BLASTOGENIC VARIATIONS. 143 



seven generations. Japanese waltzing mice are mostly 

 black and white, i. e., piebald, in colour, but their 

 crosses and reciprocal crosses with the albino race 

 yielded a most unexpected result. The whole of the 

 offspring produced were of a gray colour, indistin- 

 guishable in respect either of colour or of size from the 

 common house mouse. The waltzing action was en- 

 tirely wanting, the reversion being apparently com- 

 plete. Heacke had obtained a similar result on cross- 

 ing the same races.* In the third generation, how- 

 ever, the type was broken, for the 44 young produced 

 by the mating of 4 pairs of the reverting gray mice 

 consisted of 8 waltzers (albino, spotted, gray, and 

 black), 11 pure albinos, and 25 gray mice. In the sub- 

 sequent generations, the albinos and also the gray and 

 the spotted mice were found to breed true. Gray mice 

 crossed with white yielded mostly gray offspring, but a 

 certain number of waltzers. 



Of the occurrence of reversion there can thus be no 

 question. In fact, its appearance in the offspring of 

 crossed races is by no means an infrequent phenomenon. 

 The reversion of hybrids and mongrels to one of their 

 pure parent forms, after an interval of two or more 

 generations, is especially common. Hence it would 

 seem that the act of crossing in itself gives an impulse 

 towards reversion. Why and how this is the case must 

 be more or less a matter of conjecture. Indeed, this is 

 equally true for all the phenomena of reversion, but I 

 think that a brief consideration of certain presumptions 

 regarding the germ-plasm as the bearer of hereditary- 

 characters will show that, after all, we are not dealing 

 *Biol. Central., Bd. xv. p. 44, 1895. 



