THE FACTORIAL HYPOTHESIS 227 



embryology have been directed for several years 

 toward this goal. It may even be true that this 

 information, when gained, may help us to a better 

 understanding of the factorial theory — we can not 

 tell; for a knowledge of the chemistry of all of the pig- 

 ments in an animal or plant might still be very far 

 removed from an understanding of the chemical 

 constitution of the hereditary factors by whose 

 activity the pigments are ultimately produced. 

 However this may be, the far-reaching significance 

 of Mendel's principles remains, and gives us a 

 numerical basis for the study of heredity. Although 

 Mendel's law does not explain the phenomena of 

 development, and does not pretend to explain them, 

 it stands as a scientific explanation of heredity, 

 because it fulfils all the requirements of any causal 

 explanation. 



