Dec. 12, 1878] 



NATURE 



125 



abundant evidence of more than one kind that Europe 

 and America were connected then, and that animals and 

 plants passed between them. Besides that, the floras 

 themselves contain both American and European types, 

 and during the middle eocene a great number of plants 

 were common to both coniinents. 



In the next place the temperature of the eocene period 

 in Europe was much hotter than that of the miocene, 

 and therefore presumably more favourable to the growth 

 of such floras in northern latitudes. To call them mio- 

 cene we have to admit the former existence of a climate 

 sufficiently uniform to hare enabled the same species of 

 plants to grow simultaneously from Italy and the United 

 States to the 70th parallel, a state of things not in accord- 

 ance with our present experience of plant distribution. 

 But if we assume them to be eocene, the decreas- 

 ing temperature which prevailed from that time to 

 the miocene would have gradually and naturally driven 

 the forms southward, and thus the very similarity of 

 the miocene floras of America and Switzerland to those 

 of the Arctic regions, renders it most unlikely that they 

 ■were of the same age, and almost certain that the latter 

 were considerably older. 



In comparing the eocene and miocene temperatures 

 we find, as already stated, that the former most readily 

 accounts for the growth of temperate floras in high lati- 

 tudes. Taking Heer's estimate that the miocene tem- 

 perature in the latitude of Switzerland at the sea-level 

 was only (f C. warmer than at the present day, the pro- 

 gressive decrease of heat to the north is not so much in 

 accordance with that of the present time, as it is found 

 to be on the supposition that they belong to the eocene ; 

 we have to suppose that the mean temperature diminished 

 in a less degree. 



Speaking roughly, the present decrease in the isotherm 

 from latitude 50"^ of south England, to that of Spitzbergen, 

 is about 10° Fahr. for every 10° of latitude. This is as 

 nearly as possible the ratio of decrease between England 

 and Greenland in eocene times as implied by the floras, 

 supposing them to be of one age. If, as I assume from 

 all the data I can collect, England in middle eocene 

 times possessed a mean annual temperature of 70° P., 

 Greenland would naturally have had one of 50°, which is 

 that assigned to it by Heer (9° C). The decrease thence 

 to Spitzbergen and Grinnell-land is hardly less rapid, 

 being about 1° F. {^ C.) of cold for each degree of lati- 

 tude. Heer calculates— principally on the mean tem- 

 perature required by Platanus — that between Greenland 

 and Spitzbergen, _8'' lat., it was 4" C., but very unac- 

 countably fancies^that between Spitzbergen and Grin- 

 nell-land no further decrease took place, and upon this 

 assumes that trees might have extended to the very Pole 

 itself. The evidence against it, however, seems perfectly 

 clear, for all the planes and limes, and more temperate 

 forms have disappeared, and the genera found there, with 

 a single exception, have representatives which at the pre- 

 sent day live within the Arctic Circle. This exception, 

 Taxodium, judging from its present habitat, Mexico and 

 the Southern United States, would necessitate a climate 

 completely different from that required by all the other 

 plants with which it is associated. They form a distinctly 

 Arctic assemblage : the spruce, especially, is never met 

 •with fossil to the south, except in inter-glacial beds. The 

 fossil Taxodium must, therefore, only be looked upon as 

 an allied extinct species, whose resemblance to living 

 forms does not imply identity of habit, since all other 

 considerations are against it. A similar instance is found 

 in the willow, which is generally characteristic of the 

 north; yet Salix humboltiana is found in the Amazon 

 ^stricts and S. safsafm Eg^'pt; and similarly, although 

 Cassia is eminently characteristic of tropical and sub- 

 tropical zones, C. marylandica flourishes on the banks of 

 the Lake of Geneva. As it is essential to get rid of the 

 evidence of Taxodium, if we are to suppose this former 



climate followed the present natural laws, I shall refer to 

 some remarks by Lesquereux upon the nearly allied red- 

 woods. 



In describing the pliocene plants of California, he con- 

 cludes that they are related to the present flora of the 

 Atlantic slope, and not to that of California. He accounts 

 for their destruction on the eastern side by the powerful 

 agencies of glacial action, marine submersion, and long- 

 sustained volcanic cataclysms. When these had ceased 

 the sheets of water between the Missouri River and the 

 Rock}- Mountains and the mountains themselres pre- 

 vented the old flora from again occupying the Western 

 area. Some of the pliocene species, however, were pre- 

 served through the glacial epoch in California, but modi- 

 fied, for the most part, by the cold conditions they had 

 undergone. " The two species of Sequoia— one the more 

 predominant, the other the more remarkable, of the flora 

 of California — are evidently also remnants of the plio- 

 cene. ..^T. giganiea, which in aU probability covered the 

 higher slopes of the mountains of that epoch, has been 

 destroyed everywhere, except in some deep valleys. . . . 

 The other, S. sempei-virens, left here and there, has again 

 taken the ascendency under more favourable physical 

 circumstances. Its present distribution explains its pre- 

 servation until the present epoch. According to Prof. 

 Bolander, "the distribution of the redwood depends upon 

 sandstone and oceanic fogs. Where either one of these 

 conditions is wanting there is no redwood. The redwoods 

 begin in the northern part of Monterey County, in 

 isolated groups, in deep, moist canons. A short distance 

 south of Monterey City, on the Monterey Bay, a white 

 bituminous slate sets in, and extends nearly to Pajaro 

 River. On this no redwood is found, but Pinus insigiiis. 

 At Pajaro River, eight to ten miles from the ocean, they 

 set in again, and extend to nearly twenty-eight miles 

 south of this city (San Francisco), either in deep caiions, 

 or in groves extending over several ridges eastward, as 

 far as the fog may reach. Then they continue in similar 

 localities to-latitude 42°, the state boundary." 



The existing allied species withstood a glacial period in 

 California ; there is no improbability in supposing that 

 older and extinct species may have habitually supported 

 a cold temperature. It appears that they belong to a 

 very old type, now confined to a limited area, and be- 

 coming extinct, at whose survival we cease to wonder 

 when we reflect that individual trees have been calcu- 

 lated to be 3,000 years old. To pass through the life of 

 such a species, an enormous period must be required, for 

 only 100 generations might carry us back 300,000 years, 

 with as little modification as an annual plant might 

 undergo in 100 years. The sandstone soil and damp sea 

 fogs required by them in their native habitat, may explain 

 the difficulty in getting them to grow under cultivation 

 except in comparatively warm latitudes — and it is upon 

 plants under cultivation Heer's estimate in Europe is 

 based — but lessens our surprise that they should have 

 existed in Greenland or farther north during the eocene 

 time. 



Apart from Taxodium, therefore, there is every evi- 

 dence, in the disappearance of temperate forms and the 

 preponderance of conifers of boreal type, that, as at the 

 present day, there was a natural and progressive decrease 

 of temperature to the north between Grinnell-land, 

 Spitzbergen, and Greenland. 



In the next place I would call attention to the possibility 

 that the respective temperatures thought to be requisite 

 for the growth of such associations of plants as are foimd 

 fossil in these various lands may be in excess of the 

 minima which would have sufficed. If this were the 

 case, it would of course remove to a slight extent an 

 argument I have just brought forward against the miocene 

 age of the deposits. One of the conditions peculiarly 

 favourable to the growth of trees in northern latitudes is 

 the protracted length of the summer days, and it is an 



