58 Heredity and Environment 



epigenesis, which was in its main features founded upon the di- 

 rect 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 begin- 

 nings 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 dis- 

 coverer of the circulation of the blood, and to Caspar Friedrich 

 Wolff, whose doctoral 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, homo- 

 geneous and undifferentiated, and that differentiation and or- 

 ganization 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 formativa 

 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 develop- 

 ment 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 struc- 

 tures, the other upon outside forces and conditions. 



3. Endogenesis and Epigenesis. Modern students of develop- 

 ment recognize that neither of these extreme views is true adult 

 parts are not present in the germ, nor is the latter homogeneous 

 but there are in germ cells many different structures and func- 

 tions which are, however, very unlike those of the adult, and by 

 the transformation and differentiation of this germinal organi- 



