THEORIES OF INHERITANCE AND DEVELOPMENT " It is certain that the germ is not merely a body in which life is dormant or potential, but that it is itself simply a detached portion of the substance of a pre-existing living body." " Inheritance must be looked at as merely a form of growth." DARWIN? " Ich mochte daher wohl den Versuch wagen, durch eine Darstellung des Beobachteten Sie zu einer tiefern Einsicht in die Zeugungs- und Entwickelungsgeschichte der organischen Korper zu fuhren und zu zeigen, wie dieselben weder vorgebildet sind, noch such, wie man sich gewohnlich denkt, aus ungeformter Masse in einem bestimmten Momente plotzlich ausschiessen." VON Every discussion of inheritance and development must take as its point of departure the fact that the germ is a single cell similar in its essential nature to any one of the tissue-cells of which the body is composed. That a cell can carry with it the sum total of the heritage of the species, that it can in the course of a few days or weeks give rise to a mollusk or a man, is the greatest marvel of biological science. In attempting to analyze the problems that it involves, we must from the outset hold fast to the fact, on which Huxley insisted, that the wonderful formative energy of the germ is not impressed upon it from without, but is inherent in the egg as a heritage from the parental life of which it was originally a part. The development of the embryo is nothing new. It involves no breach of continuity, and is but a continuation of the vital processes going on in the parental body. What gives development its marvellous character is the rapidity with which it proceeds and the diversity of the results attained in a span so brief.
But when we have grasped this cardinal fact we have but focussed our instruments for a study of the real problem. How do the adult characteristics lie latent in the germ-cell ; and how do they become patent as development proceeds ? This is the final question that looms in the background of every investigation of the cell. In approaching it we may well make a frank confession of ignorance ; for in spite of all that the microscope has revealed, we have not yet penetrated the mystery, and inheritance and development still remain in their fundamental aspects as great a riddle as they were to the Greeks. What we have gained is a tolerably precise acquaint
ance with the external aspects of development. The gross errors of the early preformationists have been dispelled.' We know that the germ-cell contains no predelineated embryo ; that development is manifested, on the one hand, by a continued process of cell-division, on the other hand, by a process of differentiation, through which the cells gradually assume diverse forms and functions, and so accomplish a physiological division of labour. But we have not yet fathomed the inmost structure of the germ-cell, and the means by which the latent adult characters that it involves are made actual as development proceeds. And it should be clearly understood that when we attempt to approach these deeper problems we are compelled to advance beyond the solid ground of fact into a region of more or less doubtful and shifting hypothesis, where the point of view continually changes as we proceed. It would, however, be an error to conclude that modern hypotheses of inheritance and development are baseless speculations that attempt a merely formal solution of the problem, like those of the seventeenth and eighteenth centuries. They are a product of the inductive method, a direct outcome of accurately determined fact, and they lend to the study of embryology a point and precision that it would largely lack if limited to a strictly objective description of phenomena.
All discussions of development are now revolving about two central hypotheses, a preliminary examination of which will serve as an introduction to the general subject. These are, first, the theory of Germinal of Wilhelm His ('74), and second, the Idioplasm Hypothesis of Nageli ('84). The relation between these two conceptions, close as it is, is not at first sight very apparent ; and for the purpose of a preliminary sketch they may best be considered separately.