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 
