24 THE METHODS AND 



factors be perceived. So in the mouse if 

 no other colour-factor is present, the fur is 

 chocolate. If the next factor in the series be 

 there, it is black. If still another factor be 

 added, it has the brownish grey of the common 

 wild mouse. Conversely, by the variation 

 which dropped out the top factor, a black 

 mouse came into existence. By the loss of 

 the black factor, the chocolate mouse was 

 created, and for aught we can tell there may 

 be still more possibilities hidden beneath. 



In the disentanglement of the properties 

 and interactions of these elementaiy factors, 

 the science we must call to our aid is 

 Physiological Chemistry. The relations of 

 Genetics with the other branches of biology 

 are close. 8uch work can only be conducted 

 by those who have the good fortune to be able 

 to count upon continual help and advice from 

 specialists in the various branches of Zoo- 

 logy, Physiology, and Botany. Often we have 



