xvi Life and Living Creatures 315 



two are precisely like each other. General similarity is 

 associated with small variations of structure. Sometimes 

 these variations produce no influence on the life of the 

 organism, and may pass unnoticed. But when they happen 

 to make one individual better fitted for obtaining food or 

 escaping danger than the others, that one has a better chance 

 of living, thriving, and handing on its fortunate peculiarities 

 to its descendants. If the variation of structure throws an 

 individual out of harmony with its environment, making it 

 weakly or stupid, that individual has a smaller chance of 

 surviving and leaving offspring. According to Darwin's 

 view the constant struggle for life is always weeding out the 

 weak and improving the position of the strong, leading by a 

 process of natural selection to the survival of the fittest. 

 But climate, and even the outline and configuration of the 

 land, are not constant ; hence organisms, hitherto victorious 

 in the struggle for existence, have to contend with an altered 

 environment, and their development, according to natural 

 selection, must after a time take place in a new direction 

 with great sacrifice of life, and possibly the extinction of 

 some species. This subject is far from simple, many of the 

 facts have still to be discovered, and none of the hypotheses 

 as yet can compare for certainty with theories that admit of 

 mathematical proof. An excellent idea of the difficulties 

 and the fascinating interest of biological facts and theories 

 will be obtained from Professor Geddes's Modern Botany^ 

 and Mr. J. Arthur Thomson's Animal Life, in this series. 



404. Conditions of Plant Distribution. Plant life, as 

 a rule, is most luxuriant where there is abundant sunlight, 

 high temperature, copious rainfall, and soil abounding in the 

 soluble salts necessary for nutrition. In the course of the 

 ages plants have gradually been modified, so as to adapt 

 themselves to their environment. Thus not only the com- 

 parative luxuriance, but also the species of plants, depends 

 to a large extent on the conditions of their growth. Where 

 natural conditions change abruptly, as, for example, on the 

 sea-coast, on the slopes of a snow-clad mountain, or the 

 edge of a desert, the kinds of creatures inhabiting the two 

 regions differ in a very marked way. If such barriers are 



