Inconstancy of Improved Races 791 



of selection here finds its limit and that cen- 

 turies and perhaps geologic periods of contin- 

 ued effort in the same direction are not capable 

 of adding anything more to the initial effect. 

 Some illustrative examples may suffice to 

 prove the validity of this assertion. Every bot- 

 anist who has studied the agricultural practice 

 of plant-breeding, or the causes of the geo- 

 graphic distribution of plants, will easily recall 

 to his mind numerous similar cases. Perhaps 

 the most striking instance is afforded by culti- 

 vated biennial plants. The most important of 

 them are forage-beets and sugar-beets. They 

 are, of course, cultivated only as biennials, 

 but some annual specimens may be seen each 

 year and in nearly every field. They arise 

 from the same seed as the normal individuals, 

 and their number is obviously dependent on 

 external conditions, and especially on the time 

 of sowing. Ordinary cultures often show as 

 much as 1^ of these useless plants, but the exi- 

 gencies of time and available labor often com- 

 pel the cultivator to have a large part of his 

 fields sown before spring. In central Eu- 

 rope, where the climate is unfavorable at this 

 season, the beets respond by the production of 

 far larger proportions of annual specimens, 

 their number coming often up to 20^ or more, 

 thus constituting noticeable losses in the prod- 



