84 HEREDITY 



The breeder therefore confines himself to the growing 

 of large numbers of seedlings, and searching amongst 

 them for valuable new types ; or else simply to watching 

 for bud variations. Among plants of this type are the 

 potato and a great number of fruit plants and flowers, 

 such as the apple, strawberry, and carnation. It is 

 in such species that perhaps the most remarkable im- 

 provements have been made, the absence of any neces- 

 sity to fix new types having rendered improvement 

 easier and more rapid. 



There are other species of plants which are repro- 

 duced regularly by cross-fertilisation, or in which cross- 

 fertilisation is at least so frequent that a material loss 

 of vigour occurs when it is prevented, yet in which 

 systematic crossing on the lines described for maize 

 would be impracticable, owing to the male and female 

 organs occurring mixed together. In such cases separa- 

 tion of pure types would be useless, as their vigour 

 would be materially less than that of a mixed, cross- 

 bred " population." In such cases all that can be 

 done, apparently, is to select continuously for the 

 special characters required and also for vigour. In 

 sugar-beet, where a very thorough system of selection 

 has been followed with regard to the percentage of 

 sugar in the root, this has been increased from about 

 eight or nine to seventeen or eighteen. The improve- 

 ment was fairly rapid at first, but soon slowed down, 

 and now, in spite of the improved methods latterly 

 used, the increase seems to have ceased. In fact, it 

 seems that rigorous selection is necessary to maintain 

 the sugar content at the level which it has reached. 

 This is but what we should expect, for the probability 

 is that one is continually selecting hybrids, and the 

 hybrid condition as such is unfixable. 



A similar process of selection, on a somewhat less 

 elaborate scale, is carried out with other species which 

 have a similar method of reproduction to that of the 

 sugar-beet. The method is somewhat costly and 

 laborious, yet science shows us no more simple method 

 of securing the desired end. 



