FACTS AND FACTORS OF DEVELOPMENT 37 



This doctrine of preformation was not only an attempt to solve the 

 mystery of development, but it was also an attempt to avoid the theolog- 

 ical difficulties supposed to be involved in the view that individuals are 

 produced by a process of gradual development rather than by super- 

 natural 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 absurdi- 

 ties of which contributed to the downfall of the entire doctrine of pre- 

 formation, 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 ani- 

 mals 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 organi- 

 zation 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 fol- 

 lowers, and they were compelled to assume some extrinsic or environ- 

 mental 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 alto- 

 gether; 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 rec- 

 ognize 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 differ- 

 entiation of this germinal organization the complicated organization of 

 the adult arises. Development is not the unfolding of an infolded organ- 



