THE CELL-DOCTRINE 145 



shuffled about as in Mendelian heredity, it is necessary to 

 suppose not only that the adult is in some manner contained 

 within the germ but also that something within the germ- 

 cells can be shuffled in a corresponding manner. These 

 somethings, which are called determiners, genes, or factors, 

 appear to be located in the chromosomes. Superficially, 

 development is a process of building the adult organism a 

 step at a time epigenesis. When examined more closely, 

 it is a coming into being of what is potentially existent, just 

 as the dealing of hands at cards is the production of an end 

 result foreshadowed by an arrangement within the pack. 

 To this extent the concept of preformation is applicable to 

 development. 7 



If the cell must furnish clues to the resemblances be- 

 tween generations it must also furnish clues to variation. 

 Heredity and variation are simply different aspects of the 

 reproductive process. Like begets like, but not just like. 

 We often speak of heredity and variation as conflicting 

 forces. They are merely the two sides of development. 

 Heredity and variation are the initial processes in evolution- 

 ary change. Explain these two phenomena and you explain 

 the starting points of evolution. Cell-theory and evolution- 

 ary theory here meet. The discovery of a cellular mechan- 

 ism for Mendelian heredity establishes a continuity between 

 fields of biological knowledge which are at first glance 

 distinct. 8 Investigators continue to approach the evolution- 



7 A brief discussion of the modern concept of the relation between preforma- 

 tion and epigenesis in development appears on p. 193 of the present volume. 

 Cf. also: Parker, G. H., "Biology and Social Problems," Chapt. Ill, 

 Reproduction. 



8 Recognition of cell problems as related to evolutionary problems first 

 appears in the writings of August Weismann (1834-1914). The nature of the 

 genetic continuity between parent and offspring having been established, Weis- 

 mann realized that once in each generation the potentialities of the individual, 

 and so of the race, are encompassed within the limits of single cells the ovum 

 and the spermatozoon, and the oosperm formed by their union. Whatever 

 restrictions subsequent investigation may place upon his conclusions, Weis- 

 mann will remain a commanding figure, because he first brought into correla- 

 tion the two major lines of biological interest. 



