144 Controversial Questions. 



variability serves to place the antithesis between muta- 

 bility and individual variability in an even stronger light. 



I shall therefore briefly touch on a few examples of 

 partial variability. 



One of the most valuable pieces of w^ork in biology 

 are Stahl's classical researches on the effect of sunny 

 and shady positions on the development of foliage leaves.^ 

 The insolated leaves are generally smaller, stouter, poorer 

 in air spaces and richer in chlorophyll and have stronger 

 veins; they are in fact adapted to turn the strong sun- 

 lio-ht to best account. The shaded leaves are broader and 

 thinner, with larger air spaces and delicate epidermis — 

 in fact eminently adapted to make the most of the meagre 

 supply of light at their disposal. Lactuca, Iris, Fagus 

 are the best known examples. The more a species has 

 become specialized as a ''sun-plant" as Pinus, or a ''shade- 

 plant" as Chclidoiiium, the less is its power of adaptation 

 in this direction. 



Still more important, if possible, than Stahl^s re- 

 searches are Gaston Bonnier's recent investigations on 

 the adaptation of plants to arctic and alpine climates.- 

 Both authors dealt with partial variability; the former 

 dealing with the problem from a comparative standpoint, 

 the latter from an experimental one. In Bonnier's ex- 

 periments a single individual of each species that was 

 dealt with was divided into two. One of the parts was 

 then grown on the Alps or Pyrenees ; the other on low 

 lying land. In the course of a very short time the former 



^ Jenaische Zeitschrift fur Naturw., XVI, N. F., IX, i, 2, 1883. 



^G. Bonnier. Rccherches expcrimcntalcs siir Vadaptation dcs 

 plantcs au cUmat alpin. Ann. Sci. nat. 7. Serie, T. 20. Lcs plantes 

 arctiqucs coinparccs aux mhnes cspcccs dcs Alpes ef des Pyrenees. 

 Revue generale de Botanique, Tome 6; Influence de la lumicre elec- 

 triqiie sur la forme ct la structure des plantes. Ibid., T. 7, 1896. 



