1 NATURAL SELECTION " AND "SURVIVAL OF THE FITTEST. 95 



the warmer temperate zone of the northern hemisphere. It is a 

 trimorphic plant, but only one form, the "short-styled," is known in 

 the north. It never sets seed, though it might be thought capable 

 of doing so by the pollen falling upon the stigmas ; but it is solely 

 propagated by bulbs. Indeed, Italian botanists record that the fruit 

 has never been seen ; the whole of the flower articulates, leaving the 

 pedicel. 



As a tropical example of natural selection, an equatorial rain-forest 

 might be taken, in which a constantly uniform condition of moisture and 

 heat stimulated growth, every square foot being pretty well occupied by 

 some kind of vegetation. Consequently an intense struggle for existence 

 has obtained, till something like equilibrium has been established. The 

 trees are tall, more or less unbranched till the stems have attained 

 a great height, when the struggle for light goes on. 



Other plants have acquired the habit of climbing, which enables them 

 to reach the light as well. 



Another group with very light seeds, as orchids and rhododendrons, 

 or spores, as of ferns, and easily diffusible by the wind, have located 

 themselves on the trunks and branches of trees, and so secured a home, 

 which they could not have done on the ground in competition with more 

 powerful plants. They exist and thrive by having acquired xerophytic 

 structures. 



In dry tropical and subtropical regions, the character of the vegetation 

 is quite different ; and though many may grow together, spinescence and 

 succulency have become characteristic features. 



With these plants the struggle for existence is, perhaps, more 

 generally with inhospitable conditions of soil and climate. These are 

 great aridity, a sandy and rocky ground. 



In such surroundings, like the epiphytes in a rain-forest, the plants 

 exist by adapting themselves to these by becoming xerophytes, the chief 

 adaptation being the acquisition of great succulency, by means of which 

 water is stored up against the dry season, which lasts during the greater 

 part of the year. 



In many instances where the soil is sandy for great depths, plants 

 have developed long roots by means of which they can reach subterranean 

 water. In the case of the Narras plant of Damaraland, a member of the 

 Cucurbitacece, the roots sometimes attain a length of forty feet. 



Very similar conditions prevail in high Alpine and Arctic conditions, 

 as far as their effect upon plants is concerned. The struggle for existence 

 is more often with the physical conditions than in competition with other 

 plants ; and those which characterise such inhospitable regions have 

 secured their positions through adaptations, by responding to them. 



Numerous experiments have proved this interpretation to be true. 

 Plants of lowlands or of lower latitudes have been grown on the Alps and 

 Pyrenees or in high Arctic latitudes, and vice versa ; and they have 

 always acquired in varying degrees all the characters and habits of plants 

 native to those regions respectively. 



In Arctic regions the struggle for existence against the inhospitable 

 conditions of existence is very intense. The causes of the dwarf habit due 

 to arrested growth are the low temperature in summer, and the cold soil ; 



