Exa}uplcs of Constant Races. 77 



vioiisly made under conditions which excluded the effects 

 of crossing. 



In spite of the existence of these experimental data, 

 it is still the general view that varieties are inconstant 

 forms. That which distinguishes them from true spe- 

 cies is supposed to be their faculty of giving rise to 

 occasional and not even rare reversions to the type of 

 the species. This, moreover, is supposed to be a proof 

 of their assumed relation to their species. 



Every one of course is free to choose his own defi- 

 nition of a variety. But he wdio makes inconstancy an 

 essential part of the definition will have to exclude a very 

 large number — and perhaps the most important — of our 

 horticultural varieties, and regard them as elementary 

 species. 



I have spent much time in the endeavor to test the 

 constancy of horticultural and also of wild varieties witli 

 a view partly of directly satisfying myself as to their 

 purity and partly of finding inconstant forms for sub- 

 sequent experiments. I have usually started from seed 

 but sometimes, in the case of perennial varieties, from 

 bought plants. Whenever possible the visits of insects 

 were excluded and the plants artificially pollinated. But 

 in the great majority of cases pollination has to be left 

 to bumblebees and moths, and we must be content in 

 providing complete isolation. 



The most important point is the extent of the experi- 

 ment. Absolute constancy can obviously never be di- 

 rectly demonstrated. The space and time needed for 

 other experiments seldom permit the bringing to flower 

 of more than a few thousands of plants of one sort. 

 And even if this is continued for several years the possi- 

 bility of the occurrence of rare cases of atavism (e. g.. 



