778 Fluctuations 



selection is not limited to a single choice, but 

 may be applied in two or more succeeding gen- 

 erations. Obviously such a repetition affords 

 a better chance of increasing the progression of 

 the average and of ameliorating the race to a 

 greater degree than would be possible by a sin- 

 gle choice. This principle of repeated selection 

 is at present the prominent feature of race- 

 improvement. Next to variety-testing and 

 hybridizing it is the great source of the steady 

 progression of agricultural crops. From a 

 practical standpoint the method is clear and as 

 perfect as might be expected, but this is not the 

 side of the problem with which we are concerned 

 here. The theoretical analysis and explana- 

 tion of the results obtained, however, is sub- 

 ject to much doubt, and to a great divergence 

 of conceptions. So it is also with the applica- 

 tion of the practical processes to those occur- 

 ring in nature. Some assume that here, repeated 

 selection is only of subordinate importance, 

 while others declare that the whole process of 

 evolution is due to this agency. This very im- 

 portant point however, will be reserved for the 

 next lecture, and only the facts available at pres- 

 ent will be considered here. 



As a first example we may take the ray-florets 

 of the composites. On a former occasion we 

 have dealt with their fluctuation in number and 



