PERPETUATION OF MUTATIONS 189 



eliminated. If the improbable happened, however, as it 

 may do sometimes, and the mutation or large individual 

 variation in a character gave the individual an advantage, 

 the fact that it is transmitted in an alternative manner 

 would prevent that disappearance or diminution of the 

 character that would be produced by blending. Conditions 

 in an environment are conceivable where such a thing 

 might happen. A race of moths, for instance, might occupy 

 a large area of country in which the environment differed 

 abruptly in a particular locality. Suppose that for some 

 reason the physical surroundings became darker rapidly in 

 this restricted locality than in the rest of the area, if a 

 sudden large variation towards a darker colour occurred 

 in an individual moth it might give it a considerable 

 advantage. This changed locality might have harboured 

 but few dark variants previously, because they were 

 too conspicuous. The dark variation would survive its 

 lighter fellows, in its descendants the dark character would 

 segregate, and so a dark race would be established in this 

 dark area much more rapidly than could happen if only 

 small variations were available. Such an occurrence, how- 

 ever, must be extremely rare, and would generally be due 

 to artificial conditions, such as the interference of man. He 

 might not have intentionally changed the environment, but he 

 might nevertheless have brought this result very effectually. 

 Apparently then we have characters that are transmitted 

 alternatively from parent to offspring, and there seems to be 

 evidence that such transmission occurs only in the case of 

 individual characters. We also have characters that blend 

 in the offspring. Racial characters behave in this manner. 

 There is a mechanism in cell division by which certain parts 

 of the cell are distributed in an alternative manner to the 

 gametes, and while the bulk of the cell-substance mingles 

 when two gametes fuse, these particular parts are distributed 

 to the cells subsequently produced in such a manner as to 

 suggest very strongly that they are intimately connected 

 with the phenomenon of alternative inheritance of characters. 



