CLIMATIC CHANGES 85 



Organic Evidence of Change of Climate. This is in many 

 respects the most reUable, since organisms are the most sensitive 

 indicators of cHmatic conditions. 



I. Plants. Coal swamp vegetation, as indicative of cooler and 

 moister climates, has already been referred to. The various types 

 of swamp vegetation preserved in the peats of different coun- 

 tries, serve as an excellent index to the gradual change of climatic 

 conditions since the last glacial period. Thus for Germany it has 

 been ascertained that the first fioral mantle following the retreat of 

 the ice was of the tundra type wathout any true arboreal growths 

 whatever. In many places a lower horizon, with the mountain or 

 arctic dryas. Dryas octopetala, and the arctic dwarf willow, Salix 

 polaris. and a higher one, with Salix pliylicifolia and ^. reticulata, 

 besides Dryas octopetala, can be determined. Aquatic plants are 

 rare in this period, but several species of Potamogeton occur regu- 

 larly in the upper beds, together with Myriophyllmn spicatum, Hip- 

 puris vulgaris and Batrachium aquatilc confcrvoidcs. During the 

 Dryas period, even in the earliest epoch, the climate could not have 

 been an arctic one in North Germany, for the aquatic plants require 

 a July temperature of approximately 6° C. and need 4 to 5 months 

 with a temperature of at least 3° C. in order that their seeds may 

 ripen. The rapid amelioration of the climate during the Dryas 

 period is shown by the presence of Phragmites communis in the 

 upper layers formed during this period, followed by heavy deposits 

 of decayed vegetation, indicating a rapid increase in the plant and 

 animal life of the waters. With this appears the first arboreal vege- 

 tation with birches and pines. 



The two epochs of arctic floras, /. c., the earlier colder one with 

 Salix polaris and the later milder one with Salix reticulata, are rec- 

 ognized in many regions in Scania and elsewhere in southern 

 Sweden. In Finland the arctic Dryas flora (Dryas, Salix polaris, 

 Betula nana, Batrachium confcrvoidcs) and the moss Sphccro- 

 ccphalus turgidus, characteristic of the modern arctic region, to- 

 gether with the arctic beetle Ptcrosticlius vcniiiculosus, occur in a 

 sandy deposit between Lake Ladoga and the Gulf of Finland, in- 

 dicating a climate, during the period of the melting of the ice, com- 

 parable to that now found in northern Russia and the neighbor- 

 hood of the polar sea. The Salix polaris flora has also been found 

 in Norway and Denmark, this arctic flora everywhere forming the 

 first or tundra type of flora to appear during the period of melting 

 of the glaciers. Gunnar Andersson lays especial stress on the oc- 

 currence of an aquatic flora with these arctic plants, which, though 

 consisting of few species, is nevertheless rich in numbers. He con- 



