THE ORIGIN OF SPECIES 



limited an area. In the case of young endemics, the answer is obvious: 

 they simply have not had time to achieve the range extension which may 

 be expected in the future. Whether this is a sufficiently frequent occur- 

 rence to have general importance will be discussed below in connection 

 with the Age and Area theory of Willis. The more usual situation appears 

 to be that endemics are relics of ancient groups, like the Sequoias and the 

 Cypress. Such endemics generally appear to have very little genetic vari- 

 ability, with the result that they are adaptable only to a narrow range of 

 environments. This must be based in part upon a very low mutation rate. 

 Also, it may be that much variability which such species possessed in the 

 past may have been lost by genetic drift as the population contracted. 



CONTINENTAL DISTRIBUTION 



But island life and endemism are specialized phenomena. The continental 

 distributions of most organisms must be based upon other principles. 

 Simpson has shown that the history of the mastodonts, an extinct family 

 closely related to elephants, illustrates clearly the more general principles 

 of distribution. Mastodonts first appear in the fossil record of north Africa 

 in the Oligocene. As other Oligocene faunas from all parts of the world 

 are well known, it is quite certain that north Africa was the place of origin 

 of the mastodonts, and that they did not occur elsewhere at that time. 

 But they immediately began to expand their range by active migration, 

 and by the beginning of the Miocene period they had reached mid- 

 continental Africa to the south, the Baltic area to the north, and India 

 to the east. By mid-Miocene, they occupied about half of Africa and most 

 of Europe and Asia except the most northerly parts. Late in the Miocene, 

 they crossed the Bering Strait to North America, over which they spread 

 during the Pliocene. Only toward the end of the Pliocene did the masto- 

 donts reach South America, over much of which they spread during the 

 Pleistocene. About this time, the mastodonts became extinct in the Old 

 World, and by the late Pleistocene, the Americas were their last remain- 

 ing refuge. Finally, the American mastodonts also disappeared. 



This history may be restated in more general and more explanatory 

 terms. A species (or larger group) originates in a definite, more or less 

 restricted area ( north Africa in the case of the mastodonts ) , It then tends 

 to spread by active migration in all directions, occupying whatever suit- 

 able habitats it may find, until it reaches an impassable barrier ( the hmits 

 of the continents of Africa, Europe, and Asia in the present example). 

 Further distribution may then stop, or the group may find a way to cross 

 a barrier, as the mastodonts did cross the Bering Strait to North America. 

 This may be accomplished by removal of the harrier by geologic or cli- 

 matic changes. Once such a major barrier is crossed, the point of crossing 

 becomes a new center of dispersal from which the group again expands 

 its range until it meets an impassable barrier. After a time, climatic, topo- 

 graphical, or biotic conditions change sufficiently that the group can no 

 longer compete adequately over all or part of its range. Local populations 

 then become sparse, then altogether extinct. Finally, only an isolated 



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