THE CARRIERS OF THE HERITAGE 29 



entities, and is there such a thing as a mechanical 

 basis for heredity as the German embryologist 

 Wilhelm His suggested years ago when he wrote: 

 "It is a piece of unscientific mysticism to suppose 

 that heredity will build up an organism without 

 mechanical means " ? Can we find these determiners 

 by the aid of microscopes and differential stains, 

 or are they some sort of intangible entities, such as 

 enzymes or hormones or the like, which only the 

 chemist can detect ? 



Whatever the answer to these questions, it may at 

 least be affirmed that the determiner represents the 

 adult structure without resembling it. It is something 

 which controls the unfolding of the developing or- 

 ganism with respect to both quantity and quality, 

 and which also governs the time and rate of appear- 

 ance of its various characteristics so that certain 

 combinations rather than others shall come about 

 in definite sequence. To use the words of Conklin : 

 "The mechanism of heredity is the mechanism of 

 differentiation." 



12. THE CHROMOSOME THEORY 



Certain investigators, who seek a morphological 

 basis for heredity, regard the chromosomes as the car- 

 riers of the heritage; in other words, as the source 

 of the determiners of ontogeny or the effective 

 factors in the process of differentiation. 



A few of the grounds for this theory are briefly 

 indicated below. 



First: In spite of the great relative difference in 



