;8 MENDELISM chap. 



other the normal equal distribution of the various 

 factors is upset, and some of the gametes receive 

 a factor less than others. From the union of two 

 such gametes, provided that they are still capable 

 of fertilisation, comes the zygote which in course of 

 growth develops the new character. 



Why these mutations arise : what leads to the 

 surmised unequal division of the gametes : of this 

 we know practically nothing. Nor until we can 

 induce the production of mutations at will are we 

 likely to understand the conditions which govern 

 their formation. Nevertheless there are already 

 hints scattered about the recent literature of experi- 

 mental biology which lead us to hope that we may 

 know more of these matters in the future. 



In respect of the evolution of its now multi- 

 tudinous varieties, the story of the sweet -pea is 

 clear and straightforward. These have all arisen 

 from the wild by a process of continuous loss. 

 Everything was there in the beginning, and as the 

 wild plant parted with factor after factor there came 

 into being the long series of derived forms. Exquisite 

 as are the results of civilisation, it is by the degrada- 

 tion of the wild that they have been brought about. 

 How far are we justified in regarding this as a 

 picture of the manner in which evolution works ? 



There are certainly other species in which we 

 must suppose that this is the way that the various 

 domesticated forms have arisen. Such, for example, 

 is the case in the rabbit, where most of the colour 

 varieties are recessive to the wild agouti form. Such 

 also is the case in the rat, where the black and albino 

 varieties and the various pattern forms are also reces- 



