Theory of Plant Breeding 



In nature there can be but very seldom so sudden and 

 complete a change of climate as that between the Sahara 

 and an English garden. As Dr. Wallace points out in 

 the most recent of his addresses * (so far as the author is 

 aware), there seems to have been but little change in the 

 European flora since the Glacial Period profoundly 

 altered and disturbed all the climates and conditions of 

 tertiary Europe. 



As a rule plants inherit not only ancestral habits and 

 structures, but also the climate of their ancestors. Their 

 harmony with the environment is of a general business- 

 like character ; it is not, in most cases, exact and 

 mathematical. 



Such genera as the hawk-weeds, brambles, roses, 

 Euphrasia, and several others seem to those who have 

 specialised on them to consist of an enormous number 

 of little species, very difficult to define or to separate, 

 but nevertheless distinct. These might be considered as 

 tentative attempts at a new species, of which a single one 

 might manage not only to survive but to kill out all the 

 others if it was in every way distinctly the best. 



What, however, produces these changes which dis- 

 tinguish the little species from one another ? 



It is upon this point that there is so much uncertainty. 

 Lamarck and especially the Neo-Lamarckians believe 

 that any change in the environment must necessarily 

 affect not one but all the plants which are subjected to 

 the change. 



On a particularly dry and sandy knoll where the soil 

 is poor, every plant will be affected by the conditions 

 and all will become dwarf and stunted. 



But if this change of climate is marked, and especially 

 if it is continued for generations, the effect is not only 

 external but the whole internal condition of the plant 



* Pharmaceutical Journal^ 1909. 

 296 



