BIRDS AND EVOLUTION 397 



index of the blood. It has been shown by Raymond Pearl 

 and John W. Gowen (1914) that there is a definite, character- 

 istic, and permanent difference between the refractive index 

 of the serum of the common fowl (Gallus) and that of the 

 guinea-fowl (Numida meleagris). When a genus-hybrid 

 is successfully obtained (between common cock and guinea 

 hen) it approximates in its refractive index to the guinea 

 parent. In other words, as regards the physico-chemical 

 constitution of the blood, in so far as that is expressed in the 

 refractive index, the guinea-parent is dominant. 



