there are forces at work that are of the highest importance, and are
essentially of the same kind as instinct.

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 arcellae, 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