PERIODIC GLACIATION. 909 



selves over the lowlands, so also if the temperature rises they are able to retire to 

 the heights again. No more need be said to show that the advance and retreat of 

 vegetation has taken place, and does still take place, pari passu with the growth 

 and melting away of glaciers. 



Very various notions concerning the cause of the periodical return of an ice-age 

 have prevailed from time to time. Several prominent experts of the present day 

 believe that alterations in the eccentricity of the earth's orbit are the cause of the 

 phenomenon. When the eccentricity increases the earth's surface is considerably 

 cooled, and as the eccentricity diminishes the heat increases. A period of great 

 eccentricity must have begun about 240,000 years before our era and have lasted 

 16,000 years. Similarly the gi-eat eccentricity which existed 850,000 and 2,500,000 

 years before our era must have brought about repeated glaciation. By others an 

 alteration of the position of the pole is considered to be the cause of the phenomenon 

 in question. Much may no doubt be urged against this explanation, but several 

 phenomena in the plant-world are more easily reconciled with it than with any 

 other. One example of these is the existence of lofty plants with large foliage in 

 the Arctic region during the Miocene, Cretaceous, and Carboniferous periods, as is 

 proved by the discovery of numbers of fossil remains. In the Miocene and Cretaceous 

 periods. Tulip-trees, Magnolias, Limes, Planes, Bread-fruit trees, and Water-lilies 

 flourished in North Greenland, Grinnell Land, Iceland, and Spitzbergen. None of 

 these plants can live there now, for the two following reasons. Firstly, the con- 

 ditions in respect of solar illumination which obtain there would not permit of their 

 healthy development; and, secondly, there is not sufBcient warmth to enable them 

 to grow hardih^ Since the most eminent geologists of the day have declared 

 against the idea of the interior of the earth being in a fiery, molten condition it will 

 not do to attribute to that source the high temperature necessary for great Planes, 

 Magnolias, and Bread-fruit trees to flourish in such high latitudes. On the other 

 hand, the presence of large-leaved Angiospermous trees in North Greenland, 

 Grinnell Land, Iceland, and Spitzbergen would be satisfactorily explained if it 

 were assumed that the spot which now forms the North Pole — and with it the 

 whole region now called Ai-ctic — then occupied a different position relatively to the 

 earth's orbit, and consequently received a different amount of light and heat. 



As regards the history of plants prior to the Eocene and Cretaceous periods no 

 data are afforded by the investigation of the distribution of living plants, and we 

 are thrown back on the fossil remains derived from those older periods. These are 

 unfortunately comparatively scanty, and they no doubt represent but a small 

 proportion of the species which lived before the Cretaceous period. Two conclusions 

 may, however, clearly be drawn from these remains, viz.: firstly, that no single main 

 division existed at that time which is not still represented at the present day; and, 

 secondly, that some very conspicuous genera of particular groups have died out and 

 been replaced by other genera of the same groups. Specially noticeable in this 

 connection are the tree Club-mosses of the Carboniferous period and the Calamites, 

 species of Horse-tail which must have formed extensive forests in the Carboniferous 



