326 RELATIVE AGES OF THE CRAG 



we should now only meet with in more equatorial latitudes, as 

 Lingula, Pholadomya, Pyrula, nearly related to Pyr. reticu- 

 lata, a large Voluta, &e. They are accompanied moreover 

 by many stone corals of extinct genera, and one of these, the 

 Anthophyllum, now occurs within the tropics. 



Some explanation, perhaps, of the apparent anomaly of 

 these associations may be sought in the analogy of the present 

 state of parts of the southern hemisphere, which enjoy a mild 

 and equable climate. In South America, for example, Mr. 

 Darwin has shown that certain tropical species and genera 

 will range to very high latitudes, provided their progress be 

 not arrested by severe winter's cold. Thus he found on the 

 east coast of South America, in latitude 39° S., three species 

 of olive (one of large size), a Voluta and a Terebra, among 

 the most abundant shells on the mud-banks of Bahia Blanca ; 

 and a large species of volute has been traced as far south as 

 lat. 45°, or, according to some accounts, much farther. 1 — 

 Such forms in the northern hemisphere would be characteris- 

 tic of tropical seas. It is moreover said that in the southern 

 hemisphere at present, the transition is very sudden from a 

 latitude to which tropical forms extend, to one not far to the 

 south, where there is extreme cold. But we have yet to learn 

 how far such circumstances alone can give rise in the ocean 

 to abrupt lines of demarcation between distinct geographical 

 provinces of Testacea. 



It appears to me impossible to account for the specific dif- 

 ference of the marine Faunas of Suffolk and Touraine, as- 

 suming them to be contemporaneous, without speculating on 

 some other cause which co-operated perhaps with a state of 

 climate like that above suggested, so as to prevent a free 

 range of northern species towards the south, or of the south- 

 ern species towards the north. Thus, for example, some ge- 

 ographical barrier, such as an isthmus, may formerly have 

 existed between Dover and Calais. If Great Britain, thus 

 joined to the European continent, stretched continuously far 

 to the north, beyond the Shetland islands ; while at the same 

 time the Land's End, in Cornwall, was prolonged for some 

 distance in a southerly direction, the two gulfs then placed 

 on the opposite sides of the supposed isthmus might, in the 

 course of time, become the habitations of very distinct assem- 

 blages of marine animals, the isthmus constituting the extreme 

 boundary, on one side, of the range of certain tropical ani- 

 mals, and, on the other, of many arctic species. 



1 Journal of Travels in S. America, in Voy. of H. M. S. Beagle, p. 611. 



