CHAPTER TWENTY 



Distribution of Species 



From the time of Linnaeus until well into the post-Darwinian era, bio- 

 geography was a very active field, in which many of the most distinguished 

 biologists worked. New discoveries of major importance could be expected 

 as the reward of competent work in this field. New phyla were described 

 fairly frequently, new classes and orders with regularity. But, by the end 

 of the nineteenth century, all of the major biogeographical realms were 

 fairly well known, their floras and faunas were catalogued, and only prob- 

 lems of detail and revision confronted young biogeographers. Meanwhile, 

 the rise of experimental biology made biogeography and taxonomy rather 

 passe, and their practitioners were often regarded as hack workers. The 

 modern revival of evolutionary studies has brought with it a renewed 

 interest in taxonomy, and more recently biogeography has shared in this 

 renaissance. There are problems in evolution which can be profitably 

 attacked only if detailed biogeographical information, both present and 

 past, be studied. Some of these problems will be considered below. 



In one sense, problems of distribution overlap those of isolation. The 

 minimum biogeographical requirement for relationship between two spe- 

 cies is that their ancestors must at some time have lived in the same area. 

 In other words, permanent isolation and relationship are mutually exclu- 

 sive. Thus, if the magnolias of southeast China are related to those of 

 southeastern United States, their present isolation must not have charac- 

 terized all past ages. As a matter of fact, it is well established that their 

 distribution was continuous in the Tertiary. On the other hand, the devel- 

 opment of geographic isolation between related populations is certainly 

 an important aspect of biogeography. 



ISLAND LIFE 



It was the study of the flora and fauna of the Galapagos Islands that first 

 caused Darwin to consider the possibility that species might be mutable; 

 and the island life of the Malay Archipelago played a major role in bring- 

 ing Wallace to the same conclusion. Since their time, the study of oceanic 

 island life has always been an important aspect of the study of evolution. 

 Many features inherent in the island locale combine to cause this. The 

 fact that the inhabitants of any island generally resemble those of the 



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