CHAPTER XII 

 EVIDENCES FROM GEOGRAPHIC DISTRIBUTION 



PRINCIPLES OF GEOGRAPHIC DISTRIBUTION 



Just as palaeontology may be said to be a study of the vertical 

 distribution (distribution in time) of organisms, so geographic distribu- 

 tion may be called a study of the horizontal distribution of organisms, 

 on the earth's surface at any given time (spatial distribution). We are 

 chiefly to be concerned with the present spatial distribution of animal 

 and plant species, but equally interesting studies have been and still 

 may be made of the horizontal or contemporaneous existence of 

 extinct forms. Much new knowledge has been gained by combining 

 the data of palaeontology with those of geographic distribution. In 

 fact, neither field can be studied profitably without recourse to the 

 other. This fact was clearly perceived by J. A. Thomson in his little 

 manual on Evolution when he combined the two types of evidence in 

 one chapter under the title "Evidences of Evolution from Explorer 

 and Palaeontologist." 



It was a consideration of the present and of the past distribution 

 of Edentates that led Charles Darwin to his first clear concept of 

 descent with modification. In his voyage on the "Beagle" he found 

 that present-day Edentates (armadillos, sloths, anteaters), a very 

 peculiar group of archaic mammals, are practically confined to South 

 America. When he also found that the only fossil Edentates, resem- 

 bling but also differing from the existing types, are also confined to 

 South America, he easily arrived at the only inference permitted by 

 the facts: that the present Edentates are the modified descendants 

 of the Edentates of the past. 



The following quotations from both an older and a recent writer 

 will give the reader a clear idea of the ways in which the general facts 

 of geographic distribution bear witness to the truth of the evolutionary 

 principle. 



"The theory," says Wallace, 1 "which we may now take as estab- 

 lished — that all the existing forms of life have been derived from other 

 forms by a natural process of descent with modification, and that this 

 same process has been in action during past geological time — should 



1 From A. R. Wallace, Darwinism (1889). Used by special permission of the 

 publishers, The Macmillan Company. 



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