624 Original Articles. [Oct., 



Having given a brief outline of the history of our subject, I 

 must endeavour to explain the reasons that induced Professor Edward 

 Forbes to give utterance to the sentence I have just quoted, and 

 that instigated his successors to revive his opinion. Let us suppose 

 that during the deposition of the most ancient fossiliferous rocks, the 

 world was peopled by a single assemblage of animals and plants; 

 in other words, that the same aquatic species existed in every part of 

 the ocean under the same conditions, and the same land-species on 

 the whole surface of the dry land, also under the same conditions. 

 In this case it is obvious that the strata of that period could now, 

 all over the world, be absolutely identified by their organic remains. 

 But if we take a survey of the fauna and flora of the globe as they 

 now are, we shall find that the very reverse of our supposition is what 

 occurs at the present time ; for the surface of the earth consists of a 

 number of zoological and botanical provinces, each one of which has 

 an assemblage of animals or plants more or less peculiar to it. It is 

 therefore very unlikely that a geologist of some Post-Quaternary 

 period will be able, with our means and appliances, to identify and 

 correlate by their organic remains distant deposits of our period ; such, 

 for instance, as are being formed in the Baltic and on the shores of 

 the Indian Ocean. 



But this is by no means the whole of the case for the opponents of 

 the old view. The views of all geologists on this question must 

 depend more or less on their belief in the origin of species by descent 

 with modification, and from single " specific centres." Taking first 

 the affirmative view of these theories, it must be allowed that it 

 takes a considerable time for a species to extend itself over a large 

 area from the spot where the modified descendant could first be called 

 a new species ; but it takes a still longer time, after the full geo- 

 graphical extension of the species has been reached, for that species 

 to emigrate to a distance in consequence of a change in the conditions 

 of existence, such as alteration of climate, in the depth of the sea, in 

 the abundance and quality of food, &c. In illustration of this point 

 I may cite several facts, especially the occurrence of the Beindeer 

 in the South of France during a late, but still pre-historic period, 

 and its existence at the present day in Arctic regions. Again, Gyrena 

 fluminalis (Plate, fig. 1) "occurs abundantly in the brick-earths and 

 gravels of the Thames Valley, and exists now in the rivers of more 

 southern latitudes, ranging from Egypt to China ; and a parallel 

 instance is afforded by Argonauta Mans (Plate, fig. 2), which occurs 

 fossil in Pliocene European deposits, and ranges now as far as the 

 coasts of China. A stronger case is, perhaps, afforded by the fact 

 that a large number of species which occur fossil in the Miocene 

 deposits of Europe (for instance, Plate, fig. 3) live now in tropical 

 latitudes, chiefly on the shores of the Indian Ocean, so that the fossils 

 of the strata now being deposited in that region cannot be very 

 different in facies from those of the Miocene formation of Europe. 



One apparent objection to this doctrine becomes a corroboration, 

 when subjected to a searching examination, as is not unfrequently the 

 case with unsound objections to true theories. Opponents to the 



