throughout. And the methods by which its missing parts are restored differ profoundly from the regenerative processes of the crystal. That the regenerative processes in organisms and crystals are closely related phenomena cannot, therefore, be considered as proven, or perhaps even as probable, at the present time. For the vitalist, regeneration is a phenomenon that is inexplicable in terms of physical and chemical laws. Driesch, for instance, would interpret form production of all kinds as the result of an entelechy, or vital principle, which shapes the constructive processes in an organism so that they realize a certain end result. One of the striking peculiarities of formative processes is that they may produce a given organ by a variety of methods. While there are generally certain similarities between the way in which organs are regenerated and the way in which they were originally formed in the embryo, there are many cases in which these processes differ profoundly. Organs formed from one germ layer are sometimes regenerated from a different germ layer. The regeneration of the lens of the eye of a Triton, for instance, has been found to take place by a method that is very different from its original production. In the embryo, the lens of the eye arises as a thickening of the ectoderm where the optic cup, which grows out from the brain, comes in contact with the outer covering of the head. This thickening sinks in, becomes detached from the surface layer, and transforms into the lens. If the lens of the adult Triton is removed, the cells of the upper margin of the iris proceed to multiply and form a mass out of which a new lens is gradually differentiated. The epithelium of the iris and the embryonic epithelium from which the lens originally developed are both ectodermic; nevertheless, they have had a very different embryonic history from an early period of development. Here is an impressive illustration of the resourcefulness of the organism in replacing a missing structure. It is not surprising that the vitalists frequently cite it with an air of triumph in supporting their position. Much experimental investigation has been devoted, during the last thirty years, to the attempt to ascertain the causes of developmental and restorative processes. Not content with describing what takes place, the modern biologist is endeavoring to ascertain why the phenomena take place as they do. This leads to experimentation with the aim of analyzing the phenomena into their component factors. Much has been learned in this way in regard to the mutual dependence of parts during development and regeneration. Perhaps with increasing knowledge and insight, the orderly building of the organic body may some day be explained, but at present this is only a pious hope. Undoubtedly biologists are a long way from giving a physical or chemical explanation of the production of organic form. A closer approach toward an understanding of this enigma is made, we believe, by those theories which view form production as the result of physiological interadjustments. The doctrine of metabolic gradients put forward by Dr. C. M. Child has brought out an important relation between intensity of metabolism and differentiation in regenerating and developing organisms. The part of a planarian, for instance, in which the rate of metabolism is the highest, tends to differentiate into a head, and this, according to the theory, dominates the differentiation of the parts lying behind it. It is doubtless true that metabolic gradients are intimately associated with the localization of differentiating activities. They perhaps afford what Herbst has called formative stimuli in development and regeneration. According to a physiological theory of form regulation propounded a few years ago by the present writer the production, regeneration, and maintenance of the normal form of the organism are the outcome of an essentially symbiotic relation subsisting between its various parts. Each part of the organism is assumed to derive certain advantages from the substances or stimuli received from neighboring parts, and these, in turn, receive corresponding advantages from it. If a small part of an organism is removed, and cells of a relatively unspecialized character are produced in its stead, as in fact they commonly are, in what direction would these cells tend to differentiate? As a result of the environment of these cells a sort of premium would be placed upon development in the direction of the missing part, since this would secure whatever advantages were afforded by the symbiotic relation. Regeneration in an organism would therefore have a certain analogy with regeneration in social groups. If all bricklayers should suddenly disappear, their ranks would soon be filled by other people who would respond to the increased demand that would be created for this particular kind of workers. The similarities in the processes of regeneration and adjustment that go on in the individual and in the social group represent something more than mere analogy. Both societies and individual organisms are composed of units which tend to grow and perpetuate their kind. The units of both have a considerable degree of plasticity, especially in early stages, and what each unit may become is largely determined in accordance with the principle of supply and demand as well in the individual organism as in human industrial society. That "the fate of a cell is a function of its position" is precisely what would be expected in accordance with the theory here set forth.1 Position means a certain complex of stimuli from adjacent parts which determine the direction in which a part develops. REFERENCES CHILD, C. M., Individuality in Organisms. University of Chicago Press, 1915. -, Physiological Foundations of Behavior. N. Y., Holt, 1924. DRIESCH, H., The Science and Philosophy of the Organism. 2 vols. London, A. and C. Black, 1907-8. HERTWIG, O., The Biological Problem of To-day. N. Y., Macmillan, 1896. JENKINSON, J. W., Experimental Embryology. Oxford, Clarendon Press, 1909. LOEB, J., Regeneration. N. Y., McGraw-Hill, 1924. 1 The theory which is here very briefly and inadequately expounded is more fully elaborated in an article on “The Problem of Form Regulation,” published in Roux's Archiv f. Entw. Mech., 17:265–305 (1904). MORGAN, T. H., Regeneration. N. Y., Macmillan, 1901. THOMPSON, D'ARCY W., Growth and Form. Cambridge University Press, 1917. WEISMANN, A., The Germ Plasm. London, Scott, 1893. WILSON, E. B., The Cell in Development and Heredity (3rd ed.). N. Y., Macmillan, 1925. CHAPTER XIV HEREDITY AND VARIATION The field of biology which has seen the most rapid advancement during the last quarter century is unquestionably genetics, or the science which is concerned with heredity and variation. Balzac once remarked that heredity is "a maze in which science loses itself." This statement correctly described the situation at the time it was written, but it is true no longer. So rapidly indeed has genetics advanced that it has become the one department of biology which perhaps approaches most nearly the status of an exact science. Professor Bateson has stated that "an exact determination of the laws of heredity will probably work more change in man's outlook on the world and in his power over nature, than any other advance in natural knowledge that can be clearly foreseen." This was written in 1900 just after the re discovery of Mendel's law of heredity. The remarkable progress in genetics to which I have referred has gone far toward justifying Professor Bateson's prophecy. The man primarily re sponsible for bringing genetics out of its previous Egyptian darkness was Gregor Johann Mendel, and it is no exaggeration to say that more has been learned about heredity and variation since Mendel's discoveries became generally known than had been learned in all preceding time. Mendel was an Austrian priest who was engaged as a teacher of natural science for several years at Brünn, Austria. During his leisure time he occupied himself with experiments on hybridizing plants, and he published the results of his researches in two papers that appeared in the Proceedings of the Natural History Society of Brünn for 1865 and 1869. Although these papers contained the announcement of the most important generalization ever made in the study of heredity, they remained |