302 DR. J. W. GREGORY ON THE PALEONTOLOGY [Aug. 1895, 



do so. Thus Dr. A. Bussel Wallace, 1 relying mainly on the evidence 

 of the fishes, maintains that the isthmus -was certainly submerged 

 more than once in Tertiary times, and that the last separation of 

 the two Americas in this way may have helped the oncoming of 

 the glacial conditions. Bicketts has twice expressed 2 the views 

 of English glacialists who accept this explanation of the cause of 

 the ' Ice Age,' but while he admits that there is no absolute proof 

 of the submergence of the isthmus, he urges its probability owing 

 to the lowness of the ridge, to the ' identity ' of the fauna on 

 both sides of it, and to the great earth-movements that have taken 

 place in this region. 



Still more recently the subject has been fully discussed by 

 Jukes-Browne and Harrison. 3 They conclude with a belief in a 

 ' free communication between the Atlantic and Pacific at a late 

 period in Tertiary time.' This ' late period ' they distinctly refer 

 on the same page to the Pleistocene. 



Nevertheless, all recent zoological work has tended to show that 

 though at one time the Caribbean Sea must have been in direct 

 communication with the Pacific, this was at a very distant period, 

 certainly prior to any part of the Pleistocene. There have always 

 been advocates for this view. Thus Carrick Moore, 4 who has been 

 so often quoted as showing the connexion across Panama, pointed 

 out the great differences in the faunas of the two sides. Similarly, 

 Belt, 5 in 1874, pointed out that the occurrence of less than 50 

 species common to the two faunas was really a proof that the 

 isthmus could not have been submerged since Pliocene times ; for 

 the number is very small in proportion to the size of the faunas. 

 Moreover, instead of P. P. Carpenter's list of species found on both 

 sides of the isthmus having grown with further work, it has shrunk 

 considerably. Pischer calls it ' une infime minorite (3 %),' and 

 therefore carries back the close of the separation to the Miocene. 6 



The same change has come in the interpretation of the evidence 

 of the fishes, thanks to the works of Jordan, 7 and of Evermann and 

 Jenkins. By the study of much larger collections than those avail- 

 able to Dr. Giinther, Jordan reduced the number of species common 

 to the two faunas to 6 per cent., and declared that they were so 

 ' substantially distinct ' that no recent connexion between the seas 

 had existed. This estimate has been still further reduced by 



1 A. E. Wallace, 'Island Life,' 2nd ed. 1892, p. 151. 



2 C. Bicketts, ' The Cause of the Glacial Period with reference to the British 

 Isles,' Geol. Mag. 1875, pp. 573-580, and ' Some Phenomena which occurred 

 during the Glacial Epoch,' Proc. Liverpool Geol. Soc. vol. vi. (1891) pt. iii. 

 pp. 244-247. 



3 Op. jam cit. pt. ii. Quart. Journ. Geol. Soc. vol. xlviii. (1892) p. 224. 



4 J. Carrick Moore, ' On some Tertiai-y Beds in the Island of San Domingo, 

 from notes by J. S. Heniker, Esq., with remarks on the Fossils,' Quart. Jo n. 

 Geol. Soc. vol. vi. (1850) p. 43. 



5 T. Belt, ' Naturalist in Nicaragua,' 1874, pp. 263, 264. 



6 P. Fischer, ' Manuel de Conchologie,' 1887, p. 168. 



7 Dav. S. Jordan, ' A List of Fishes known from the Pacific Coast of Tropical 

 America, from the Tropic of Cancer to Panama,' Proc. U.S. Nat. Mus. vol. viii. 

 (1885) pp. 361-394, especially pp. 393, 394. 



