1 64 MENDELISM chap. 



to be the inevitably lengthy and laborious business of re- 

 covering the original variety and refixing it, even if in the 

 process it was not altogether lost. That danger Mendel- 

 ism 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 sex- 

 ual 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 de- 

 teriorate in a disappointing 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 inter- 

 polation of a sexual generation. Unfortunately this 

 generally means that the variety is lost, for owing to the 

 haphazard way in which new kinds of potatoes are repro- 

 duced it is probable that most cultivated varieties are 

 complex heterozygotes. Were the potato plant subjected 

 to careful analysis and the various factors determined 

 upon which its variations depend, we should be in a posi- 

 tion to remake continually any good potato without 



