68 INVASION 



endemic until the natural regions in which they are found 

 have been accurately defined, a work which has barely be- 

 gun. In the much simpler matter of distribution, upon 

 which the accuracy of statistical methods depends directly, 

 there are few regions suflBciently well known at the present 

 time to yield anything like permanent results. 



POLYGKNESIS. 



Before Darwin, the question of the origin of species at 

 two or more places on the earth's surface, either simultan- 

 eously or successively, was merely one of single or multiple 

 creation, and the answer was usually determined by the bias 

 for or against migration. In the Origin of Species, the ques- 

 tion was summarily dismissed, and migration was invoked to 

 explain all the phenomena of distribution. With rare ex- 

 ceptions, this has been the position of most botanists since 

 Darwin. DeCandoIIe (1820:52), without insisting upon the 

 creation of new species at a single center, seems to have 

 been of this opinion, as the following will indicate: "Si Ton 

 r^fl^chit maintenant a Faction perpetuelle des quatre causes 

 de transport de graines que je viens d' indiquer, les eaux, 

 les vents, les animaux et I'homme, on trouvera, je pense, 

 qu' elles sont bien sufBsantes pour expliquer ce petit nombre 

 de v^g^taux qu'on retrouve semblables dans les continens 

 divers." Schoaw (J824:I22), as an opponent of what he termed 

 the migration theory, must have favored the view that 

 species were created at two or more centres, though he 

 failed to express himself definitely, as the following wiU 

 show : ' 'Ob diese Pflanzen in beiden Theilen der Erde ihre 

 ursprtingliche Heimath haben oder nicht, ist in pfianzen- 

 geographischer Hinsicht gleichgtiltig. " Henfrey (1852:47), 

 discussed briefly the "opinion that species were created in 

 different places, at different centres, either simultaneously 

 or at different epochs," but dismissed it as unsatisfactory 

 because of the possibility of diffusion. A. DeCandoIIe (1855: 

 1114) considered the single or multiple origin of species 

 merely with reference to the number of individuals of each 

 species originally created, reaching the conclusion that both 

 methods were probably to be found in nature. Darwin 



