200 GEOGRAPHICAL AND GEOLOGICAL DISTRIBUTION. 



This condition is somewhat surprising in view of the free- 

 swimming character of the molluscan embryo, and the fact that a 

 number of forms of tropical habit have in some way managed to 

 gain access to the opposite shores of the oceanic basin. Many of 

 the forms belonging to the Caribbean province recur on the west 

 coast of Africa, and the Dolium galea of the Mediterranean is also 

 a member of the Brazilian and Antillean faunas. In what particular 

 manner the transport was effected in the case of these few favoured 

 species it is impossible even to conjecture, but it appears to point 

 to a remarkable vital tenacity on the part of the embryo. Still 

 more surprising, and entirely inexplicable, is the distribution of 

 the fifteen or more si^ecies (including a dozen species of the genus 

 Triton) whose common habitat is the Indian Ocean and the West 

 Indian sea, when no connecting representative is found in the 

 intermediate area. This is perhaps the most remarkable instance 

 of specific areal discontinuity known, and contrasts sharply with 

 what is observed on the west coast of America, where, of the 

 very rich fauna of the Panamaic province, with no intervening 

 barrier, an only equally small number of species is held in common 

 with the Indo-Pacific region. The species of the far south, on 

 the other hand, are largely similar, despite the absence of existing 

 land-connection. From the occurrence of identical forms in New 

 Zealand, the Magellan district, and the isolated tracts represented 

 by the Kerguelen, Marion, Crozet, and Prince Edward islands, 

 Fischer argues "^ that we have represented here the disrupted parts 

 of a former Antarctic continent, along which specific diffusion was 

 primarily effected. But there seems to be no reason why the pres- 

 ent distribution might not at least in part be explained on the 

 assumption of diffusion along the oceanic bottom— where the differ- 

 ence between surface and bottom temperature is no longer extreme 

 — seeing that we have here also a number of distinctive Arctic or 

 northern types represented (Chiton Belknapi, Lastea rubra, Tere- 

 bratulina septentrionalis, Terebratula vitrea, var. minor). The most 

 southerly mollusk thus far met with is a pteropod, Limacina ? 

 cucullata, which was obtained by the Wilkes Exploring Expedition 

 on the sixty-sixth parallel of south latitude. 



Instances of specific limitation are exceedingly numerous, and 

 particularly characterise insular faunas. To such an extent is this 

 the case that it may be said that almost every oceanic island or 



