1888.] Evolution as illustrated by the Geogr. Distrib. of Plants. 277 



These are as follows . 



(1.) There is no essential difference, but one of degree only, 

 between variety and species and between species and genus. 

 (2.) Existing specific forms ha ve in some way been evolved from 



specifically different pre-existing forms. 

 ,(3.) The process by which existing species have been evolved is 

 wholly or partially what Darwin has called " natural 

 selection " (including in this term for convenience sexual 

 selection also). 

 The points in which the two theories differ are as follows : 

 (1.) According to the migration theory variations are due to 

 environment only or to causes so obscure that they may conveniently 

 be ascribed to chance. They are all-round in character and almost 

 infinite in number, and there is no natural tendency or capability of 

 varying in any prescribed directions. 



Similar variations may be produced simultaneously in different 



regions from similar environment, bat as the combinations of external 



circumstances are practically infinite, as are likewise the possible 



variations of the species, the chances of the same varieties and species 



. beingevolved simultaneously and independently are infinitesimal. 



The theory of simultaneous evolution on the other hand depends 

 on the following hypothesis : 



(2.) Variations, being correlative, are only possible in a compara- 

 tively limited number of directions and naturally tend to follow certain 

 lines, so that amidst circumstances not too dissimilar similar variations 

 will occur independently and simultaneously both in the different parts 

 of the same area and even in the different areas over which a species 

 is diffused ; so that a varying species without any necessary inter- 

 communication evolves throughout its area either the same or closely 

 allied species. 



According to the migration theory species originating locally have 

 become more or less widely disseminated by migration. The seat of the 

 species being generally assumed to be the region in which the species 

 and its congeners are most numerous. Going back to this region and 

 applying the same theory, we have to conclude that the species now 

 distributed over it originated in some limited locality, and continuing 

 the process the species is ultimately traced back either to a number of 

 individuals of the species somewhere simultaneously and independently 

 evolved, or to a single individual ancestor or pair of ancestors, from 

 which all the individuals of the species have come by lineal descent. 



