THE FACTORIAL HYPOTHESIS 281 
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. 
