470 AGRICULTURAL REPORT 



eouthom hemispheres, nor even in the same zone in tlie Old and in the New 

 World, certain classes of plants being exclusively circumscribed within certain 

 regions, such as the magnolia and cactus in America, and certain animals, as 

 the kangaroos in Australia, and the elephant, rhinocer-^s, and hippopotamus in 

 Asia and Africa, which are limited to their respective districts. 



Although there exists an intimate correlation between climate and vegetation, 

 the temperature and other influences which constitute climate do not reveal all 

 the causes which are operating or have produced these differences as they arc 

 repeated under the isothermal lines, or lines of equal heat, between the eastern 

 and western shores of the Old "World in the same order as along the eastern 

 ind western shores of North America. The similarity of the climates of 

 eastern North America and eastern Asia, and of western North America and 

 western Europe, originating in similarity of position as respects latitude, expo- 

 Bare to ocean currents, pi'otection by mountain barriers, &:c., does not excite 

 surprise ; but that there should exist in Japan many genera and species of 

 plants unknown except to the eastward of the Rocky mountains in North 

 America — that the district of the lower Amoor river, on the shore of the Pacific, 

 should produce several species identical with peculiarly eastern North American 

 kinds, and many others nearly allied, if not identical therewith, which are not 

 found in northwestern America nor elsewhere on the globe, is more worthy of 

 remark ; and when on further inquiry we learn that the Canary islands and 

 Azores possess American genera not found in Europe nor in Africa, and that 

 the lofty mountains of Borneo contain Tasmanian (Van Dieman's Land) and 

 Himalayan representatives, that the Himalayas contain Andean, Rocky 

 mountain, and Japanese genera and species, and the Alps of Victoria (a dis- 

 trict of Australia) and Tasmania, assemblages of New Zealand, Fuegian, 

 Andean, and European genera and species, the interest becomes greatly 

 enhanced, and the query arises, How shall all these seeming anomalies of a 

 ♦similarity of product, under similarity of climate, yet vvnde disseverance of 

 epecies identical with each other, over districts between which there seems to 

 exist no possible means of communication, be satisfactorily explained] Causes 

 now in operation cannot be made to account for a large assemblage of flower- 

 ing plants characteristic of the Indian peninsula, being also inhabitants of 

 tropical Australia, while not one characteristic Australian genus has ever been 

 found in the peninsula of India. Still less will these causes account for the 

 presence of Antarctic and European species in the Alps of Tasmania and 

 Victoria, or for the reappearance of Tasmanian genera on an isolated lofty 

 mountain in Borneo.* 



The above-cited and a multitude of analogous facts have led to the study of 

 the agencies which may be reasonably supposed to have had an influence in 

 determining the distribution of plants. Among these agencies may be named 

 supposed changes of climate, which geological research seems to exhibit, 

 arising either from direct cooling of the entire mass of the earth, or from new 

 positions and elevations of the surface, which we cannot doubt have occurred 

 at various epochs of the history of the progress of the earth. "We shall not 

 pause to dwell upon these most interesting problems which are tasking the 

 learning and acumen of the most profound and acute philosophers, and which 

 have not yet found a satisfactory solution, nor enlarge upon the nature of the 

 facts and reasoning brought up from the study of extinct plants and animals, 

 whose remains, entombed in the solid rock, bear witness of their former life, 

 and of the conditions demanded for its support, but will pass to more practical 

 applications of the general law of distribution under the regularly recurring 

 changes of temperature now influencing the life of the globe. 



* Dr. J. D. Hooker, Tasmanian Flora in Silliman's Jouinal of Science, vol. XXES, pp. 

 10, 21. 



