GEOGRAPHICAL DISTRIBUTION 391 



be the means of maintaining the race in harmony with its 

 environment. Although vegetative mutation appears to be 

 comparatively infrequent, it should be borne in mind that groups 

 like Bacteria and Cyanophyceae, which multiply exclusively by 

 vegetative means, often do so with great rapidity. An excep- 

 tionally large number of generations is therefore formed in a 

 short time, and so presumably the opportunities for mutation 

 are proportionately great. 



Intimately bound up with the subject of the origin of new 

 forms, considered in the preceding pages, is that of the geo- 

 graphical distribution of plants. This is mainly related to differ- 

 ences of climate, the distribution of a species often depending 

 on its tolerance of cold or heat, dryness or moisture, etc. Under 

 more or less uniform climatic conditions, such as obtain in our 

 own country, soil differences play a large part in determining 

 the regional distribution of plants. Most frequently it is the 

 physical properties of the soil that are important in thus modi- 

 fying the effects of climate. Of two areas receiving the same 

 annual rainfall, for instance, that with sandy soil may be a 

 desert, whilst that with peaty soil forms a bog. 



The effect of climate is well illustrated by the distribution 

 of forest, grassland, and desert over the earth's surface. In 

 general, forest occupies the regions of highest, and desert those 

 of lowest, rainfall, whilst grassland occurs in areas of medium, 

 rather uniformly distributed, rainfall ; the influence of man or 

 of winds may, however, profoundly modify the applicability of 

 this generalisation. In northern latitudes forests are composed 

 of such trees as Birch, Pine, and Spruce, whilst in temperate 

 zones deciduous trees, such as Beech, Oak, and Ash, predominate. 

 On the slopes of high mountains, like those of Switzerland, the 

 same relative distribution occurs, with deciduous forests below 

 and Coniferous ones above. 



But, apart from such cases in which the range of species is 

 limited by their inability to withstand competition under adverse 

 climatic conditions, certain species, genera, or even families are 

 found to be restricted to particular areas of the earth's surface, 

 though others, are equally well suited to their existence. This 



