onsistent 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,[167] "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
habitual that they are performed, whether or not of any service,
whenever the same desire or sensation is felt, even in a very weak
degree."[168] The modes of expression which fall under this head have
become instinctive through the hereditary transmission of acquired
habit. "As far as we can judge, only a few expressive movements are
learnt by each individual; that is, were consciously and voluntarily
performed during the early years of life for some definite object, or
in imitation of others, and then became habitual. The far greater
number of the movements of expression, and all the more important
ones, are innate or inherited; and such cannot be said to depend on
the will of the individual. Nevertheless, all those included under our
first principle were at first voluntarily performed for a definite
object,--namely, to escape some danger, to relieve some distress, or
to gratify some desire."[169]

"Our second principle is that of antithesis. The habit of voluntarily
performing opposite movements under opposite impulses has become
firmly established in us by the practice of our whole lives. Hence, if
certain actions have been regularly performed, in accordance with our
first principle, under a certain frame of mind, there will be a strong
and involuntary tendency to the performance of