490 A. Blytt on the probable Cause of 



come to a different conclusion ; I should regard it as a probable 

 supposition that the formation of deposits went on more rapidly 

 in Pala30zoic times than later on. If the moon at that time 

 were nearer to us and the sidereal day shorter, as Darwin 

 thinks, the tidal wave must both have been stronger and have 

 acted more frequently than at jiresent. The coasts would be 

 destroyed much more rapidly, and the sea would have much 

 more material to deposit. A cycle of this period would 

 be thicker than the younger cycles, and the fossils would 

 extend through a greater thickness of strata than in the latter. 

 For I see at present no probable ground for the supposition 

 that the development of new species would be accelerated in 

 the same degree as the formation of deposits. 



There is therefore reason to assume that it is owing to these 

 great changes in the form of the earth, occurring at long 

 intervals, that we can distinguish between geological forma- 

 tions. But such great changes in the distribution of land and 

 sea must necessarily also bring with them considerable changes 

 of climate, and at the same tiine also changes of living forms. 

 I have already, in one of my memoirs, put forward the opinion 

 that the glacial period had its origin in a change of the dis- 

 tribution of land and sea. If the land gained a great extension 

 in the middle and higher latitudes, especially if there should 

 be a formation of bridges across the sea such as the supposed 

 bridge through the Faroes and Iceland from Scotland to 

 Greenland, the warm sea-currents would be excluded from the 

 higher latitudes. The northern seas would then become icy 

 seas, and where the snow-fall is sufficient inland ice would be 

 formed. In a memoir entitled " Natiirliche Warmwasser- 

 heizung als Princip der klimatischen Zustande der geolo- 

 gischen Formationen " (in Abhandl. Senckenb. Gesellseh. vol. 

 xiii. p. 277 et seqq.), J. Probst (hke Sartorius von Walters- 

 hausen, in his Untersuchungen fiber die Kiimate der Gegenwart 

 und Vergangenheit, 1865) has with justice pointed out the 

 great importance which warm sea-currents have, and have 

 had, in rendering milder the climate of high latitudes. 



It has been generally accepted among geologists that during 

 the older formations animal- and plant-life was more uniform 

 over the whole earth than at present. But this opinion must 

 be changed according to recent investigations. Thus J. VV. 

 Judd says ('Nature,' March 1, 1888, pp. 424 et seqq.), with 

 regard to the oldest fossiliferous deposits (the Cambrian) : — 

 " Even at that early period there were life-provinces with a 

 distribution of organisms in space quite analogous to that 

 which exists at the present day." Examples of geographical 

 provinces are indicated by him in the Silurian, Trias, Jura, 



