1052 LIFE : OUTLINES OF GENERAL BIOLOGY 



He finds a frequent occurrence of small mutations, transmissible 

 in their entirety in Mendelian fashion, often showing themselves 

 in "pure lines", and sometimes suggesting an enhancement of vigour 

 as when a mutant appears with deeper green in its leaves. According 

 to Baur, wild species of snapdragon, like garden races, have often 

 arisen by the summation of small mutations, the summation being 

 to the credit of natural selection, which has sifted the mutations in 

 reference to the changeful conditions of locality and climate. 



CHANGE OF CLIMATE. — So far we have considered what may 

 happen when organisms are naturalised, perhaps acclimatised, in a 

 new and different environment. But similar biological problems are 

 raised when we think of the changes that occur or have occurred in 

 the fauna and flora of a country after some drastic alteration of the 

 climate — towards aridity or humidity, towards markedly higher or 

 lower temperature. This is a very important inquiry: To what 

 extent have changes of climate functioned as factors in organic 

 evolution ? (i) In extreme cases, as when a country is covered with 

 glaciers, there may be an almost complete elimination of life, as 

 happened over the greater part of Britain during the Quaternary 

 Glacial Periods. (2) In less severe conditions the gradual setting in 

 of unfavourable climatic conditions would exert a selective influence. 

 Thus xerophytic plants with reduced transpiration would tend to 

 survive when arid conditions enervated; quickly flowering and 

 fruiting plants, entrenched below ground in winter, with reserves 

 in rhizome and bulb, would tend to survive when the snow began 

 to cover the ground for many months of the year. In a country 

 becoming warmer there might be survival value in summer growth ; 

 in a country becoming colder, the advantage might be with the 

 hibernators. In scores of ways a gradual change of climate would 

 sift the fauna and flora. 



(3) Some animals, able to move about for considerable distances 

 and not too slowly, would be prompted by the spur of climatic 

 change to shift their quarters. As the severity of the Ice Ages spread 

 southwards in Europe, many northern mammals came with it, 

 thus remains of reindeer, lemming, and Arctic fox are found in 

 deposits far to the south. As milder climates set in and the glaciers 

 melted, the descendants of some of the Arctic types, like Reindeer 

 and White Fox, were able to trek for the north. Some, however, 

 remained as refugees on the mountains, like the Snow Vole 

 (Microtus nivalis) of the high Alps, which seems to have been a 

 "tundra" mammal that came far south in Europe in glacial times, 

 but was unable to return. Some of the true bird migrations, with a 

 seasonal ebb and flow, may owe their origin in part to distant 

 climatic changes such as those of the Quaternary Ice Ages (see 

 Migration). 



