THE ORIGIN OF SPECIES. 13 



sterility of hybrid progeny interposes an effectual 

 barrier against the blending of the original species 

 by crossing. 



From this generally-accepted view the well-known 

 theory of Agassiz and the recent one of Darwin diverge 

 in exactly opposite directions. 



That of Agassiz differs fundamentally from the 

 ordinary view only in this, that it discards the idea of 

 a common descent as the real bond of union among 

 the individuals of a species, and also the idea of a local 

 origin — supposing, instead, that each species origi- 

 nated simultaneously, generally speaking, over the 

 whole geographical area it now occupies or has occu- 

 pied, and in perhaps as many individuals as it num- 

 bered at any subsequent period. 



Mr. Darwin, on the other hand, holds the orthodox 

 view r of the descent of all the individuals of a species 

 not only from a local birthplace, but from a single 

 ancestor or pair ; and that each species has extended 

 and established itself, through natural agencies, wher- 

 ever it could ; so that the actual geographical distri- 

 bution of any species is by no means a primordial ar- 

 rangement, but a natural result. He goes farther, 

 and this volume is a protracted argument intended to 

 prove that the species we recognize have not been in- 

 dependently created, as such, but have descended, like 

 varieties, from other species. Varieties, on this view, 

 are incipient or possible species : species are varieties 

 of a larger growth and a wider and earlier divergence 

 from the parent stock ; the difference is one of degree, 

 not of kind. 



The ordinary view — rendering unto Ca?sar the 



