31-i The Unit// of flic Organism 



men of science — that the investigation of .causes is the main 

 if not the exclusive function of science, this can mean noth- 

 ing else than that in the eyes of natural science at least one- 

 half of all nature is of less importance and interest than 

 the other half, and that science is privileged to decide which 

 is the important part. This is another way of criticizing 

 the practice now so dominant in biology of exalting the con- 

 cept of causality and degrading that of description. An 

 important fruit of the present discussion will be, it is hoped, 

 an exposure of the injury that has befallen theories of hered- 

 ity from this practice. Nor does our position imply the 

 notion, held by a few men of science, that the category of 

 causality is useless in science and ought to be dropped. Far 

 from it. The causal explanation of heredity is not only a 

 legitimate but an exceedingly important object of investi- 

 gation. But it is by no means the whole problem or even 

 the most important problem, the subject of heredity being 

 regarded in the large and for all time. Perhaps in the 

 present stage of progress of biology it is the most im- 

 portant ; but if so I would maintain that at some later time, 

 when the knowledge of causes shall have been advanced out 

 of proportion to other aspects of the subject, some of these 

 retarded aspects will become for a period the center of 

 genetic interest. 



The Definition of Heredity Adopted in This Discussion, 



•with Remarks on Its Application, Especially with 



Reference to the Chromatin Theory 



We are now in position to fix upon a definition of heredity 

 which shall recognize that its mechanism is as much a part 

 of and subordinate to the organism as are all its other parts 

 and organs. Or, employing the form of expression current 

 in recent genetics, a definition which shall tactily recognize 

 that chromosomes, even though bearers of heredity, are 

 causally explained by the organism in the same sense that 



