394 T. Bland on the Geographical Distribution of Mollusca. 



"Another point of interest extensiv^ely elucidated by the col- 

 lections of the Expedition is the occurrence of analogous species 

 in co-ordinate regions. It is now a received fact that the animals 

 and plants of the northernmost zones are, for the most part, iden- 

 tical throughout the whole circuit ; and that the species gradually 

 diverge from each other towards the equator, on the three conti- 

 nents ; and that after passing the equator towards the south, there 

 is not a return to the same species, and rarely to the same genera, 

 as we should expect if variation of forms depended mainly on 

 difference of ten:iperature. There is, however, a return to mol- 

 lusks of a kindred character and form, and oftentimes to the same 

 genera. The analogies of specimens from distant regions are 

 much stronger when reckoned by isothermal longitude than by 

 isothermal latitude. In the latter case we may have analogous 

 genera. Along our northern seas, some of the most characteristic 

 shells are Bucdnnm, Tritonium^ Fusus, Terebratula, Rimula, 

 &c. Around Cape Horn are shells of the same types, so closely 

 allied that they have not yet been separated as distinct genera, 

 though peculiar in many important respects. But this resem- 

 blance does not descend to species. In the first case, however, 

 not only have we the same genera, but the species seem to repeat 

 each other: so that species brought from great distances east or 

 west, are scarcely to be distinguished upon comparison." — p. xii. 



Dr. Gould gives, in illustration, a list of species from Oregon, 

 and from the Eastern States, and observes that "mingled with 

 these are others very different in ty))e, which mark the two locali- 

 ties as constituting very different zoological regions." He adds, 

 "the same comparison holds good between the shells of the Gulf 

 of California and the Gulf of Mexico." 



We would quote also the following : — 



" From a consideration of the land shells collected on the Pa- 

 cific Islands, it seems possible to draw some fair inferences as to 

 the relations of the lands which once occupied the area of the 

 Pacific Ocean, and whose mountain peaks evidently now indicate 

 or constitute, the islands Vvith which it is now studded. By ob- 

 observation of the species, we think there are strong indications 

 that some groups of islands have an intimate relation to each 

 other, and belonged, at least, to the peaks of the same mountain 

 ranges, before they were submerged ; while the indications are 

 equally strong that other groups had no territorial connection." 

 — p. xiv. 



Drawing inferences from the land shells, Dr. Gould considers 

 that the Samoa and Friendly Islands are more intimately rela- 

 ted to the Society Islands, though at a much greater distance, 

 than to the Feejee Islands, and that the Feejees are more nearly 

 allied to the islands to the westward, — such as the New Hebrides, 

 than to the Friendly Islands on the east, though so much nearer. 



