Another important group of internal environmental factors may be termed factors of position. With favorable material, such as the sea urchin, the two cells resulting from the first division of the fertilized egg (or even the four-celled and eight-celled stages of later divisions) may be shaken apart. Each cell then produces a normal surface layer over the area where it has previously been in contact, and restores the respiratory conditions necessary to, and characteristic of, the normal egg. The two cells, which would have produced the right and left halves of a single individual, now proceed to develop into complete organisms. With other material, separation of the cells results in the production of right- and left-half organisms. With still other material - the frog is a good example - the egg itself may be cut into two parts; and each part will produce a half individual. What is the cause of these peculiar results? The answer given by the biologists best able to judge is this: Organisms vary somewhat in the detailed maneuvers by which they differentiate; but in every case development is a process of sorting out the various cytoplasmic materials and is directed by the genes under certain restrictions offered by cell position.

The conclusion that the course of development taken by any cell lineage depends upon its relations to other cells is not a speculation or an indirect inference; it is a direct conclusion based on experimental evidence. In the early stages of embryonic development, cells which would have produced parts of the central nervous system will produce skin if transplanted to a region that would normally produce skin. An organization center exists at a given place in the hollow ball of cells comprising the gastrula, and what each cell becomes depends upon its position with reference to this center. The organization center itself can be shifted experimentally, and, following the change, the whole organization pattern of the embryo is altered.

In later stages of development this formative power is lost. Differentiation having passed a certain point, the fate of any given structure is more firmly fixed. Liver cells, or spleen cells, or brain cells, or skin cells remain the cells of these types, no matter where they may be placed. Yet an opportunity for a limited degree of variable development still exists in individuals having identical gene patterns.

In the first place, the developmental machinery is not perfect. Slips may occur; and from these slips some diversity of structure and function may ensue. These workshop errors may affect organs having little influence on general physiological activity, and the result be negligible; but if they retard or accelerate the energy of such essential organs as the ductless glands, the consequences may be more serious. In the second place, an alteration in the physical environment may change the rates at which chemical changes take place, and thus entail poor development or superior development, as the case may be; but the changes induced by such causes are quantitative rather than qualitative. In fact, one should not expect to have the normal development pattern upset markedly by either of these causes. An organism is a dainty piece of machinery; and the higher it is in the evolutionary scheme, the more delicate it turns out to be. If slips in development occur, therefore, or if inhospitable conditions are met, growth is more likely to stop altogether than to continue in an abnormal manner.

These general considerations give us a clearer idea of the processes collectively known as growth; but, after all, they do not answer the question of greatest interest. We want to know precisely what variations can be expected - due to the deviations in environmental factors that are ordinarily confronted during development - in a series of individuals having identical gene patterns. We may admit that the gene equipment makes it possible for an egg to develop into a potato, or a fish, or a man, as the case may be, with the appropriate characteristics which make these organisms distinguishable from one another. We may admit that the genetic constitution even marks out rather definitely what kind of potato or fish or man shall develop. But what are the extreme developmental variations attributable to the environment as a whole?

It is difficult to give an answer that will cover all cases, for various classes of organisms have different powers of adjustment and different factors of sensitivity. The type of plant or animal concerned must therefore be taken into consideration. In general, capacity for adjustment diminishes and sensitivity increases as we go up in the evolutionary scale. Thus the lower orders of plants are less sensitive than the higher orders; aquatic animals are less impressionable than land animals; cold-blooded animals are less responsive than warm-blooded animals. The mammals, and in particular man, are extremely sensitive. These facts give rise to an interesting situation. Experimental alteration of the environment can cause greater effects on the lower organisms than on the higher organisms simply because the latter have developed natural devices which preclude any effective modification of conditions. It is not to be assumed, however, that even the lower organisms have unlimited powers of response. They are able to undergo only such changes as are permitted by the gene pattern; and these changes are limited in type and degree. An experimenter can modify the chemical constitution and the temperature of sea water quite materially and thus obtain bizarre fishes having Cyclopean eyes or two heads or divided tails; but these monstrosities are not chaotic expressions of change in every direction; on the contrary, they exhibit the natural potentialities of the gene patterns under the novel conditions presented. We need not dwell, however, on the character, degree, or frequency of the variations produced in such special cases. The matter to be emphasized, it seems to me, is that such animals as the higher mammals cannot have their environmental conditions altered materially and live. Just consider for a moment the extreme nicety of man's regulatory apparatus. Temperature is controlled; moisture is regulated; and the types of food permitted are severely restricted. If his temperature rises half a degree, he is ill. If he does not get a balanced ration consisting only of fats, carbohydrates, proteids, essential minerals and vitamines, growth is prohibited. He cannot even synthesize his own protoplasm unless he has all the necessary amino-acids. And if his blood absorbs a tiny fraction of any one of a million different chemicals, he dies. One should not expect to find great plasticity under such conditions. Moreover, man is a delicate machine in another sense. When developmental errors occur and monstrosities are formed, these deviants are seldom viable. Generally speaking, therefore, the effect of any ordinary change of environment is negligible. It is usually confined to the production of well-nourished or ill-nourished offspring; and this is true, whether one is speaking of plants or animals, of unspecialized forms or highly specialized forms. But it should not be forgotten that even in man himself an occasional serious aberration is attributable to the environment. An accident may prevent normal maturation of the central nervous system, resulting in imbecility or feeble-mindedness; absence of iodine in the diet may affect the growth of the thyroid gland, resulting in a condition resembling cretinism. These inductions are supported by thousands of experiments in which the genetic constitution has been held constant, while the environment has been varied; they are supported by thousands of other experiments in which the genetic constitution has been varied, while the environment has been held constant.