The least illogical attempt to justify the position of the absolute environmentalist is the comment that since the tremendous range in intellectual attainments shown by different individuals is not paralleled by detectable physical variations, the environment must have more to do with mental than with physical development. But this argument also is fallacious. One may also say that training brings out innate differences in mentality and thus offers a method of detecting the physical differences.

It is regrettable that so many social workers, physicians, and psychologists should tend to minimize the role which heredity plays in human affairs merely because they jump to the erroneous conclusion that if this role is trivial, their own vocations are of more importance. On the contrary, the business of alleviating sorrow, sickness, and mental maladjustment is as momentous from the genetic point of view as from the environmentalist point of view. But even if this were not so, we should still have to face the facts. And the facts are roughly these. Individuals, because of their diverse gene patterns, exhibit marked differences in aptitudes, temperaments, and all other varieties of mental capacity. They have this groundwork laid down at the very beginning of development; and it is altogether likely that, owing to the rigidity of the environment entailed by mammalian physiology, there is great constancy in the unfolding processes which are carried on during the more plastic period of the life history. At puberty, or a little after, growth is finished; and the mind, as well as the body, has reached the state of complete organization known as maturity. But this does not mean that either the mind or the body becomes rigid and inflexible. It may very well be, as Watson claims, that, in a certain sense, the very young child has no mind. It has merely a rather empty brain which can be filled and its behavior conditioned by what fills it. But the brains of children, like clay pots, are of various sizes and qualities. What they will hold, and how much, is largely determined by the genic materials which went into them.

Though to those who are familiar with modern genetic data it is as redundant as two tails to a dog, direct evidence exists on this very point. It is found in various statistical studies of eminent men and their relatives, in studies of identical twins, and in studies of intelligence by means of psychological tests.

Among the extensive studies of eminent men and their relatives are those of Galton, De Candolle, Ellis, Cattell, and Woods. Naturally, such studies are full of pitfalls. It is difficult to select a series of names for study in a wholly objective manner; it is impossible to make accurate allowances for differences in opportunity; and eminence is certainly not a precise measure of intelligence. Yet, in spite of their deficiencies, these analyses have yielded some very significant results. Each investigator finds that the relatives of great men have a very much greater chance of becoming eminent than people selected at random from the general population. The difference is too great to be accounted for by any assumed difference in opportunity. Moreover, it is to be noted that the chance of becoming great varies directly with the closeness of genetic relationship. One must agree with Cattell, of course, that environment holds a veto over mental development, but one cannot avoid the conclusion that environment merely harnesses the forces of heredity.

Such investigations go far toward establishing Galton's important generalization that no man is likely to attain eminence unless he possess innate ability of a high order. They also support his further conclusion, though perhaps they do not establish it, that"few who possess these very high abilities can fail in achieving eminence." In other words, it is hard to keep a good man down. Or woman either. An instance is Helen Keller. Cattell once said that if Isaac Newton had been born among Hottentots, he would have announced no Laws of Motion. True enough. But he would have been the intellectual leader of the tribe. A few Hottentot Isaac Newtons, and the group would have had a different cultural history.

The evidence from identical twins points in the same direction. Identical twins come from a single fertilized egg. Each member of the pair, therefore, has the same heredity. And such twins exhibit mental attributes as well as physical characteristics which are much more nearly alike than are the traits exhibited by other children of the same family. They even tend to show this same similarity when they are separated at an early age and reared under different conditions, though the facts are not altogether harmonious in the few cases of this type that have been adequately studied.

The intelligence tests add to this evidence. Intelligence tests measure acquirements, it is true, and they have been criticized severely on this score. But the critics should realize that there is no more reason why properly safeguarded measures of intellectual acquirements should not be interpreted in terms of genes than that developed physical characters should not be thus interpreted. Two ten-year-old children, for example, show significant differences in their intelligence quotients. They are given the same studies and are tested again at the end of four years. They are now further apart mentally than they were in the earlier test. The more intelligent has advanced further than the less intelligent. What conclusion can be drawn other than that they differed in genetic constitution?

From the sum total of all the evidence geneticists have concluded that mentality is a complex trait inherited like any other complex character. Some of the numerous genes concerned presumably have varied into what one may call plus and minus types; and it takes only 20 of such genic differences, inherited independently, to give the possibility of 1,000,000 different recombinations."If the Thinker requires 20 plus genes and the Simpleton 20 minus genes, then the Average Man may be supposed to have about 10 plus and 10 minus genes. If a family stock, by selective matings, gathers together a preponderant proportion of plus genes, its average worth will rise; conversely, if a family puts its efforts into accumulating minus genes, its social value will drop. But even the mediocrities may produce Thinkers - or Simpletons - if the constitutions of the fusing germ cells are such as to bring together the required genetic complex. Thus there is no difficulty in accounting for emergent individuals like Carlyle and Abraham Lincoln in otherwise undistinguished families. The proportion of eminent persons in such families will be low because of great differences in quality among the parental germ cells. On a percentage basis, selected high-grade families will often produce ten, twenty, or even fifty times as many notables as mediocre families. Yet the absolute number of geniuses appearing in families of the latter type will always be high because mediocrity is plentiful."