350 THE HISTORY OF CREATION. 
CHAPTER XIV. 
MIGRATION AND DISTRIBUTION OF ORGANISMS. 
CHOROLOGY AND THE ICE-PERIOD OF THE EARTH. 
Chorological Facts and Causes.—Origin of most Species in one Single 
Locality : ‘Centres of Creation.””—Distribution by Migration.—Active 
and Passive Migrations of Animals and Plants.—Means of Transport.— 
Transport of Germs by Water and by Wind.—Continual Change of the 
Area of Distribution by Hlevations and Depressions of the Ground.— 
Chorological Importance of Geological Processes.—Influence of the 
Change of Climate.—Ice or Glacial Period.—Its Importance to 
Chorology.—Importance of Migrations for the Origin of New Species. 
—Isolation of Colonists——Wagner’s Law of Migration—Connection 
between the Theory of Migration and the Theory of Selection Agree- 
ment of its Results with the Theory of Descent. 
As I have repeatedly said, but cannot too much emphasize, 
the actual value and invincible strength of the Theory 
of Descent does not lie in its explaining this or that single 
phenomenon, but in the fact that it explains all biological 
phenomena, that it makes all botanical and zoological 
series of phenomena intelligible in their relations to one 
another. Hence every thoughtful investigator is the more 
firmly and deeply convinced of its truth the more he 
advances from single biological observations to a general 
view of the whole domain of animal and vegetable life. 
Let us now, starting from this comprehensive point of view, 
survey a biological domain, the varied and complicated 
