230 



THE AMERICAN NATURALIST 



[Vol. XLV 



experimental data now available makes it necessary to 

 recognize a clear distinction between tlie evolutionary 

 changes in types, on the one hand, and the fluctuations 

 within each type, on the other hand. 



Quite naturally the first experimental evidence of the 

 existence of permanent hereditary types involved only 

 such characteristics as are clearly distinguishable upon 

 inspection. Thus Jordan was able to demonstrate that 

 within the systematic species Draba verna there are in- 

 cluded as many as two-hundred hereditary forms, whose 

 distinguishing characteristics appear unchanged from 

 generation to generation, in such manner that his pedi- 

 grees of these forms were clearly and permanently dis- 

 tinguishable from each other by easily defined morpho- 

 logical features. Such " petit es especes" or " little 

 species" (afterwards called by De Vries 1 'elementary 

 species," and by Johannsen "biotypes" or "geno- 

 types"), have since been observed by Wittrock and his 

 students, and by many others, in a great number of wild 

 species, and they are now quite generally supposed to be 

 of almost universal occurrence. 



About 1890 N. H. Nilsson made a similar discovery in 

 connection with his breeding of wheat, oats, barley and 

 other grains at Svalof, Sweden, but his work remained 

 practically unknown to the scientific world until it was 

 brought to light several years ago by De Vries. Nilsson 

 found in these grains elementary species, each with its 

 own morphological characters and its own specific ca- 

 pacity to yield crops of given size or quality under given 

 external conditions. More recently, sharp-eyed taxon- 

 omists have been rapidly raising many of the elemen- 

 tary species of wild plants to the rank of systematic 

 species. 



It was natural that the earliest genotypes recognized, 

 such as those of Jordan and Nilsson, should have pos- 

 sessed visibly discrete characteristics, and that they 

 should first have become familiar in normally self-fertil- 

 ized plants, among which little confusion is occasioned 



