268 THE SURVIVAL OF THE UNLIKE. [xv. 



mentative paper, it should be explained that a half 

 century ago there was no satisfactory explanation of the 

 means by which plants and animals have become widely 

 disseminated over the earth. This was particularly true 

 respecting the curious phenomena of disconnected dis- 

 tributions, or the fact that some species occur in widely 

 separated and isolated places. Certain plants occur 

 only in eastern America and in Japan, and there may 

 be no other representatives of the genus extant; that is, 

 the genus is monotypic, and has a peculiarly disjointed 

 distribution. There are also certain bitypic genera, of 

 which one species occurs only in eastern America and 

 the other in Japan. There are equally strange distribu- 

 tions of plants and animals in other parts of the world. 

 There were few general hypotheses in vogue at the time 

 Gray wrote, to account for these detached distributions. 

 One was Agassiz's theory, which has been called the 

 autochthonal hypothesis, from the fact that it supposes 

 that each species was borne or brought forth upon the 

 area which it occupies {autochthon, one borne of the 

 land itself). It "maintains, substantially," says Gray, 

 "that each species originated where it now occurs, 

 probably in as great a number of individuals occupy- 

 ing as large an area, and generally the same area, or 

 the same discontinuous areas as at the present time." 

 Much the same view was held by Schouw, of Co- 

 penhagen, who advanced the hypothesis of the double 

 or multiple origin of species, but he supposed that the 

 species had the power of greatly distributing itself 

 when it was once created in a given region. It was 

 even then (Schouw wrote in 1837) maintained by vari- 

 ous naturalists that species had sprung from one pro- 

 genitor, but Schouw declared that "when we look at 



