350 THE HISTORY OF CHEATION. 



CHAPTER Xiy. 



MIGRATIOTT AND DISTRIBUTION OF ORGANISMS. 

 CHOROLOGY AND THE ICE-PERIOD OF THE EARTH. 



Chorological Facts and Canses. — 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 Elevations 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 



