SUN AND CLIMATIC VARIATION 49 



which the idea of cataclysms created in the mind of Cuvier, 

 that an exceptionally high temperature and an especially 

 humid atmosphere charged with carbonic acid were essential 

 to their origin. Nothing of the kind, however, was required. 

 It needed only slopes capable of supporting a dense vegetation, 

 a uniform temperature, and a normally humid atmosphere, 

 to permit the rapid and continuous growth of vegetation. And, 

 in fact, there is not the slightest trace, in cross-sections of tree- 

 trunks of this period, of those concentric circles which in the 

 cross-section of contemporaneous trees indicate yearly growth 

 in obedience to the seasons. During the Carboniferous Period 

 the earth enjoyed a perpetual spring, somewhat mitigated in 

 high latitudes, recalling those regions which to-day are repre- 

 sented in Europe by the Alps with their perpetual snows and 

 glaciers. All this accords with the hypothesis of a then greater 

 solar diameter. The absence of flowering plants must have 

 caused a greater uniformity in the vegetation than there is 

 to-day, and the newness of the land flora also explains why there 

 was as yet so little differentiation into species. The same plants, 

 in fact, were distributed over the entire North Atlantic 

 continent. A similar uniformity characterized the whole 

 Gondwana continent ; but their flora is entirely distinct. 

 The relatively poor flora of the Gondwana, known as the 

 Glossopteris flora, seems to suggest, especially in the south, a 

 lower mean temperature than that of the North Atlantic 

 continent, at least, if the vegetation of this continent was not of 

 more recent origin. This flora later extended to other regions. 

 At the end of this epoch the beautiful flora of the North 

 Atlantic continent began to become impoverished. The climate 

 then no longer allowed an abundant vegetation. Over a large 

 part of northern Germany, the southern Alps, eastern Russia, 

 and the United States, heavy rains, sweeping along with them 

 into the sands, now transformed into sandstone, chemical 

 substances which they dissolved, alternated with long periods 

 of drought and heat which gave to these sandstones the colour 

 so characteristic of desert formations. The species that were 

 typical of the North Atlantic continent could not protect them- 

 selves against the invasion of the more resi stent species from 

 the Gondwana continent which had not had to endure the 

 test of a dry heat comparable to that of the present Sahara. 

 This period of impoverishment in the northern flora coincides 



