foundations of modern distributions 155 



Some Effects of Relatively Recent Climatic Changes 



At the close of the Tertiary or beginning of the Quaternary, 

 ahhough the vegetation of the tropical and adjacent zones continued 

 in considerable luxuriance, there was a marked lowering of tempera- 

 ture in most other regions, that led to the covering of some by 

 glaciers and to complete changes in the floras of others. Climate 

 being the primary controller of vegetation, the flora in favourable 

 warm areas probably continued, as it does to the present day, much 

 as it had done at least through the middle Tertiary. But in the 

 less favoured belts to the north and south, a lowering of temperature 

 and decrease in precipitation — for instance in much of the Mediter- 

 ranean Basin, in eastern Europe, and in northern and central Asia 

 — led to the widespread replacement of the warmth-loving floras 

 by hardier types. It may be presumed that there was at the same 

 time a reduction in numbers of species and individuals, and in the 

 general luxuriance of the vegetation. And whereas in the boreal 

 regions, and even in the Arctic, there had formerly flourished (and 

 according to some authorities evolved) a vast array of warmth-loving 

 or at least mesothermic {i.e. liking moderate temperatures) types, 

 these were in time pushed far south or, in some cases, doubtless 

 exterminated. 



Although evolution in general is a gradual process, it seems to 

 be accelerated by sudden changes in habitat conditions. Thus the 

 violent upheavals of the earth's crust or direct changes of clim.ate 

 appear to have induced sudden inherent and hereditary changes 

 involving the abrupt creation of new races and, in time, of new 

 species. Contemporaneously and for similar reasons, floras have 

 been caused to migrate. These variations of conditions over long 

 periods of time have probably led to a far more intricately adjusted 

 and highly evolved general flora and vegetation than would have 

 obtained if conditions had remained as they were, for example, in 

 the Carboniferous. Indeed, some authorities have pictured a world 

 of gigantic Reptiles instead of Mammals, and of giant Club-mosses 

 and Horsetails instead of Angiosperms, as continuing today if the 

 Carboniferous climatic and other conditions had persisted. 



Besides direct climatic changes and others depending on such 

 geological revolutions as the thrusting up of mountain ranges, there 

 have evidently been changes in the conformation of land and sea 

 — even, according to some authorities, in the positions of the con- 

 tinents with regard to one another and in relation to the geographical 



