GEOGRAPHICAL DISTRIBUTION PAST AND PRESENT. 217 



In sharp contrast to the limited faunas of a north-and-south 

 extension is the fauna of the Indo-Pacific region, whose domain 

 covers about forty- five degrees of latitude, mainly comprised within 

 the Tropical Zone, and extends over fully three-quarters of the cir- 

 cumference of the globe. The influence of an equable climate is 

 here plainly manifest. The molluscan species occurring in this tract 

 have evidently a very broad distribution, for we find that, out of 

 an estimated total of five or six thousand species, the Philippine 

 Islands alone possess twenty-five hundred species, and New Cale- 

 donia an equal number. Upwards of one hundred of the East 

 African species have been identified by Cuming in the faunas of 

 the Philippines and the coral seas of the Pacific, or over an ex- 

 panse of seventy to one hundred degrees of longitude, and Fischer 

 enumerates twenty-one species whose range takes up practically 

 every part of the province. On the other hand, only one hundred 

 and sixty-five species are known to connect this fauna with the 

 Japanese, although the two are separated from each other, in a 

 north and south line, by an interval of only ten to fifteen degrees of 

 latitude. 



Facts such as have been here presented clearly demonstrate 

 how all-powerful in its influence upon distribution is temperature, 

 and warrant us in assuming that it was this agent, likewise, which 

 primarily controlled distribution in the past as well as in the pres- 

 ent. "Were evidence of a nature other than that which is derived 

 from purely zoogeographical considerations needed to prove the 

 existence of more equable climatic conditions in the earlier periods 

 of the earth's history, we have the testimony to this effect of the 

 ancient reef-building corals, whose remains are so abundantly im- 

 planted in the deposits of the temperate and frozen north, and, in 

 no less striking degree, of the flora of the coal. 



It is, however, a significant fact, that many parts of the oceanic 

 surface which may be said to enjoy practically identical climates 

 hold at the present day very dissimilar faunas (viewed from the 

 stand-point of species). Thus, of the four hundred or more spe- 

 cies of mollusks inhabiting the Japanese waters, it appears that not 

 more than twenty are found on the west coast of North America 

 (Oregon, California, Mexico). Nearly all the east-coast species of 

 the United States, south of Cape Cod, differ from the species of the 

 corresponding regions on the opposite side of the Atlantic, probably 



