Elementary Species m Nature 37 



respectively. Other authors have made still 

 greater numbers of species in the same groups. 



It is very difficult to estimate systematic dif- 

 ferences on the ground of comparative studies 

 alone. All sorts of variability occur, and no 

 individual or small group of specimens can 

 really be considered as a reliable representa- 

 tive of the supposed type. Many original diag- 

 noses of new species have been founded on di- 

 vergent specimens and of course, the type can 

 afterwards neither be derived from this indi- 

 vidual, nor from the diagnosis given. 



This chaotic state of things has brought some 

 botanists to the conviction that even in syste- 

 matic studies only direct experimental evidence 

 can be relied upon. This conception has in- 

 duced them to test the constancy of species and 

 varieties, and to admit as real units only such 

 groups of individuals as prove to be uniform 

 and constant throughout succeeding gener- 

 ations. The late Alexis Jordan, of Lyons in 

 France, made extensive cultures in this direc- 

 tion. In doing so, he discovered that syste- 

 matic species, as a rule, comprise some lesser 

 forms, which often cannot easily be distin- 

 guished when grown in different regions, or by 

 comparing dried material. This fact was, of 

 course, most distasteful to the systematists of 

 his time and even for a long period afterwards 



