The meaning of heredity

The meaning of heredity becomes clear, then, as the prenatal environment induces more and more specificity in a growing mass which accepts increasing specificity as it accepts oxygen or water. This increase of specificity involves the gradual freeing of each tissue, step by step, from the intimate interdependence which at first characterizes its relations with other tissues. Inherited dispositions become clearer and clearer as more and more of the particularized make-up of the individual, its way of interacting with the environment, becomes evident. The degree of independence of each tissue from other tissues--the degree to which it carries out its own task without submitting to the control of environing tissues--is largely a question of its maturity, as demonstrated in the work of experimental embryologists. Tissues that are grafted very early in embryonic development, say in the period of gastrulation, take on in rather complete detail not only the functional role but the anatomical form which their position requires. Cells which when left to their own devices in saline solution become skin or hair cells may, when grafted in a region destined to become an eye, become typical cells of the eye structure. Let this transplantation be done a little later, and a considerable number of cells will fail to yield to this environing pressure. If it is attempted later still, none of the cells will thus yield. In the process of achieving maturity, each cell has become more refractory to influences from without, except those it accepts in terms of its own basic rhythm. If belly ectoderm from the newt is grafted over the head region of the young axolotl early enough, it takes on the form of the little rods or "balancers" which, in newt organization, characterize the head surfaces, despite the fact that the axolotl itself develops no such head decorations. "The cells have reacted to the lateral head field of the axolotl to the best of their newt knowledge, according to which a balancer is the proper thing to form on the side of the head." Within certain limits, a cell, if young enough, does what the environment tells it to do. There is, perhaps, no more universal generalization about organismic development than the generalization that the younger the organism, the less specific its identity, the more plastic its modalities.

All this illustrates the gradual transformation of any tissue or tissue system into something else; at no point is anything ever "added" to the organism; no "acquired" traits are affixed to it. The genes establish inexorable limits, but what will be done with their potential depends on growth circumstances. Environing pressures do what they can do because tissue potentialities await them; the latter, however, are nothing but potentialities, have no clear destiny of their own. There are many things which a given tissue cannot become, but there is no one thing which it is inexorably destined to become.

It must be noted in the comparisons just made that individual differences still remain within each of the contrasting groups. We never find ourselves obliterating these differences or reducing all humanity to a common pulp. There is a profound response to changed environing conditions--by and large, the earlier the influence the more profound its effects--but we never find infinite plasticity, the complete emptying of individuality into the great sink of a cosmic process. The developmental process that underlies all organismic structuring is a process within specific individual tissues which can be molded into superficially identical forms, but only with varying degrees of reluctance. Traits display varying degrees of modifiability, depending partly upon which traits are involved, partly upon the severity of the environmental pressure, and partly upon the life period during which the pressure is applied. There is always individuality, even in the most extreme environmental pressure, just as there is always a basic humanity appearing through all the individualization which human stuff possesses, in whatever cultural arrangements men may contrive.

These individual differences call at times for a statistical evaluation in which it is legitimate and important to separate (for purposes of abstraction) the contributions of nature and nurture. There is a large difference between the method most suitable for approaching the individual and the method most suitable for reaching a quantitative generalization about the roles of nature and nurture in a given population. It is true that in every individual case nature-nurture is a single indivisible whole; if we describe an individual, making no comparison with other individuals but simply following the chromosome pattern and the embryonic growth, there is nothing hereditary in the organism, and nothing attributable to environment. Since, however, many creatures come from the same stock--indeed, identical twins come from the same germ cell--there is an advantage in abstracting from the field the aspect which we may call genetic similarity. Similarly, since many organism of different stock may be exposed to essentially the same temperatures, pressures, or hydrogen-ion concentrations, we may abstract from many life histories the aspect of environmental uniformity. We may then suitably set up statistical evaluations of the concrete effects of allowing the stock to vary to such and such a degree in any given case, or allowing the environment to vary to such and such a degree. We may say, for example, that with a given group of cases the stock variations produce offspring which in a given environment differ from one another in length by so many inches. Or we may say that members of the same stock (within specified limits) who are subjected to such and such widely varying temperatures have such and such variations in length of body.

No such statistical statement is ever a quantitative generalization regarding the relative importance of nature and nurture. There is no such thing as a trait that is primarily hereditary or a trait that is primarily environmental; every trait is completely and absolutely a hereditary tendency brought to its fulfillment by a specific environmental pressure. But the variability from person to person in respect to each trait within any sample of people under any sample of conditions can always be treated with respect to the question: How is the variability of individuals related to the variability of stock and of environment? Some stocks are empirically widely variable; others, especially as a result of inbreeding, are relatively invariable. When subjected to roughly uniform environmental forces, there are inevitably wider individual differences in the former than in the latter; and though in the case of the isolated individual we need apply no statistics, we may appropriately attribute the wide variability of the observed members of the first group to the stock variability. In the same way, two roughly equated samples from the same stock may be exposed to two environments, the first narrow, rigid, and homogeneous, the second wide, flexible, and heterogeneous. The observed members of the second group will display a variability in accordance with the range of environmental excitations. Combining the two principles, whenever we have known and measurable variabilities of stock and known and measurable variabilities in environmental pressures, we can calculate the approximate relative contributions made by the two types of factors to the observed variability of developing individuals. For our purposes--namely, the discussion of individual personality with relatively little statistical generalization--the concept of the unity of the specific individual nature-nurture field is as important as the question of degrees of dependence upon stock variation or upon environment variation.

Since this is so, and since the heredity of the individual is a constant, whereas the environment is a variable, our task includes the study of individual continuity through the growth process, showing the ways in which the primordial potentialities of the individual develop as a result of the interaction of the potentials with a constantly changing and expanding environing field. At every point in the expansion of the individual, new hereditary potentialities arise; but at every point in this expansion, what is realized in action is not heredity alone but the field relationships which potentialities exhibit when liberated and focused by the specific requirements of the world.