GEOGRAPHICAL DISTRIBUTION OF ORGANIC BEINGS. 115 



under past and present conditions permitted, is the most 

 probable. Undoubtedly many cases occur, in which we 

 can not explain how the same species could have passed 

 from one point to the other. But the geographical and 

 climatal changes, which have certainly occurred within 

 recent geological times, must have rendered discontinuous 

 the formerly continuous range of many species. So that 

 we are reduced to consider whether the exceptions to con- 

 tinuity of range are 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 pro- 

 duced within one area, and has migrated thence as far as 

 it could. 



ISOLATED CONTINENTS NEVER WERE UNITED. 



Origin of Whenever it is fully admitted, as it will 



Species, some day be, that each species has proceeded 

 from a single birthplace, and when in the 

 course of time we know something definite about the 

 means of distribution, we shall be enabled to speculate 

 with security on the former extension of the land. But 

 I do not believe that it will ever be proved that within 

 the recent period most of our continents which now 

 stand quite separate have been continuously, or almost 

 continuously, united with each other, and with the many 

 existing oceanic islands. Several facts in distribution, 

 such as the great difference in the marine faunas on the 

 opposite sides of almost every continent, the close rela- 

 tion of the tertiary inhabitants of several lands and even 

 seas to their present inhabitants, the degree of affinity 

 between the mammals inhabiting islands with those of 

 the nearest continent, being in part determined (as we 

 shall hereafter see) by the depth of the intervening ocean, 

 these and other such facts are opposed to the admission 



