470 Recent freshwater Mollusca, 



necessarily include descent from a single pair, because the 

 first male and the first female would, by the definition, be of 

 different species.* If we assume that geographical position 

 is of more value than specific character, it follows that we can- 

 not name Physa hypnorum, until we know the locality of the 

 specimen ; nor can it ever be ascertained whether this species 

 inhabits both continents, if the very enunciation of the fact 

 calls forth its denial.f Hence tables like that given become 

 useless to the geologist ; for should he have reason to infer 

 that certain regions were once united by a chain of islands, 

 for example, he will expect a certain community of animal 

 species ; but upon placing his specimens before a zoologist, he 

 is compelled to abandon his view by being informed that no 

 species is common to the two regions ; the assertion not being 

 founded upon the positive evidence furnished by the objects 

 themselves, but upon the mere opinion that they would have 

 been unable to traverse the intervening ocean. J 



If the same species may inhabit distant regions, the fact 

 may be accounted for in several ways, as by 



we cannot expect precision in the details of systematic botany." Prof. Heaslow, 

 Mag;. Zool. Bot. i. UG. 



*See Mag. Nat. Hist. N. S. ii. 622. Will any one contend that when " grass " 

 was first created, the meadows remained barren until covered by multiplication from 

 a single plant ? 



t In the same manner, if identity of species in the parents were a just deduction 

 from the occurrence of a prolific ofl'spring, the question touching prolific hybrids could 

 never be settled ; and if two precisely similar shells are pronounced distinct merely 

 because they are found in strata of very diflferent ages, we can never determine 

 whether a fossil species can occur in distinct formations. This point will be noticed 

 further on. 



t The circumstances of the existence of dissimilar forms of a common type are 

 parallel to those of the (Ovis ammon) equally found identical or diflTerent in Asia, 

 Africa, and the islands of the Mediterranean, which existed anciently in Spain, and 

 at this moment is spread over a great part of western North America. In no case 

 are these animals suspected to have been transported by human intervention, and yet 

 they are located in some places where, without the aid of man, they cannot have mi- 

 grated, unless we admit of changes on the surface of the earth, since the present 

 zoology was in being, of such magnitude as to include the formation of the Mediter- 

 ranean — the separation of the British Islands from the continent of Europe — of the 

 Indian Islands from that of Asia — and the formation of a channel to cut America 

 from connection with the Old World. — Col. Hamilton Smith, Nat. Hist, of 

 Horses, p. 67. 



