molecular relations of protoplasm, whether in its undifferen-
tiated form or as later in the highly conductile form of
nerve fibres. The preponderance of a certain type of
action therefore means the prevalence of a certain structure.
In this way structures are built up under the influence of
natural selection so planned that a stimulus of a given kind
inevitably produces a certain reaction. Such a reaction is a
reflex. It is mechanical in so far that it is the result of a
pre-existing structure which acts uniformly in response to
stimuli of a particular kind. Though it produces a certain
result, it is not the fact that it will produce the particular
result which brings it into being, but the fact that similar
actions have in the past produced similar results. It is
thus a consequence, not a means. It is also in some degree
mechanical in another sense, viz., that it is in part
independent of the rest of the organism and its condition.
But that is a matter of degree, and we get many variations
from the extreme type in which a reflex appears to go on
quite independently of the organic life, up to cases in
which it is dependent on the general state of the organism,
and may be subordinated to some higher type of activity.
This interdependence gives occasion for ambiguity in the
interpretation of special classes of action, but in no
way obscures the principle on which the classification
depends.

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