OF ARTS AND SCIENCES. 193 



" He regarded the subject as destined to throw light upon history ; 

 but, as far as he was connected with it, instead of applying his expe- 

 rience, he had been called upon to impart it. 



" He had constructed illustrative maps : yet, even with their aid, 

 personal inspection seemed indispensable, extending to the species 

 absent from a country, as well as those present. After various unsuc- 

 cessful trials, he had adopted a plan of numbering the species geograph- 

 ically, according to the countries arranged in a particular order : the 

 numbers will then supply the place of personal inspection, will point 

 ojit where a genus or a species commences, and in a genei-al way where 

 it terminates : by following always the same conventional route, some 

 idea of the distribution of species may perhaps be gained, without actual 

 voyaging. 



" In regard to the origin of species, Dr. Pickering's observations 

 have all tended to confirm an opinion he has long entertained. The 

 term ' local areas ' is, however, not his own ; but seems preferable to 

 that of ' centres of diffusion,' formerly used by him, and he will adopt 

 the improvement. He considers, then, that sjiecies originated upon 

 local areas ; and are spread over as much of the earth's surface as they 

 have had a chance to reach and occupy. 



" He regards the question as half decided by the fact, that very 

 many species are confined to local areas to the present day ; often, 

 too, under circumstances that seem to preclude wider diffusion at any 

 former period. 



" 1. The class Mollusca affords instances of the two extremes in 

 diffusion ; many of the land species being local, and many of the ma- 

 rine species widely diffused. At the Hawaiian Islands, each separate 

 island has in a general way its own set of land shells ; some species, at 

 least, that do not reach the other islands of the group, nor reappear 

 upon any other portion of the globe. While among the marine shells, 

 he had himself seen living specimens of the Cyprea tigris, or leopard 

 cowry, at the Hawaiian Islands, as at other points in the Pacific, and 

 throughout the East Indian Archipelago and the Indian Ocean to the 

 African coast, a distance from east to west of fifteen thousand miles. 

 The explanation probably rests in the ova ; those of the Cyprea tigris 

 being marine, and preserving their vitality in sea-water, and those of 

 the land mollusks perishing on immersion. 



" 2. The Anseridce, or species of goose, are grazing birds, and, being 



VOL. IV. 25 



