CHAPTER VIII 



GEOGRAPHIC ISOLATION AND SPECIES- 

 FORMING 



"For me, it is the chorology of organisms, the study of all the 

 important phenomena embraced in the geography of animals and 

 plants, which is the surest guide to the knowledge of the real phases in- 

 the process of the formation of species." -MoRiTZ WAGNER. 



A FLOOD of light may be thrown on the general problem 

 of the origin of species by the study of certain evidence as to 

 the l actual origin of species with which we may be familiar, or of 

 which the actual history or the actual ramifications may in 

 some degree be traced. 



In such cases, one of the first questions naturally asked is 

 this: Where did the species come from? Migration forms a 

 large part of the history of any species or group of forms. The 

 fauna of any given region is made up of the various species of 

 animals living naturally within its borders. The flora of a 

 region is made up of the plants which grow naturally within 

 its limits. Of all these, animals and plants, the inhabitants 

 of most regions are apparently largely migrants from some 

 other region. Some have entered the region in question 

 before acquiring their present specific characters; others come 

 after having done so. Which of these conditions apply to any 

 given case can sometimes be ascertained by the comparison 

 of the individuals along the supposed route of migration. 



Thus, Dr. C. Hart Merriam has undertaken to show the 

 actual origin of nine species of Californian chipmunks (Eutamias) 

 by an elaborate study of their distribution, adaptations, and 

 transformations. He finds them closely related to one another, 



1 A paper published in "Science," 1906, by the senior author, under 

 the title "The Actual Origin of Species," has been freely quoted from in 

 this chapter. 



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