ordinary twins there are two distinct chorions. Since such twins are derived from the same egg and hence from identically the same chromosomes, the view that the chromosomes are the basis of familial similarity is supported. It is, indeed, a natural extension of this finding that we should ascribe not only family but also racial resemblance to identity of genes which determine the production of these racial characters. Individual differences, on the other hand, are due to mutations in genes of minor importance. Family resemblance and racial resemblance depend on similarity of chromosome composition. Resemblance is proof of "blood" relationship; more strictly chromosomal identity.

There is, however, another set of facts which must not be neglected. In an institution for the feeble-minded one may see a number of persons having a striking similarity which enables us to say at sight that they are cretins. . . . Resemblance between them is often, in general, as great as that between members of the same European race and individuals who do not belong to the same family may be as alike as two brothers. In case of cretins the peculiar and differentiating characters that they show are generally ascribed to the fact of an endocrine disturbance. Their thyroid glands are not functioning normally and hence the similar peculiarities in form that they show.

Still another class of defectives shows an even greater resemblance. This is the class of mongoloids, so called from their resemblance in features and bodily proportions to the Southern Chinese. . . . The resemblance between these people is often very marked, so much so that the matron in a building where several of them reside and who sees them very frequently, almost constantly every day, states that she frequently confuses them. This is the same sort of resemblance that is exhibited by identical twins, the resemblance which has been used by Galton and others as the strongest evidence for the superior influence on development of nature over nurture. Yet two of the mongoloids in question belong to different European races. Children of Northern Europe and Southern Europe may be almost indistinguishable. The cause for mongoloidism has not been ascertained. It is probably due to some endocrine disturbance in the mother, since the defect is already shown in the children at birth. Whatever the abnormal condition in the mother of these mongoloids it is probably normal for the mothers of the typical Southern Chinese, for in almost every respect excepting intelligence, the resemblance between the mongoloid

dwarfs and the Southern Chinese is very close. A consideration of the facts of cretins, mongoloids and other types of the feeble-minded whose later developmental impulses, of endocrine origin, are evidently abnormal might indeed lead to the conclusion that "we are what our endocrine glands make us."

The tremendous importance of endocrine glands in controlling the later development of organisms has been brought home to us by the studies of the last two decades. It is known that the transformation of the tadpole into the frog is effected only in the presence of the thyroid gland in the tadpole. For if this gland is removed, metamorphosis fails to occur though growth continues. Again the pigmentation of the young amphibian depends upon the secretions of the pituitary gland, since when this is removed the animals are albinic. In humans, also, special disfunctioning of the pituitary gland leads to excessive growth in length of the legs, on the one hand, or to excessive obesity, on the other. Unquestionably the secretions of some of the endocrine glands have a most important effect upon the metabolism on which development depends. It is no wonder that the endocrinologist finds in the hormones the principal agency of family resemblance.

How can the difference between the views of the geneticist and the chromosomologist, on the one hand, and the endocrinologist, on the other, be reconciled? It would appear that both views cannot be true, provided they are mutually exclusive. If the chromosomes are alone responsible for resemblance then the endocrinologists must be deceived in their conclusion. If the hormones are alone responsible for resemblances then there must be something false in the scientific methods of the geneticist. Either of these conclusions is, however, untenable since both the students of chromosomes and of hormones have worked by the best of scientific methods so that their results are unassailable. Hence we must consider both views to be true and that the chromosomes and the hormones each have their rôle to play in the direction of development. The most tenable hypothesis of the nature of the chromosomes is indeed that they are packages of enzymes which activate the metabolic processes of the early stages of development, just as the hormones of the endocrine glands control metabolism in later stages. The hypothesis may be suggested that the chromosomes direct the early stages of development and create certain centers of chemical activity to which they hand over the business of differentiation of particular parts. Thus the chromosomes may work indirectly in estab

lishing certain centers of later chemical activity whose course they have determined, but in the working of whose mechanism they subsequently do not interfere. The endocrine glands of vertebrates represent perhaps a still later and highly specialized stage in the series of regulators of metabolism. In the invertebrates and in plants where such endocrine glands are unknown it is probable that there are, nevertheless, regulating centers constituting various links in the developmental chain which starts with the fertilized egg and its chromosomes and ends with the fully formed parts and tissues. These endocrine glands and other developing controlling tissues are what the chromosomes make them.

This hypothesis receives support from the observations that have been made upon physiology of the later stages of development, with their processes of folding of membranes, of concrescence, of disruption of parts by mechanical and histolytic processes, and the development of special tissues with their special kinds of form and substance such as are seen in the various connective tissues of the higher animals. These tissues are indeed responsive, in turn, not only to the hormones which seem so largely to control their development but also to external agents, such as pressure, gravity and radiant energy.

