II 



DISTRIBUTION OF SPECIES 17 



botanical works in existence, his Geographie botanique 

 raisonnee, in two thick volumes. 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 influence their distribu- 

 tion, 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 altitude, latitude, aspect, 

 humidity, geological and mineralogical causes, both in their 

 direct and indirect action, and as applying to cultivated as 

 well as wild 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 world, and 

 under various geographical conditions, and thus arrives at 

 the causes of great extension of certain species from west to 

 east in the north temperate zone, or along sea-shores or 

 river-banks in the tropics ; while the normal area is 

 considered to be " massive " rather than elongated. 



Coming then to detailed facts, he shows that about 200 

 species (out of the total then known of about 120,000) have 

 areas equal to one-third or more of the entire land surface 



i of the globe. Further, in certain Families (usually called 

 Natural Orders) there are plants which range from the 

 Arctic regions to the southern extremity of the great 

 continents. Among the former are our common Marsh 



' Marigold {Caltha palustris) and Common Sundew {Drosera 



C 



