1888.] Evolution as illustrated by the Geogr. Distrib. of Plants. 289 



These passages would naturally lead to the conclusion that Darwin 

 originally believed that varieties and species originated from some 

 casual change in an individual, but subsequently came to the con- 

 clusion that varieties and species could only establish themselves by 

 the simultaneous like variations of many. There is however a 

 passage in the sixth edition of the " Origin of Species," which occurring 

 as it does in the final summary (p. 423) seems to render this conclusion 

 again somewhat doubtful. 



Darwin is treating of the theory of the descent of organic life from 

 an individual primordial form. 



" It has, he writes, been maintained by several authors that it is as 

 easy to believe in the creation of a million beings as of one, 

 but Maupertuis' philosophical axiom of *" least action '' leads 

 the mind more willingly to admit the smaller number ; and 

 certainly we ought not to believe that innumerable beings 

 within each great class have been created with plain but 

 deceptive marks of descent from a single parent." 

 Here clearly Darwin regards the possession of similar characteristics 

 as conclusively proving descent from the same ancestor. 



The balance of evidence seems, however, on the whole to be in 

 favour of the conclusion that as far as existing species are concerned, 

 Darwin held that they have been evolved subject to selection, by the 

 tendency among individuals of pre-existing species to simultaneous 

 variation in some limited number of directions determined by a 

 subjective cause which may be termed hereditary influence. 



If then we recollect that Darwin held (p. 42) that wide-ranging, 

 much-diffused and common species vary most, it seems almost im- 

 possible to avoid the conclusion that according to Darwin's theory 

 new variations in the same direction will occur over the area or in the 

 different areas over which the species is diffused and will develope 

 into like specific forms wherever surrounding circumstances are not 

 unfavourable, whether it be in one locality or many. 



In confirmation of this view we may quote the following passage 

 (Origin of Species, p. 419) : 



" The existence of closely allied or representative species in any twO' 

 areas implies on the theory of descent with modification that 

 the same parent forms previously inhabited both areas, and 

 we almost invariably find that whenever many closely 

 allied species inhabit two areas some identical species are 

 common to both." 



