This doctrine of preformation was not only an attempt to solve the mystery of development, but it was also an attempt to avoid the theological difficulties supposed to be involved in the view that individuals are produced by a process of gradual development rather than by supernatural creation. If every individual of the race existed within the germ cells of the first parents, then in the creation of the first parents the entire race with its millions of individuals was created at once. Thus arose the theory of "emboitement," or "box in box," the absurdities of which contributed to the downfall of the entire doctrine of preformation, which, in the form in which it was held by many naturalists of the eighteenth century, is now only a curiosity of biological literature.
Epigenesis.—As opposed to this doctrine of preformation, which was founded largely on speculation, arose the theory of epigenesis, which was in its main features founded upon the direct observation of development, and which maintained that the germ contains none of the adult parts, but that it is absolutely simple and undifferentiated, and that from these simple beginnings the individual gradually becomes complex by a process of differentiation. We owe the theory of epigenesis, at least so far as its main features are concerned, to William Harvey, the discoverer of the circulation of the blood, and to Caspar Friederich Wolff, whose doctor's thesis published in 1759, and entitled "Theoria Generationis," marked the beginning of a great epoch in the study of development. Wolff demonstrated that adult parts are not present in the germ, either in animals or in plants, but that these parts gradually appear in the process of development. He held, erroneously, that the germ is absolutely simple, homogeneous and undifferentiated, and that differentiation and organization gradually appear in this undifferentiated substance. How to get differentiations out of non-differentiated material, heterogeneity out of homogeneity, was the great problem which confronted Wolff and his followers, and they were compelled to assume some extrinsic or environmental force, some vis formativia or spiritus rector, which could set in motion and direct the process of development.
The doctrine of preformation, by locating in the germ all the parts which would ever arise from it, practically denied development altogether; epigenesis recognized the fact of development, but attributed it to mysterious and purely hypothetical external forces; the one placed all emphasis upon the germ and its structures, the other upon outside forces and conditions.
Preformation and Epigenesis.—Modern students of development recognize that neither of these extreme views are true—adult parts are not present in the germ, nor is the latter homogeneous—but there are in germ cells many different structures and functions which are, however, very unlike those of the adult, and by the transformation and differentiation of this germinal organization the complicated organization of the adult arises. Development is not the unfolding of an infolded organ-
