THE SCIENCE OF BOTANY 
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in the field as to the mechanism of evolution, based chiefly 
on the two forms of variation which appear to characterise 
living organisms. 
Plants are not distributed at haphazard upon the surface 
of the earth ; each kind occupies a definite area. The 
study of the facts thus supplied by exploration and taxonomy 
forms the science of geographical distribution . The expla- 
nation of the facts involves the study of physiological, 
physical, and ecological conditions in all parts of the world, 
the past geological history of the earth, the morphology and 
phylogeny of the plants, their method of migration, their 
variation, &c., and at the same time helps to throw light upon 
all these subjects. 
It is also becoming recognised that, since geological 
evidence indicates that existing plants are descended from 
plants of warm climates, it is very important that a close 
study should be made of the existing tropical flora, and 
especially from an ecological and physiological point of view 
and in connection with fossil botany and morphology. Our 
present generalisations are too largely based on the phe- 
nomena of plants in cold countries, where the species are 
fewer and where the effect of climate is so strongly marked 
that it masks the individual peculiarities of plants. Many 
of these generalisations are only special cases of wider ones 
to be drawn from the study of tropical botany . 
Other important lines of work are also arising from the 
subdivision of those mentioned, e.g. the study of the diseases 
of plants, or vegetable pathology , which is an offshoot of 
physiology and ecology. 
All branches of Botany, then, are mutually interdepen- 
dent, and also require the aid of physics, chemistry, zoology, 
geology and other allied sciences. They must be studied 
so as to throw light upon one another’s problems, not as if 
they were independent sciences. It is from this point of 
view that the following pages are written. 
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