gained be weighed 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, consistent with the
doctrine of evolution, could in many cases be given and would in other
cases be afforded by an extension of the principles he advocated. "No
doubt," he said ("Expression of the Emotions", page 13. The passage is
here somewhat condensed.), "as long as man and all other animals are
viewed as independent creations, an effectual stop is put to our natural
desire to investigate as far as possible the causes of Expression.
By this doctrine, anything and everything can be equally well
explained... With mankind, some expressions... can hardly be understood,
except on the belief that man once existed in a much lower and
animal-like condition. The community of certain expressions in distinct
though allied species... is rendered somewhat more intelligible, if we
believe in their descent from a common progenitor. He who admits on
general grounds that the structure and habits of all animals have been
gradually evolved, will look at the whole subject of Expression in a new
and interesting light."

Darwin relied on three principles of explanation. "The first of these
principles is, that movements which are serviceable in gratifying some
desire, or in relieving some sensation, if often repeated, become
so habitu