4 HISTORICAL INTRODUCTION TO THE 



§2 



Logically, the preformationist view is associated with the notion 

 of separate particles being transmitted from parent to offspring, 

 though the converse does not hold. In preformationist theory, the 

 hypothetical particles establish the one-to-one link between the 

 corresponding organs and parts of parent and offspring, whereas 

 the modern view, which combines an epigenetic outlook on de- 

 velopment with the particulate theories of neo-Mendelism, denies 

 any such simple correspondence between hereditary germinal unit 

 and developed adult character. Darwin's theory of pangenesis re- 

 sembles that of the Hippocratic writer in this respect, the pangens 

 being supposed to come from all parts of the body of the parent and 

 to be transmitted, via the germ-cells or "semen", to the offspring 

 whose development they mould. Embryologically, however, 

 Darwin's theory is vague, and leaves the question of preformation 

 open. Weismann's theory of the germ-plasm, in which the deter- 

 minants are regarded as representing the predetermined but not 

 spatially preformed diversity of the future embryo, differs from that 

 of previous preformationists in that the particles are regarded as 

 coming, not from the corresponding parts of the body of the parent, 

 but from the germ-plasm, of which each generation of individual 

 organisms is held to be nothing but the life-custodian. Weismann 

 identified the determinants with the material in the nuclei of the 

 cells, which material he (wrongly) supposed was divided unequally 

 in the process of division or cleavage of the tgg, so as to form a 

 mosaic, the pieces (cells or regions) of which would then contain 

 different determinants and would therefore be predetermined to 

 develop in their respective different and definite directions. 



According to the writer of the Hippocratic treatise Peri Gones 

 and to Darwin, therefore, offspring resembles parent because the 

 particles responsible for the development of the parts of the off- 

 spring come from the corresponding parts of the parent. According 

 to Weismann, however, offspring resembles parent because both 

 have derived similar particles (determinants) from a common 

 source — the germ-plasm. 



The question of the origin of the particles or hereditary factors 

 and of their distribution from the parent to the offspring is one 



