HEREDITY AND EVOLUTION 107 



upon injection of testic-extract and its recession 

 to the characteristic female condition upon cessa- 

 tion of the injections, as recently described by 

 Walker. Analogous phenomena are seen in the 

 well-known effects of thyroid extracts, or in the 

 effect upon the mammary glands of injection of 

 extracts of the fetus, as described by StarUng 

 and Lane-Claypole. 



We are thus led to something more than a sus- 

 picion that the factors of determination, andj 

 therefore of heredity, are at bottom of chemical 

 nature. It is a well-known fact that correspond- 

 ing tissues of different species often show char- 

 acteristic chemical differences ; and to some extent 

 the same is known to be true of the germ-cells. 

 The conclusion thus becomes highly probable that 

 the characteristic differences of metabolism be- 

 tween different species, including those involved 

 in development, are traceable to initial chemical 

 differences in the germ-cells. In so far as the 

 chromatin theory expresses the truth, the pri- 

 mary basis of these differences may be sought 

 in the nuclear substance. There is good reason 

 to believe that some at least of the enzymes are 

 of nuclear origin. It seems a promising hypoth- 

 esis that the chromosomes may be regarded as 

 self-perpetuating magazines of specific sub- 

 stances, similar in nature to the enzymes or their 

 chemical antecedents, that play an essential role 

 in the determination of the cell-activities, includ- 

 ing those involved in development. From this 



