34 



NA TURE 



INov. 



Physical Conditions and Climate indicated by the Cre- 

 taceous Floras.— in the Jurassic and earliest Cretaceous 

 periods the prevalence, over the whole of the Northern 

 Hemisphere, and for a long time, of a monotonous 

 assemblage of Gymnospermous and Acrogenous plants, 

 implies an uniform and mild climate and facility for inter- 

 communication in the north. Towards the end of the 

 Jurassic and beginning of the Cretaceous, the land of the 

 Northern Hemisphere was assuming greater dimensions, 

 and the climate probably becoming a little less uniform. 

 Before the close of the Lower Cretaceous period, 

 the dicotyledonous flora seems to have been intro- 

 duced, under geographical conditions which permitted 

 a warm-temperate climate to extend as far north as 

 Greenland. 



In the Cenomanian we tind the Northern Hemisphere 

 tenanted with dicotyledonous trees closely allied to those 

 of modern times, though still indicating a climate much 

 warmer than that which at present prevails. In this age 

 extensive but gradual submergence of land is indicated 

 by the prevalence of chalk and marine limestones over 

 the surface of both continents ; but a circumpolar belt of 

 land seems to have been maintained, protecting the 

 Atlantic and Pacific basins from floating ice, and per- 

 mitting a temperate flora of great richness to prevail far 

 to the north, and especially along the southern margins 

 and extensions of the circumpolar land. These seem to 

 have been the physical conditions which terminated the 

 existence of the old Mesozoic flora and introduced that of 

 the Middle Cretaceous. 



As time advanced, the quantity of land gradually 

 increased, and the extension of new plains along the older 

 ridges of land was coincident with the deposition of the 

 great Laramie series and with the origination of its 

 pernliar florn, whiih indir.ites a mild climate and con- 

 siderable variety of station in mountain, plain, and 

 swamp, as well as in great sheets of shallow and weedy 

 fresh water. 



In the Eocene and Miocene periods the continent 

 gradually assumed its present form, and the vegetation 

 became still more modern in aspect. In that period of 

 the Eocene, however, in which the great nummulitic 

 limestones were deposited, a submergence of land 

 occurred on the eastern continent which must have 

 assimilated its physical conditions to those of the 

 Middle Cretaceous. This great change, affecting materi- 

 ally the flora of Europe, was not equally great in America, 

 which also by the north and south extension of its moun- 

 tain chains permitted movements of migration not possible 

 in the Old World. From the Eocene downwards, the 

 remains of land animals and plants are found only in lake 

 basins occupying the existing depressions of the land, 

 though more extensive than those now remaining. It must 

 also be borne in mind that the great foldings and fractures 

 of the crust of the earth which occurred at the close of 

 the Eocene, and to which the final elevation of such 

 ranges as the Alps and the Rocky Mountains belongs, 

 permanently modified and moulded the forms of tlie 

 continents. 



These statements raise, however, questions as to the 

 precise equivalence in time of similar floras found in 

 different latitudes. However equable the climate, there 

 must have been some appreciable difference in proceeding 

 from north to south. If, therefore, as seems in every way 

 probable, the new species of plants originated on the 

 Arctic land and spread themselves southward, this latter 

 process would occur most naturally in times of gradual 

 refrigeration or of the access of a more extreme climate, 

 that is, in times of the elevation of land in the temperate 

 latitudes, or conversely, of local depression of land in the 

 Arctic, leading to invasions of northern ice. Hence the 

 times of the prevalence of particular types of plants in 

 the far north would precede those of their extension to 



the south, and a flora found fossil in Greenland might be 

 supposed to be somewhat older than a similar flora when 

 found farther south. It would seem, however, that the 

 time required for the extension of a new flora to its 

 extreme geographical limit, is so small in comparison 

 with the duration of an entire geological period, that 

 practically, this difference is of little "moment, or at least 

 does not amount to antedating the Arctic flora of a par- 

 ticular type by a whole period, but only by a fraction of 

 such period. 



It does not appear that during the whole of the Cre- 

 taceous and Eocene periods there is any evidence of such 

 refrigeration as seriously to interfere with the flora, but 

 perhaps the times of most considerable warmth are 

 those of the Dunvegan group in the Middle Cre- 

 taceous and those of the later Laramie and oldest 

 Eocene. 



It would appear that no cause for the mild temperature 

 of the Cretaceous needs to be invoked other than those 

 mutations of land and water which the geological deposits 

 themselves indicate. A condition, for example, of the 

 Atlantic basin in which the high land of Greenland should 

 be reduced in elevation and at the same time the northern 

 inlets of the Atlantic closed against the invasion of Arctic 

 ice, would at once restore climatic conditions allowing of 

 the growth of a temperate flora in Greenland. As Dr. 

 Brown has shown (" Florula Discoana "), and as I have 

 elsewhere argued, the absence of light in the Arctic 

 winter is no disadvantage, since, during the winter, 

 the growth of deciduous trees is in any case sus- 

 pended, while the constant continuance of light in the 

 summer is, on the contrary, a \ery great stimulus and 

 advantage. 



It is a remarkable phenomenon in the history of genera 

 of plants in the later Mesozoic and Tertiary, that the 

 older genera appear at once m a great number of specific 

 types, which become reduced as well as limited in range 

 down to the modern. This is no doubt connected with 

 the greater difterentiation of local conditions in the 

 modern ; but it indicates also a law of rapid multiplica- 

 tion of species in the early life of genera. The distribu- 

 tion of the species of Salisburia, Segi/oia, Platanus, 

 Sassafras, Liriodendron, Magnolia, and many other 

 genera, affords remarkable proofs of this. 



Gray, Saporta, Heer, Newberry, Lesquereux, and 

 Starkie Gardner, have all ably discussed these points ; 

 but the continual increase of our knowledge of the several 

 floras, and the removal of error as to the dates of their 

 appearance must greatly conduce to clearer and more 

 definite ideas. In particular, the prevailing opinion that 

 the Miocene was a period of the greatest extension of 

 warmth and of a temperate flora into the Arctic, must be 

 abandoned in favour of the later Cretaceous and Eocene ; 

 and if I mistake not, this will be found to accord better 

 with the evidence of general geology and of animal 

 fossils. 



Note. — While this memoir was passing through the 

 press, the Report of Mr. Whiteaves, F.G.S., Palaeon- 

 tologist to the Canadian Survey, on the invertebrate 

 fossils of the Laramie and Cretaceous of the Bow and 

 Belly River districts appeared (" Contributions to Canadian 

 Palaiontology," vol. i. part I, S9 pp. and 11 plates). This 

 valuable Report constitutes an independent testimo.iy, 

 based on animal fossils, to the age of the beds in question, 

 and accords in the main very closely with the conclusions 

 above derived from fossil plants. Unfortunately, how- 

 ever, no animal remains have yet been found in the 

 Kootanie series, and the only fossil recorded from the 

 Mill Creek beds is a species ol hioccramus characteristic 

 in the United States of the Niobrara and Benton groups, 

 a position a little higher than that deduced from the 

 plants. 



