GEOGRAPHICAL DISTRIBUTION — PAST AND PRESENT. 221 



hundred and fifty-six species, of which number at least forty-seven 

 (or thirty per cent.), and probably considerably more, belong equal- 

 ly to the fauna of West Central Europe. Altogether, the Jurassic 

 faunas of the world, even those most widely separated, are most 

 intimately related to one another in both generic and specific charac- 

 ters. The same may be said, although to a less extent, of the faunas 

 of the Cretaceous period, many of whose most distinctive types, 

 especially of cephalopods an; I lamellibranchs, have a world-wide 

 distribution. The cephalojiodous fauna of the Cretaceous deposits 

 of India, we are informed by Stoliczka,'' holds, among its one hun- 

 dred and forty-eight species, at least thirty-eight that are common 

 in Eurojje (twenty-five and one-half per cent.), a high proportion, 

 but somewhat less than what we have seen obtains in the case of 

 the fauna of the period preceding, the Jurassic. Roemer '* has de- 

 termined fourteen out of one hundred and twenty-eight sjiecies 

 (eleven per cent.) of fossils coming from Texas to be identical 

 with European forms, while Morton has recognised no less than 

 seventeen such from among one hundred Invertebrata belonging to 

 the State of New Jersey. This proportion has since been raised 

 by Credner.'* The researches of D'Orbigny in South America, 

 of Coquand and RoUand in Africa, of Stoliczka and Waagen in 

 India, and of M'Coy, Etheridge, and others in Australia, show 

 very clearly how far specific identity is carried over the earth's sur- 

 face. 



With the beginning of the Tertiary epoch (Eocene period) a new 

 era in zoogeography sets in. The broad dispersion of species has 

 become much more of a rarity than heretofore — indeed, an excep- 

 tion — so that the widely separated regions of the earth's surface, 

 while they may yet be mutually related to one another in their 

 general faunal characters, are no longer bound together by that mul- 

 tiplicity of identical specific types which is distinctive of the earlier 

 periods. The conditions which prevail at the present day have 

 already become accentuated. This fact is clearly brought to light 

 by a study of the deposits occurring on opposite sides of the Atlan- 

 tic. Thus, it is doubtful whether, out of some four hundred to 

 five hundred species of moUusks belonging to the Eocene formation 

 of the Atlantic and Gulf borders of the United States, more than 

 twenty-five or thirty (six to eight per cent.) can be absolutely iden- 

 tified with forms occurring elsewhere. And yet we have seen that 



