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.