The science of genetics has, in the past, suffered from too narrow a point of view. The evidence of the important rôle played by the chromosomes has been so overwhelming that it has blinded us to the presence of other agencies only indirectly ruled by the chromosomes. A consideration of all the facts should enforce the view that one of the greatest fields of biology, which has been comparatively untouched, is that of the physiology of development. Embryology has indeed been studied by the morphologist. A certain beginning has been made into the field of experimental embryology; but this has been confined largely to the early stages of development. Something has been learned by transplantation experiments in later stages of the amphibia. But it remains true that the field of physiological embryology has been relatively little cultivated. The science of genetics cannot be considered well established until we have some better conception of how the chromosomes do their work, of the chemical nature of the different developmental processes and the interaction of parts of the developing organism upon each other.

The appreciation of the complicated nature of development, the knowledge of the chain of developmental processes, each link

of which is activated by a preceding link, helps us to understand more clearly the relation of heredity and environment. No physiologist can fail to recognize that all development is under the control of agencies external to the developing center. In the earliest stages of development, indeed, the processes of differentiation seem to have a remarkable independence of environment. Even though the organism be turned inside out, as in the case of the lithium larvæ of sea urchins produced by Herbst many years ago, still the spicules and other differentiating characters will be laid down in nearly normal fashion. But every student of plant genetics knows that the final form is dependent upon conditions of nutrition, temperature and the like, and students of human development are aware of the influence which the nervous system exerts upon the production of hormones. This nervous system is, of course, the organic complex which is most directly affected by external conditions and the production of hormones which has so marked an influence upon development. What is true in later stages is, no doubt, true in still earlier ones and thus one can see the basis for the conviction which has for a long time been held by thoughtful medical men that various kinds of shocks, or poisons introduced into the body, affect the development of the fetus. The striking cases of resemblance in close relatives and especially in identical twins occur where conditions of life are nearly uniform in the developmental period. Where these conditions affect differently the individuals with the same germ plasm the end result is a morphological difference. The student of genetics must take into account, therefore, chromosomes, hormones, other developmental impulses and environmental conditions if he would understand all the factors that determine development.

5. Heredity and Its Influences upon Mental Growth and Development

[CHARLES E. SKINNER.]

The human being at any time of his life is the product of two influences-heredity and environment. At the time of conception (the moment when the sperm and ovum unite to form the zygote cell) or shortly thereafter, the zygote cell possesses the finite and fixed hereditary possibilities for that organism. These possibilities might or might not be realized during the life of the organism. "The organism as a whole represents... heredity plus environment, in other words, it

is primarily the reaction to environmental factors of a protoplasm with a certain hereditary constitution. The organism is inexplicable without environment. Every characteristic of it has some relation to environmental factors. And particularly to the organism as a whole, i. e., the unity and order, the physiological differences, relations and harmonies between its parts, are entirely meaningless except in relation to an external world." 1

It must be kept in mind that the environment, whether prenatal or post-natal is never the same any two moments. Environment provides the opportunity for the development of hereditary traits and capacities. Thus our hereditary structure lies ready to be shaped in a thousand different ways-depending upon the sum total of environing forces, whether internal or external stimuli.

Heredity has too frequently been used as a "blanket" term. If a given phenomenon or characteristic could not be explained readily in any other way it was attributed to heredity. Fortunately that tendency is passing. The scientific studies in the fields of internal secretions, the vitamines, and genetics make it possible to discuss the whole problem of heredity in a much more scientific way.

The term heredity implies the presence of some determinant or factor in each developing organism that gives it an identity of its own. The certainty of an embryo developing into an organism of a certain kind is called the law of heredity. The primary fact about heredity is that "like tends to beget like," that there is an inherent tendency of the offspring to resemble its progenitors, both in structure and function.

Colored parents will bear colored children if the line is pure. Chinese parents will bear Chinese children, and white skinned parents will bear white children. Once in a very long time a "mutant," or sport, may be born as an exception to this rule. In fact, there are thousands of variations whose differential causes are to be found in the germ cells.

The teacher must constantly bear in mind that the child needs. the best possible environment to promote mental, physical, and social-moral growth which his inheritance makes possible, whatever be his heredity.

Mendelian Inheritance.-How does it happen that the characteristics of organisms are reproduced so regularly in their off

1 Charles M. Child, Physiological Foundations of Behavior (New York, Henry Holt & Co., 1924), pp. 29-30.