156 MENDELISM chap. 



fact that they have resulted from a cross but a few 

 generations back, and it is possible that they often 

 oust the older kinds not because they started as 

 something intrinsically better, but because the latter 

 had gradually deteriorated through continuous self- 

 fertilisation. Most breeders are fully alive to the 

 beneficial results of a cross so far as vigour is con- 

 cerned, but they often hesitate to embark upon it 

 owing to what was held to be the inevitably lengthy 

 and laborious business of recovering the original 

 variety and refixing it, even if in the process it was 

 not altogether lost. That danger Mendelism has 

 removed, and we now know that by working on 

 these lines it is possible in three or four generations 

 to recover the original variety in a fixed state with 

 all the superadded vigour that follows from a cross. 



Nor is the problem one that concerns self-fertilised 

 plants only. Plants that are reproduced asexually 

 often appear to deteriorate after a few generations 

 unless a sexual generation is introduced. New 

 varieties of potato, for example, are frequently put 

 upon the market, and their excellent qualities give 

 them a considerable vogue. Much is expected of 

 them, but time after time they deteriorate in a dis- 

 appointing way and are lost to sight. It is not 

 improbable that we are here concerned with a case 

 in which the plants lose their vigour after a few 

 asexual generations of reproduction from tubers, and 

 can only recover it with the stimulus that results 

 from the interpolation of a sexual generation. Un- 

 fortunately this generally means that the variety is 

 lost, for owing to the haphazard way in which new 

 kinds of potatoes are reproduced it is probable that 



