450 HEREDITY AND DEVELOPMENT 



have objected to Weismann's theory of determinants, because, 

 as they say, no one has ever seen or can ever hope to see one. 

 Determinants are scientific fictions and all discussion of them 

 is in the air. But the same sort of objection may be raised 

 against the theory of, let us say, the ether. The point is whether 

 the concept of determinants helps us to interpret visible pheno- 

 mena. Science works from beginning to end with imaginative 

 concepts which facilitate description and formulation, and which 

 are so truly representative of the invisible that we can utilise 

 them in prediction. 



Other biologists, who are aware of the impossibility of 

 4 science without imaginative concepts, object to the theory 

 of determinants on the ground that they can be done without. 

 Thus Prof. Yves Delage rejects all determinants, primary con- 

 stituents, or particules representatives, and will only postulate 

 a germ-plasm with " an extraordinarily delicate and precise 

 physico-chemical composition." " There are not," he says, 

 " in the germinative plasm any distinctive particles repre- 

 senting the parts of the body or the characters and pro- 

 perties of the organism " (1903, p. 749). What is there, then ? 

 According to Delage, the germ-cell contains a number of 

 characteristic chemical substances — which every one admits — 

 characteristic of the chief categories of cells ; and its development 

 is comparable to the flow of a river, now running deep and 

 again shallow, here forming a waterfall and there an eddy, 

 but always explicable in terms of action and reaction between 

 the flowing water and its surroundings. Given the power of 

 developing (which no one understands), given a characteristic 

 chemical composition (which every one admits), and given an 

 appropriate environment (which nobody can deny), and voild 

 tout. There is no more need to cumber biology with deter- 

 minants and biophors than there was to cumber astronomy 

 with Ptolemaic circles and epicycles. 



But even in the apparently simplest cases it seems impossible 



