GEOGRAPHICAL DISTRIBUTION. 
385 
The Geographical Distribution of the terrestrial mollusca over 
the surface of the earth is a subject fraught with many abstruse but 
interesting problems, bearing not only upon the origin of the various 
species and groups hut upon that of the entire animal and vegetable 
kingdom. 
Although the more simply organized and primitive forms are now 
so widely diffused, it is owing chietly to their vast antiquity that this 
has been accomplished, as the dispersal of the terrestrial species is but 
slow, and almost invariably across more or less intimately connected 
tracts of country, their range in time enabling them to take advantage 
of the probably nnmerons geographical changes and varied land con- 
nections to overspread the globe. 
The simpler and more primitive the species or group and the more 
ancient its origin, the wider but more discontinuous is its range in 
space, while the more complex and recent forms have a comparatively 
restricted yet more compact distribution. 
Distribution or dispersal has, however, doubtless been influenced 
by the climatal changes the globe has undergone, these fluctuations 
having been such that at no distant date a colder climate extended 
over a large part of the northern hemisphere, and although the 
severity of this epoch would appear to have been greatly exaggerated, 
jmt it was undoubtedly accompanied by the formation of extensive 
glaciers. These frigid conditions were preceded by a warmer miocene 
period, during which deciduous trees and evergreens flourished within 
ten degrees of the pole, but these changes of climate were so exces- 
sively slow, that if any power of adaptability be conceded to organized 
life, we are compelled to allow that most of the less severe changes 
would have been guarded against by snitable modifications of the 
organisms in response to the gradually changing conditions to which 
they were subjected. 
The changes of habitat by most species are, however, not due merely 
to climate, but to the evolution of more advanced races, whose migra- 
tions are the chief cause of the restriction of the less adaptable and 
more ancient forms of life to remote, inclement or isolated regions, 
where they are temporarily comparatively free from the competition 
of more advanced forms; arctic animals and plants are not following 
the colder conditions from preference, but because they are com- 
pelled by the stronger forms to fall back and adapt themselves to 
cold or barren stations not occupied by the stronger races. 
10/8/1900 
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