CHAPTER VI 



VARIATION AND LOCALITY 



In all discussions of the modes of Evolution the phenomena of 

 Geographical Distribution have been admitted to be of para- 

 mount importance. First came the broad question, were the 

 facts of distribution consistent with the Doctrine of Descent? 

 I suppose all naturalists are now agreed that they are thus 

 consistent, and that though some very curious and as yet in- 

 explicable cases remain to be accounted for, the distribution of 

 animal and plant life on the face of the earth is much what we 

 might expect as a result of a process of descent with modification. 

 Passing from this general admission to the more particular ques- 

 tion whether the facts of distribution favour one special con- 

 ception of the mode of progress of evolution rather than another, 

 no agreement has yet been reached. One outstanding feature 

 is hardly in dispute, namely that prolonged isolation is generally 

 followed by greater or less change in the population isolated. 

 Groups of individuals which from various causes are debarred 

 from free intermixture with other groups almost always exhibit 

 peculiarities, but on the other hand, cosmopolitan types which 

 range over wide areas are on the whole uniform, or nearly so 

 throughout their distribution. Examples of these two categories 

 will be familiar to all naturalists. The barriers to intercourse 

 may be seas, deserts, prairies, mountain-chains, or circumstances 

 of a much less obvious character which isolate quite as effectually. 

 The local unit is not necessarily an island, a district, or an area 

 of special geological formation, but may, as every collector knows, 

 be a valley, a pond, a creek, a "bank" in the sea, a clump of 

 trees, a group of rocks in a bay, or a particular patch of ground 

 on a mountain side. All the great groups provide examples of 

 such specially isolated forms. The botanist knows them well; 

 the conchologist, the entomologist, the ornithologist and the 

 student of marine life are all equally aware that special varieties 



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