cerebral surface is shown not only in the general size of the organ, but to a still greater extent in the irregular creasing and furrowing of the surface.  This creasing and furrowing begins to occur in the higher mammals, and in civilized man it is carried to an astonishing extent.  The amount of intelligence is correlated with the number, the depth, and the irregularity of the furrows.  A cat’s brain has a few symmetrical creases.  In an ape the creases are deepened into slight furrows, and they run irregularly, somewhat like the lines in the palm of your hand.  With age and experience the furrows grow deeper and more sinuous, and new ones appear; and in man these phenomena come to have great significance.  The cerebral surface of a human infant is like that of an ape.  In an adult savage, or in a European peasant, the furrowing is somewhat marked and complicated.  In the brain of a great scholar, the furrows are very deep and crooked, and hundreds of creases appear which are not found at all in the brains of ordinary men.  In other words, the cerebral surface of such a man, the seat of conscious mental life, has become enormously enlarged in area; and we must further observe that it goes on enlarging in some cases into extreme old age.[6]

Putting all these facts together, it becomes plain that in the lowest animals, whose lives consist of sundry reflex actions monotonously repeated from generation to generation, there can be nothing, or next to nothing, of what we know as consciousness.  It is only when the life becomes more complicated and various, so that reflex action can no longer determine all its movements and the higher nerve-centres begin to be evolved, that the dawning of consciousness is reached.  But with the growth of the higher centres the capacities of action become so various and indeterminate that definite direction is not given to them until after birth.  The creature begins life as an infant, with its partially developed cerebrum representing capabilities which it is left for its individual experience to bring forth and modify.

VI.

Lengthening of Infancy, and Concomitant Increase of Brain-Surface.

The first appearance of infancy in the animal world thus heralded the new era which was to be crowned by the development of Man.  With the beginnings of infancy there came the first dawning of a conscious life similar in nature to the conscious life of human beings, and there came, moreover, on the part of parents, the beginning of feelings and actions not purely self-regarding.  But still more, the period of infancy was a period of plasticity.  The career of each individual being no longer wholly predetermined by the careers of its ancestors, it began to become teachable.  Individuality of character also became possible at the same time, and for the same reason.  All birds and mammals which take care of their young are teachable, though in very various degrees, and all in like manner show individual peculiarities of disposition, though in most cases these are slight and inconspicuous.  In dogs, horses, and apes there is marked teachableness, and there are also marked differences in individual character.