18 THE WOELD OF LIFE 



for existence " which, as we shall see further on, is always 

 present, acts in an almost infinite number of ways, and is one 

 of the most important factors in the developmental changes 

 of the World of Life. We will now proceed to give some of 

 the numerical facts of plant distribution, in various areas small 

 and large, as well as over the whole earth; but it will be ad- 

 visable first to give a brief account of the way in which this 

 is usually dealt with by botanists. 



Four years before the appearance of the Origin of Species 

 the great Swiss botanist, Alphonse De Candolle, published one 

 of the most remarkable and interesting botanical works in 

 existence, his GeogTaphie botanique raisonnee, in two thick vol- 

 umes. He not only brought together all the then available 

 facts as to plant distribution in every part of the world, studied 

 them from almost every point of view, and grouped them in 

 relation to every known agency that might be supposed to influ- 

 ence their distribution, but at every step he most carefully 

 and ingeniously discussed the problems involved, often of a 

 very intricate nature, with a view to arriving at a more or less 

 complete explanation. 



It is impossible here to give any adequate notion of this 

 great work, but a few of the chief subjects treated may be 

 mentioned. The effects of temperature and of light upon the 

 growth and vitality of plants are first examined, and some 

 very interesting conclusions are reached, among others the great 

 importance of the time during which any particular degree 

 of heat continues. This discussion occupies the first three 

 chapters. Sixteen long chapters then deal with " Botanical 

 Geography," in which all the geographical conditions that affect 

 the distribution of plants are elaborately discussed, such as alti- 

 tude, latitude, aspect, humidity, geological and mineralogical 

 causes, both in their direct and indirect action, and as apply- 

 ing to cultivated as well as ^vild plants. The areas occupied 

 by species, both as regards size and shape, are then discussed, 

 and the causes that lead to their variations investigated. He 

 then shows what are the actual areas in various parts of the 



