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 arcellae, 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