714 



THE AMERICAN NATURALIST [Vol. XLIX 



males may exert on the egg-production of their daughters 

 can be tested only by an indirect and rather uncertain 

 process. Only in the case of females is the character di- 

 rectly measurable and then only for such females as (1) 

 are hatched "after April 1 and before June 1," (2) sur- 

 vive all the accidents of chickhood and adolescence, (3) 

 escape all attacks of disease and are kept continuously 

 free from parasites, and (4) are properly fed and housed. 

 For any bird which dies, is disabled or becomes seriously 

 ill under ten months old, the character is an unknown 

 quantity. These limitations make the proportion of 

 birds which can be accurately rated as regards the char- 

 acter extremely small, and reduce correspondingly the 

 material on which selection can be practised. 



Contrast with this situation that regarding the hooded 

 pattern of rats. This character is possessed by every in- 

 dividual of both sexes and is inherited equally through 

 either sex. The character is fully developed in its final 

 form within a week after birth, months before sexual 

 maturity is attained. This makes it possible to grade the 

 animals accurately while they are still very young and to 

 discard at once all individuals which fall below the 

 adopted standard. Selection thus has a vastly greater 

 amount of material to work with, and the variation in 

 each generation can be ascertained with a completeness 

 and accuracy quite impossible in the case of winter egg 

 production in fowls. 



It is scarcely necessary to point out that upon the com- 

 pleteness of one's knowledge of the character and extent 

 of variation depends his ability to take advantage of that 

 variation by systematic selection. By this criterion win- 

 ter egg production is very poor material on which to base 

 an experimental test of "mass selection," whereas the 

 hooded pattern of rats is material admirably adapted for 

 the purpose. Many times has the fact been commented 

 upon that Mendel's fortunate choice of peas as material 

 for his studies of hybridization was largely responsible 

 for his success where others failed. If one wishes to test 



