2 HISTORICAL INTRODUCTION TO THE 



in development from the egg, but only a realisation, expansion, and 

 rendering visible of a pre-existing diversity. Preformation is the 

 fundamental assumption of views of this type, and they are classed 

 together as preformationist theories. But the doctrine of preforma- 

 tion, however, met with even graver obstacles, both logical and 

 empirical, than the opposite view, and biological opinion is now 

 united in maintaining the existence of a true epigenesis in develop- 

 ment. In recent years, however, the discoveries of genetics have 

 reintroduced certain elements of the preformationist theory, but 

 in more subtle form. As will be seen later, the modern view is 

 rigorously preformationist as regards the hereditary constitution 

 of an organism, but rigorously epigenetic as regards its embryo- 

 logical development. 



To a large extent, the preformationist view assumes as already 

 given that which the epigenetic attempts to study and to explain ; 

 and the problem is complicated by the fact that notions of emi- 

 bryonic development have been confused with concepts of heredity. 

 This is evident in the attempt, on the part of the author of Peri 

 Gones in the Hippocratic corpus, to explain development by as- 

 suming a part-to-part correspondence between the parts of the body 

 of the parent and those of the offspring : the corresponding parts 

 being related to one another via the "semen", or, as would now be 

 said, via the germ-cells. By assuming that the embryo at its earliest 

 stage is a minute replica of the adult, its parts having been "pre- 

 formed " by representative particles coming from the corresponding 

 parts of the parent, the preformationist hypothesis attempts to solve 

 at one stroke both the problem of hereditary resemblance between 

 generations and the problem of development within each generation. 



This view was in reality shattered by Aristotle's criticism, but it 

 was revived and widely held during the seventeenth and eighteenth 

 centuries, largely owing to the fact that mechanistic explanations 

 had come into vogue, and it seemed impossible to understand 

 epigenesis on mechanistic lines. One of the foremost exponents 

 of the preformationist hypothesis was Charles Bonnet. His views 

 were freed from the crude idea that the preformation in the egg 

 was spatially identical with the arrangement of parts in the adult 

 and fully developed animal, or that the ''homunculus" in the 

 sperm, with the head, trunk, arms and legs which it was supposed 



