890 LIFE : OUTLINES OF GENERAL BIOLOGY 



sphere — in the texture, as it were, of our indispensable blanket, 

 Cr oil's famous theory referred the Ice Ages to changes in the eccen- 

 tricity of the earth's orbit, and Sir Robert Ball's venture was on 

 the same line. Again, since sunspots have their rhythm, of some 

 eleven years or thereby, may not the sun have yet larger variations 

 beyond our as yet so brief observation ? 



What is the theory of the latest authority, Coleman himself? 

 He does not think that any one of the many theories will work ; and 

 he has none of his own, beyond a combination of factors. Still, that 

 opens wide fields for further consideration; for any adequate solu- 

 tion of the compUcated problems involved must surely come from 

 a conjunction of general and local causes. Some combination of 

 astronomic, geologic, and perhaps even atmospheric conditions 

 seems to be necessary to produce such catastrophic events in the 

 world's history. The rarity of such a conjunction would account for 

 the comparatively few and irregularly spaced times of glaciation 

 which interrupt the usual monotonous continuity of mild conditions 

 shown by palaeontology. Whatever the causes of Ice Ages may have 

 been, there is no doubt as to their effects, for they are times of severe 

 sifting, when many fine forms of life are "cast as rubbish to the 

 void". To change the metaphor: an Ice Age prunes the tree of life. 



Yet it may also do much more than that — even to favouring the 

 origin and adaptation of new types along its chilly borders. See 

 for conspicuous example the vast abundance and variety of species 

 of grasses, rushes and sedges, and these along our cold temperate 

 region edge, as we approach towards Arctic climate, and we gather 

 many of these as we descend from the moors and by damp brook-, 

 banks, to marsh and pool and lakeshore. See how the rushes 

 (Juncaceas) peculiarly show reduction, both foliar and floral, and 

 each in its own way extreme, from the normal liliaceous type, of 

 which, by common consent of botanists, they are really little if 

 anything more than a sub-order. And though the grasses attain 

 more exuberance and variety, is not this but in abundance of slender 

 leafage, and with reduction of flowers, albeit multitude ? And these 

 changes the sedges carry yet further in their own ways. So, too, for 

 the coldward colonising trees, as from various conifers to birches 

 above, and wiUows and alders below. See, too, the heaths, on drier 

 areas ; and the cranberries and the like in moister ones. And, again, 

 after ferns disappear — even to their hardiest, like the deep-rhizomed 

 bracken (Pteris) or the well-coated Ceterach — see how the Lichens, 

 with their extraordinary resistance and persistence, more and more 

 cover the rocks, and even invade the pasture; thus largely over- 

 powering its grasses over vast tundras as from Lapland to North- 

 east Siberia, and onwards from Alaska as well. Do not all these 

 examples, and more, compel us to regard this iceward world-border 



I 



