7608 Notices of New Books. 



by extensive temperate and tropical lowlands, across which the mi- 

 gration of cold-loving species could not, under ordinary circum- 

 stances, take place ; the wide distvibutioo of fresh-water animal and 

 vegetable species occurring at distant stations separated by im- 

 passable barriers of land, and the occurrence of terrestrial species 

 common to islands and mainlands separated by hundreds of miles of 

 open sea acting as a barrier to free inter-migration : all such cases 

 Mr. Darwin looks upon as exceptional to a generally-pervading law, 

 that the individuals of a species have a contiguous range, and con- 

 siders that they can be accounted for by occasional and accidental 

 means of dispersal ; for instance, by variations that have taken place 

 in the level of the earth's surface, altering the form and relative con- 

 tiguity of continents and islands, by which once-existing roads of 

 migration have been obliterated ; by ocean currents effecting the dis- 

 tribution of seeds and plants ; by migratory birds carrying with them, 

 over hundreds of miles of open sea, imdigested seeds ; by wading 

 birds conveying and distributing, by simple adhesign to their feet, the 

 young of small fresh-water mollusks, and seeds of fresh-water plants ; 

 by gradual changes of temperature on the earth's surface, as evidenced 

 in the once-existing glacial condition of present temperate regions, 

 causing the migration, northward and southward, of species to and 

 from various zones of latitude. 



The occurrence of identical alpine species on widely separated 

 mountain ranges, Mr. Darwin suggests must have been effected by this 

 process during the glacial period, the alpine species then spreading 

 over the lowlands having been exterminated and replaced by temperate 

 species on the returning warmth, except where they would be able to 

 recede northward and up the higher mountains to suitable tempera- 

 tures. We cannot, however, avoid the conclusion that the tropics must 

 have undergone a corresponding diminution of temperature with the 

 more northern zones, and would probably, when central Europe was 

 ice-bound with glaciers, not have been warmer than our present tem- 

 perate regions, inevitably causing the utter extermination of all animals 

 and plants that were dependant on tropical heat for their existence, and 

 necessitating a fresh special creation to meet the altered condition of 

 the earth's surface on the return of warmth after the glacial period. 



In reading the first thirteen chapters of Mr. Darwin's work, we must 

 confess that, however widely we dissent from his conclusions, we go 

 hand in hand with him in weighing the various phases of the question, 

 and our most perfect confidence in his sincerity assures us that he is 

 striving after truth, and that, with all openness and honesty, he is fairly 



