CuAP. XI. SINGLE CENTRES OF CREATION. . 327 



by mip^raliun, the fact is g'iv^cn as something njiiarkablc and 

 exceptional. The incapacity of migrating across a wide sea is 

 more clear in the case of terrestrial mannnals than perhaps with 

 any other organic beings; and, accordingly, Ave iind no inex- 

 plicable instances of the same mammals inhabiting distant 

 points of the world. No geologist feels any dilliculty in Great 

 Britain possessing tlu^ same (juadrupeds with the rest of Europe, 

 for they were no doubt once united. But if the same species 

 can be produced at two separate points, why do we not find a 

 single mammal common to Europe and Australia or Soutlx 

 America ? The conditions of life are nearly the same, so that 

 a multitude of European animals and plants have become nat- 

 uralized in America and Australia; and some of the aboriginal 

 plants are identically the same at these distant points of the 

 northern and southern hemispheres ? The answer, as I believe, 

 is, that mammals have not been able to migrate, whereas some 

 ]>lants, from their varied means of disp(>rsal, have migrated 

 across the wide and broken interspaces. The great and striking 

 influence of barriers of all kinds is intelligible only on the view 

 that the great majority of species have been produced on one 

 side, and have not been able to migrate to the opposite side. 

 Some few families, many sub-families, very many genera, and a 

 still greater number of sections of genera, are conlined to a sin- 

 gle region ; and it has been oljserved by several naturalists that 

 the most natural genera or those genera in whicii the species are 

 most closely related to each other, are generally confined to the 

 same countr}', or if they have a wide range that their range 

 is continuous. What a strange anomaly it woidd be, if a di- 

 rectly opposite rule were to prevail, when we go down one step 

 lower in the series, namely, to the individuals of the same spe- 

 cies, and these had not been, at least at first, confined to some 

 one region ! 



Hence it seems to me, as it has to many other naturalists, 

 that the view of each species having been produced in one area 

 alone, and having subsequently migrated from that area as far 

 as its powers of migration and subsistence under past and pres- 

 ent conditions permitted, is the most jirobable. Undoubtedly 

 many cases occur, in which we cannot explain how the same 

 species could have jiassed from one point to the other. But 

 tlic geographical and dimatal dianges, which have certainly 

 occurred within recent geological times, nuist have rendered 

 discontinuous the formerly conliinious range of many species. 

 So that we are reduced to consider whether the exceptions to 



