358 THE AMEBIC AX NATURALIST [Vol. XLVI 



and again crossed with yellow. By this means the black 

 was after several generations ninch reduced. The hairs 

 were distinctly yellowish at base and the part above sooty 

 black in appearance. Recently a pink-eyed animal has 

 appeared in this race with hair light sooty-black in spots. 

 This evidently is an extreme variation in the direction 

 which selection has taken throughout the experiment and 

 probably similar in nature to the pink-eyed variation in 

 mice. There can be no question of recombination of 

 independent Mendelian factors in this experiment, be- 

 cause (aside from albinism) only a single Mendelian fac- 

 tor is involved. The heterozygotes, as regards black, 

 have consistently behaved as simple heterozygotes, and 

 the experience of all observers agrees with my own that 

 black in guinea-pigs is a simple Mendelian unit. If so it 

 is clearly a unit modifiable under selection. 



In yellow animals, as in blacks, individuals of varying 

 intensity occur, the darkest known as reds, the lightest 

 as creams. A complete series of intermediates can be 

 obtained if so desired. If we select any two widely sep- 

 arated stages in this series fairly stable in their breeding 

 capacity and cross these, they Mendelize, i. e. they behave 

 as if there were a single unit-character difference be- 

 tween them. Now this fact is instructive, for it throws 

 light on the nature of unit-characters in such cases. 

 They are not things in themselves distinct and separate 

 from the organism concerned; each is a quantitative 

 variation plus or minus in some one character possessed 

 by the organism. Each quantitative condition of a char- 

 acter tends to persist from cell-generation to cell-genera- 

 tion. When two quantitatively unlike conditions of a 

 character are brought together in a fertilized egg, they 

 tend to keep their distinctness in subsequent cell-gen- 

 erations and to segregate into different gametes at re- 

 production, i. e., they Mendelize. Only by a figure of 

 speech are we justified in recognizing a unit difference 

 between them. That difference might equally well be 

 half as great as it is, or a quarter as great, or a thou- 

 sandth part as great. A mono-hybrid ratio would result 



