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.