These differences among men are too plainly marked to be overlooked. The usual judgment is, however, that they are due to heredity. This claim I will not argue; I shall merely show that the differences lend themselves to another interpretation. As the mind goes through successive stages in its development after conception, may not each stage have an environmental complement which reacts on it and helps or hinders its growth? In any case, when we examine the two contrasted physical types from this point of view, some claims may be made as to their genetic meaning. The first type has been retarded early in pregnancy; the other at a later period. The retardation in the one case may be due to defective nutrition or excessive sex excitation; in the other it is perhaps the result of irritants in the mother's system. Facts that make satisfactory evidence in support of these suppositions are hard to obtain, but a justifiable theoretical position is taken by assuming that, in the one case, the child is carried an abnormally long time in the womb, while in the other birth is premature.
To render my classification clear it is important to contrast the stages in a child's development that occur before and after birth. The prenatal stages are physical, and physical defects are cases either of prenatal retardation or acceleration before birth. Postnatal development, on the other hand, is mainly mental, and mental defects have their origin in the association of ideas, which comes necessarily after birth. This simple distinction students of development fail to make; consequently, they confuse relations which would otherwise be obvious.
Let me carry my contrast one step further. The sensory development of a child is prenatal; the motor development is postnatal. The delay of motor development is due to the fact that bones are needed to serve as fulcrums on which the muscles act. These bones can not harden until after birth. The head is formed before birth; the bones solidify after birth. It is, of course, the difficulty of child-bearing that causes the delay of motor development. The sensory stage precedes the motor stage of growth by several years, and from this fact important consequences follow. At birth the sensory powers are fairly complete. The stomach is ready for food, and the circulatory system is active. The early impressions of the child come from these sources alone; it lacks the motor coordinations which make adjustment to the environment effective. Immediately after birth, all impressions are sensory, and are bound together by mental associations in which there are no motor elements. Such associations may easily become disjustive.
The mental life of a child should be pictured as arising from the activity of a number of partially organized psychic centers. Each center has stored up some latent energy which becomes active when adjacent centers are aroused. A stimulus started by any external disturbance excites these centers to activity with the result that a mental impression is formed. A succession of these arousals fix definite grooves along
