Page:Popular Science Monthly Volume 80.djvu/279

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in a thousand ways, physical and social. Types are easy to recognize, but hard to measure. We use many uncertain characteristics to distinguish them, and yet must expect that any one of them will fail if it is put to a definite test.

Some changes, however, are true variations due to the evolution of germ cells. To get at these, different reasoning must be used, but the outcome is the same. As a starting point I shall take a contrast employed by the late Dr. John Eyder, of the University of Pennsylvania. He was fond of asking his students whether the hard parts of the body determined the soft parts or whether the reverse is true, thus making the soft parts determine the hard parts. The ordinary assumption is that bony structures are manifestations of the germ cell determinants. This gives the static measurements on which statistics are based, and from which the ordinary view of heredity is derived. Dr. Eyder's view was the opposite of this. He held that the bony structures were the consequence of the activity of the soft parts and were laid down later. Those parts became solid and unyielding in which the metabolism was defective. The solid ingredients of the blood were deposited there; bony structures thus came into being and seemed a part of heredity, when in reality they were a consequence and not a cause.

This view has not won general acceptance. There is, however, enough truth in it to make certain that these are dynamic characters, which must be measured not in terms of structure, but in bodily activity and its effects. An illustration of this is the contrast between anabolism and katabolism, as is also the increased plasticity manifested in the prolongation of childhood. Plastic brain cells do not result in a single mental trait, but in a change in the whole range of mental activity. The races of slow maturity differ from those rapidly maturing in many traits, and yet they may all be the result of a single variation involving the increased plasticity of brain cells. Among the psychic characters fear is an example of this kind. Cowardice, deceit, falsehood, humility and other traits are clearly the outcome of one fundamental variation. The supplanting of fear by courage would transform a whole civilization, and modify its best known characteristics. Dynamic variations are thus like environmental modifications. Groups of traits change or appear together, due to one primary cause, innate or external. Types are thus formed that differ in a thousand ways and yet are readily referred back to a few ultimate causes.

If types are formed in this way, single visible traits can not be altered unless a change is made in other traits that are due to the same cause. The changes from upland to lowland, from cold to hot climate, from damp to dry regions, or from meager to abundant food modifies many external traits at the same time. Paces of men are formed by each external change which continues long enough to compel an adjustment to it. Each single visible trait does not have an independent