328 • SINGLE CENTKES OF CKEATION. Cii.vr. XI. 



contiiuiity of rani^c arc so numerous and of so grave a nature, 

 that we ought to give up the belief, rendered probable by 

 general considerations, that each species has been produced 

 within one area, and has migrated thence as far as it could. It 

 ^vould be hopelessly tedious to discuss all the exceptional cases 

 of the same species, now living- at distant and separated points; 

 nor do I for a moment i)retend that any explanation could l)c 

 olVercd of many instances. But after some jireliminary remarks, 

 I will discuss a few of the most striking classes of facts ; namely, 

 the existence of the same species on the summits of distant; 

 mountain-ranges, and at distant points in the arctic and ant- 

 arctic regions; and secondly (in the following chapter), tlic 

 wide distribution of fresh-water productions ; and thirdly, the 

 occurrence of the same terrestrial species on islands and on the 

 main-land, though separated by hundreds of miles of open sea. 

 If the existence of the same species at distant and isolated 

 points of the earth's surface, can in many instances be ex- 

 plained on the view of each species having migrated from a 

 single birthplace; then, considering our ignorance Avith respect 

 to former climatal and geographical changes, and to the various 

 occasional means of transport, the belief that a single birthplace 

 is the law, seems to me incomparably the safest. 



In discussing this subject, we shall be enabled at the same 

 time to consider a point equally important for us, namely, 

 whether the several species of a genus, which must on the 

 iheorj- all be descended from a common progenitor, can have 

 migrated, undergoing modification during their migration, from 

 some one area. When most of the species inhabiting one re- 

 gion are diflcrcnt from those of another region, but are closely 

 allied or belong to the same genera, if in all such cases it can 

 be shown that there probably has been at some former period 

 migration from the one region to the other, our general view 

 will be much strengthened ; for the explanation is obvious on 

 the principle of descent with modification. A volcanic island, 

 for instance, upheaved and fomied at the distance of a few 

 hundreds of miles from a continent, would prol^ably receive from 

 it in the course of time a few colonists, and their descendants, 

 though modified, would still be related by inheritance to the 

 inhabitants of that continent. Cases of this nature are common, 

 and are, as we shall hereafter see, inexplicable on the tlicory of 

 indi'pendent creation. This view of the relation of the species 

 of one region to those of another, does not differ much from 

 that advanced by Mr. "Wallace, who concludes that " every spc- 



