Horticultural Wirictics Ariscu Suddenly. 103 



either because they produce no seed or yield too small 

 a harvest when self-fertilized or because they cannot be 

 artificially fertilized on a sufficiently large scale or be- 

 cause the number of seeds produced, even under normal 

 conditions, is too small. Moreover one is almost abso- 

 lutely confined to annual or biennial species or to such 

 perennial ones as flower freely in the first year. 



But in spite of these difiiculties and of the incom- 

 pleteness of the observations made hitherto, we may 

 safely conclude from them the possibility of an experi- 

 mental study of the origin of horticultural varieties.^ 



* I shall describe an experiment of this kind with Linaria vul- 

 garis pcloria in § 20. 



