air vesicles, and come presently to fill a considerable part of the hollow of the shell, thereby driving part of the protoplasm outside it. After from five to twenty minutes, the specific gravity of the arcella is so much lessened that it is lifted by the water with its pseudopodia, and brought up against the upper surface of the water-drop, on which it is able to travel. In from five to ten minutes the vesicles will now disappear, the last small point vanishing with a jerk. If, however, the creature has been accidentally turned over during its journey, and reaches the top of the water-drop with its back uppermost, the vesicles will continue growing only on one side, while they diminish on the other; by this means the shell is brought first into an oblique and then into a vertical position, until one of the pseudopodia obtains a footing and the whole turns over. From the moment the animal has obtained foothold, the bladders become immediately smaller, and after they have disappeared the experiment may be repeated at pleasure.
The positions of the protoplasm which the vesicles fashion change continually; only the grainless protoplasm of the pseudopodia develops no air. After long and fruitless efforts a manifest fatigue sets in; the animal gives up the attempt for a time, and resumes it after an interval of repose.
Engelmann, the discoverer of these phenomena, says (Pflüger’s Archiv für Physologie, Bd. II.): “The changes in volume in all the vesicles of the same animal are for the most part synchronous, effected in the same manner, and of like size. There are, however, not a few exceptions; it often happens that some of them increase or diminish in volume much faster than others; sometimes one may increase while another diminishes; all the changes, however, are throughout unquestionably intentional. The object of the air-vesicles is to bring the animal into such a position that it can take fast hold of something with its pseudopodia. When this has been obtained, the air disappears without our being able to discover any other reason for its disappearance than the fact that it is no longer needed. . . . If we bear these circumstances in mind, we can almost always tell whether an arcella will develop air-vesicles or no; and if it has already developed them, we can tell whether they will increase or diminish . . . The arcellæ, in fact, in this power of altering their specific gravity possess a mechanism for raising themselves to the top of the water, or lowering themselves to the bottom at will. They use this not only in the abnormal circumstances of their being under microscopical observation, but at all times, as may be known by our being always able to find some specimens with air-bladders at the top of the water in which they live.”
If what has been already advanced has failed to convince the reader of the hopelessness of attempting to explain instinct as a mode of conscious deliberation, he must admit that the following considerations are conclusive. It is most certain that deliberation and conscious reflection can only take account of such data as are consciously perceived; if, then, it can be shown that data absolutely indispensable for the arrival at a just conclusion cannot by any possibility have been known consciously, the result can no longer be held as having had its source in conscious deliberation. It is admitted that the only way in which consciousness can arrive at a knowledge of exterior facts is by way of an impression made upon the senses. We must, therefore, prove that a knowledge of the facts indispensable for arrival at a just conclusion could not have been thus acquired. This may be done as follows: [111] for, Firstly, the facts in question lie in the future, and the present gives no ground for conjecturing the time and manner of their subsequent development.
Secondly, they are manifestly debarred from the category of perceptions perceived through the senses, inasmuch as no information can be derived concerning them except through experience of similar occurrences in time past, and such experience is plainly out of the question.
It would not affect the argument if, as I think likely, it were to turn out, with the advance of our physiological knowledge, that all the examples of the first case that I am about to adduce reduce themselves to examples of the second, as must be admitted to have already happened in respect of many that I have adduced hitherto. For it is hardly more difficult to conceive of à priori knowledge, disconnected from any impression made upon the senses, than of knowledge which, it is true, does at the present day manifest itself upon the occasion of certain general perceptions, but which can only be supposed to be connected with these by means of such a chain of inferences and judiciously applied knowledge as cannot be believed to exist when we have regard to the capacity and organisation of the animal we may be considering.
An example of the first case is supplied by the larva of the stag-beetle in its endeavour to make itself a convenient hole in which to become a chrysalis. The female larva digs a hole exactly her own size, but the male makes one as long again as himself, so as to allow for the growth of his horns, which will be about the same length as his body. A knowledge of this circumstance is indispensable if the result achieved is to be considered as due to reflection, yet the actual present of the larva affords it no ground for conjecturing beforehand the condition in which it will presently find itself.
As regards the second case, ferrets and buzzards fall forthwith upon blind worms or other non-poisonous snakes, and devour them then and there. But they exhibit the greatest caution in laying hold of adders, even though they have never before seen one, and will endeavour first to bruise their heads, so as to avoid being bitten. As there is nothing in any other respect alarming in the adder, a conscious knowledge of the danger of its bite is indispensable, if the conduct above described is to be referred to conscious deliberation. But this could only have been acquired through experience, and the possibility of such experience may be controlled in the case of animals that have been kept in captivity from their youth up, so that the knowledge displayed can be ascertained to be independent of experience. On the other hand, both the above illustrations afford evidence of an unconscious perception of the facts, and prove the existence of a direct knowledge underivable from any sensual impression or from consciousness.
This has always been recognised, [113] and has been described under the words “presentiment” or “foreboding.” These words, however, refer, on the one hand, only to an unknowable in the future, separated from us by space, and not to one that is actually present; on the other hand, they denote only the faint, dull, indefinite echo returned by consciousness to an invariably distinct state of unconscious knowledge. Hence the word “presentiment,” which carries with it an idea of faintness and indistinctness, while, however, it may be easily seen that sentiment destitute of all, even unconscious, ideas can have no influence upon the result, for knowledge can only follow upon an idea. A presentiment that sounds in consonance with our consciousness can indeed, under certain circumstances, become tolerably definite, so that in the case of man it can be expressed in thought and language; but experience teaches us that even among ourselves this is not so when instincts special to the human race come into play; we see rather that the echo of our unconscious knowledge which finds its way into our consciousness is so weak that it manifests itself only in the accompanying feelings or frame of mind, and represents but an infinitely small fraction of the sum of our sensations. It is obvious that such a faintly sympathetic consciousness cannot form a sufficient foundation for a superstructure of conscious deliberation; on the other hand, conscious deliberation would be unnecessary, inasmuch as the process of thinking must have been already gone through unconsciously, for every faint presentiment that obtrudes itself upon our consciousness is in fact only the consequence of a distinct unconscious knowledge, and the knowledge with which it is concerned is almost always an idea of the purpose of some instinctive action, or of one most intimately connected therewith. Thus, in the case of the stag-beetle, the purpose consists in the leaving space for the growth of the horns; the means, in the digging the hole of a sufficient size; and the unconscious knowledge, in prescience concerning the future development of the horns.