s
coordination of behaviour is accompanied by a correlation of the modes
of primary experience. We may classify the instinctive modes of
behaviour and their accompanying modes of instinctive experience under
as many heads as may be convenient for our purposes of interpretation,
and label them instincts of self-preservation, of pugnacity, of
acquisition, the reproductive instincts, the parental instincts, and
so forth. An instinct, in this sense of the term (for example the
parental instinct), may be described as a specialised part of the
primary tissue of experience differentiated in relation to some
definite biological end. Under such an instinct will fall a large
number of particular and often well-defined modes of behaviour, each
with its own peculiar mode of experience.

It is no doubt exceedingly difficult as a matter of observation and of
inference securely based thereon to distinguish what is primary from
what is in part due to secondary acquisition--a fact which Darwin
fully appreciated. Animals are educable in different degrees; but
where they are educable they begin to profit by experience from the
first. Only, therefore, on the occasion of the first instinctive act
of a given type can the experience gained be regarded as _wholly_
primary; all subsequent performance is liable to be in some degree,
sometimes more, sometimes less, modified by the acquired disposition
which the initial behaviour engenders. But the early stages of
acquisition are always along the lines predetermined by instinctive
differentiation. It is the task of comparative psychology to
distinguish the primary tissue of experience from its secondary and
acquired modifications. We cannot follow up the matter in further
detail. It must here suffice to suggest that this conception of
instinct as a primary form of experience lends itself better to
natural history treatment than Darwin's conception of an impelling
force, and that it is in line with the main trend of Darwin's thought.

In a characteristic work,--characteristic in wealth of detail, in
closeness and fidelity of observation, in breadth of outlook, in
candour and modesty,--Darwin dealt with _The Expression of the
Emotions in Man and Animals_. Sir Charles Bell in his _Anatomy of
Expression_ had contended that many of man's facial muscles had been
specially created for the sole purpose of being instrumental in the
expression of his emotions. Darwin claimed that a natural explanation,
c