CHAPTER VI 



THROWING BACK, OR REVERSION 



In tlie crosses which have hitherto been dealt with, 

 the result of mating two individuals which differed 

 in respect of a particular characteristic has been a 

 hybrid which has resembled one of the parents so 

 closely as to be indistinguishable from it ; or it has 

 been intermediate between them, as in the texture of 

 the pod in peas, and the colour of the feathers in the 

 fowl. We shall now proceed to cases in which the 

 hybrid differs from either parent, but is not inter- 

 mediate between them. Into this class fall those 

 cases in which the result of mating two varieties is 

 the production of the features of the wild ancestral 

 form from which the two varieties are supposed to 

 have descended. A very characteristic instance of 

 such a result is afforded by the result of crossing 

 the ordinary albino mouse with the so-called Japanese 

 waltzing mouse. These two varieties differ from 

 one another in respect of their colour, and in respect 

 of their customary movements ; the one walking and 

 running normally, the other exhibiting the so-called 

 waltzing movements which are indicated by its 

 name. We are concerned at present solely with the 

 colour of the two forms ; the question of the mode 



of progression must be banished from the reader's 



72 



