150 DARWINISM AND HUMAN LIFE 



two daminant features; and when these are self- 

 fertilised (which is equivalent to inbreeding), 

 out of 16 offspring there are 9 tall yellows (Dd), 

 3 tall greens (Dr), 3 dwarf yellows (Rd), and one 

 dwarf green (Rr). When a rabbit of the wild 

 grey colour is crossed with an albino, the offspring 

 are all grey, and these, if bred together, give in 

 certain cases 9 greys, 3 blacks, and 4 albinos, 

 which is a slight modification of the ordinary 

 9:3:3, 1 ratio due to the impossibility of dis- 

 tinguishing, by external appearance, between two 

 different kinds of albino. 



Unit Chaeaoters. — We do not at present know 

 with certainty how many qualities and parts can 

 be called " unit characters " in the Mendelian 

 sense. The only criterion is the experimental one : 

 can the character be lost, as a whole, in cross- 

 breeding ? Prof. W. E. Castle ' gives an illustra- 

 tion : " If we cross a black guinea-pig with one 

 which lacks black — say a brown one — we obtain 

 only black offspring ; but these bred inter se produce 

 black offspring and brown ones, in the proportion 

 three black to one brown. We thus learn that 

 black is a imit character. It was contributed by 

 one parent to the cross, but not by the other, and 

 transmitted by the cross-bred individual to half 

 its offspring, but not to the other half. This is 

 Mendel's explanation of the 3 : 1 ratio, now 

 familiar to every biologist. 



" But if we cross the same black parent in the 

 foregoing case, not with a brown individual, but 

 with a white one or with a yellow one, we may 

 obtain, not black offspring, but wild-coloured 



1 " The Behaviour of Unit Characters in Heredity," in " Fifty 

 Years of Darwinism " (1909), p. 148. 



