Chap. XI. SINGLE CENTRES OF CEEATION. 355 



lias probably received at some former period immigrants 

 from this other region, my theory will be strengthened ; 

 for we can clearly understand, on the principle of 

 modification, why the inhabitants of a region should be 

 related to those of another region, whence it has been 

 stocked. A volcauic island, for instance, upheaved and 

 formed at the distance of a few hundreds of miles from a 

 continent, would probably receive from it in the course 

 of time a few colonists, and their descendants, though 

 modified, would still be plainly related by inheritance to 

 the inhabitants of the continent. Cases of this nature 

 are common, and are, as we shall hereafter more fully 

 see, inexplicable on the theory of independent creation. 

 This view of the relation of species in one region to 

 those in another, does not differ much (by substituting 

 the word variety for species) from that lately advanced 

 in an ingenious paper by Mr. Wallace, in which he con- 

 cludes, that "every species has come into existence 

 coincident both in space and time with a pre-existing 

 closely allied species." And I now know from corre- 

 spondence, that this coincidence he attributes to gene- 

 ration with modification. 



The previous remarks on " single and multiple centres 

 of creation " do not directly bear on another allied 

 question, — namely whether all the individuals of the 

 same species have descended from a single pah*, or 

 single hermaphrodite, or whether, as some authors 

 suppose, from many individuals simultaneously created. 

 With those organic beings which never intercross (if 

 such exist), the species, on my theory, must have de- 

 scended from a succession of improved varieties, which 

 will never have blended with other individuals or varie- 

 ties, but will have supplanted each other ; so that, at each 

 successive stage of modification and improvement, all 

 the individuals of each variety will have descended from 



