GEOGRAPHICAL DISTRIBUTION 141 



fern of almost universal occurrence, in both temperate 

 and torrid latitudes, eastern and western hemispheres, 

 could be easily explained on the supposition that it had 

 gradually spread from the center of distribution where 

 it was created, or on the theory that it had been indepen- 

 dently "created" in many different localities. 



The idea that the same species was "created" indepen- 

 dently in different localities, from which it might spread, 

 was taught by Gmelin as early as 1747. It is often 

 referred to as Schouw's hypothesis, from the Danish 

 botanist who elaborated and urged it in the first part of 

 the nineteenth century. 1 Reasoning from the facts of 

 discontinuous distribution (to be noted in following para- 

 graphs) Schouw argued for the hypothesis of the multiple 

 origin of species, that is, that there were originally many 

 primary individuals. The existing vegetation of the 

 globe was not created at once, argued Schouw, but by 

 degrees, since the surface of the earth has only gradually 

 become fitted for the growth of plants, and moreover 

 certain plants (e.g., parasites) depend upon the existence 

 of others, and therefore the latter must have previously 

 existed. 2 The hypothesis of multiple origin was also, at 



1 Schouw, J. F. Desedibus plantarum originariis, Hauniae, 1816. His 

 memoir On the origin of plants was published in Danish in 1847, and the 

 English translation by N. Wallich was published in Hooker's Journal of 

 Botany, 2:321-326,373-377. London, 1850; and Ibid, 3: 11-14, 1851. 



2 This is an interesting illustration of how the same kind of evidence may 

 lead one student toward the truth and another toward error. Schouw was 

 proposing the ideas here set forth at the same time that Darwin was 

 elaborating his theory of natural selection, and only twelve years before 

 the appearance of the Origin. Raising the question as to whether new 

 species continue to be created, or whether the existing vegetable kingdom 

 has been finally completed, he argues that, "The most rational mode for 

 accounting for new species being possibly created, seems to be by suppos- 



