THE TERRACE EPOCH. 45 



Still following Dana's geological views, he imagines a third ejioeh (the Terrace Epoch), wliich 

 is the transition between his fluvial eiioch and the present state of things, during which he supijoses 

 that the interchange of migrations by which the preponderating affinity between the East of North 

 America and the East of Asia over that of "Western America w^as eifected, took place by, and in virtue 

 of, the isothermal Hnes. To use his own words, — " The interchange of plants between the East of 

 North America and Eastern Asia has mainly taken place in high northern latitudes, and that the 

 isothermal Hnes have, in earlier times, turned northward on our eastern, and southward on our 

 north-western coast, as they now do, arc points which go far towards explaining why Eastern North 

 America, rather than Oregon and California, has been mainly concerned in it, and why the 

 temperate interchange, even with Europe, has principally taken place in Asia." 



I am a little sceptical about the sixpposed fluvial epoch ; and its climate being milder than 

 that of the present day. The presence of the Megatherium might be worth something as evidence 

 of a warm climate, seeing that the typical South American species lived in a warm climate, and 

 that the species itself lived previous to the glacial epoch ; but the company of the Mammoth, and this 

 not a tropical Mammoth, but the woolly fur-clothed Elephas primigenius, adds nothing to the force 

 of that fact. If the maxim "iioscitiir a sociis" is to be applied, its association with the species 

 of Megatherium and Mylodon in question would be fatal to the idea of a mild climate. For, although 

 the kindred of the Megatheres lived in a warm climate, so did the kindred of the Mammoth : and 

 yet we are as sure almost as we can be of anything depending on paloBontological evidence that 

 the Siberian Mammoth (Elefhas primigenius) was an animal fitted for a cold climate. More- 

 over, the evidence of extinct Megatheres having survived the glacial epoch is not altogether beyond 

 suspicion. The reader will find the question more fully discussed when we come to the Edentata. 

 If they did, they did not survive it long, and only in the most southern and warmest parts of North 

 America. 



The argument from the isothermal lines is more satisfactory. There is, however, one purpose 

 to which it is applied by Professor Gray in which I cannot concur, and that is, to explain the 

 cause why species whicli, according to him, have crossed from Asia, have passed by, or omitted 

 to enter, north-west America, and travelled on to Eastern America. His theory explains most 

 happily and ingeniously how species, whose habitat is in Eastern Asia and Eastern America at as 

 low a latitude as 50° N., may yet have been able to cross from one continent to the other at a 

 latitude of upwards of 60°, but having once crossed, I do not see they shoidd have a greater range 

 of latitude in east than west America, nor why the species should be absent in the latter altogether. 

 The fact is, however, that they have not spread into north-west America. That district — that 

 strip of land lying to the west of the Rocky Mountains — has been passed by the plants, whether 

 from Asia or Eastern America? If the plants really touched the north of this territory, there 

 must have been some other reason for their going past it than the difference in temperature. There 

 must have been a barrier there which they could not pass ; and I think the hypothesis which 

 I have above suggested, that at that time there was a strait or sea to the north of Vancouver's 

 Island, joining, what I may call the Missouri-Mackenzie tertiary sea, is the true explanation of 

 the phenomenon. Such a barrier would hem in and preserve North- West America as a com- 

 paratively isolated region, in which, as Dr. Hooker points out, — " wc have, as in an oceanic 

 island, a great mixture of types (Asiatic, Eurojjcan, East and West American) and paucity of 

 species."* 



* HooKKK, " On the Distribution of Arctic Plants," in Liunean Society's Transactions, vol. xxiii., p. 270. 1S61. 



