DISTRIBUTION A^B EVOLUTION 105 



qiuntuple it. But in all such comparisons we require a large 

 number of fairly comparable cases to give a trustworthy 

 average, and the materials for this do not seem to exist. Yet 

 there is a striking general agreement between the numbers 

 of the species in the various kingdoms, states, or colonies of 

 Europe, North America, and Australia, requiring only slight 

 allowances for greater area, better climate, or geological history 

 to bring them into line with one another to a really remarkable 

 degree. 



It appears, then, that, whether we take small areas roughly 

 approximating 100 square miles, or much larger areas of from 

 100,000 to 200,000 square miles, there is, over the whole world, 

 an unexpected amount of agreement in the numbers of the 

 flowering plants, but always showing a moderate increase from 

 the colder to the w^armer parts of the earth. 



Differences of Temperate and Tropical Vegetation 



One of the chief differences between the floras of the colder 

 and of the warmer parts of the earth (already referred to) is 

 the greater prevalence in the former of gregarious plants. To- 

 wards the northern limit of vegetation we find continuous 

 forests of pines or firs, the same species often extending for 

 hundreds or even thousands of miles ; while woods of birches 

 extend even farther north almost up to the limits of perpetual 

 snow, and in this case a single species — our common birch — • 

 extends entirely across northern Europe and Asia, with allied 

 species in North America. Earther south, forests of beeches, 

 oaks, chestnuts, etc., are common, but seldom covering such 

 large areas, being dependent on conditions of soil as well as 

 of climate; while in the warmer parts of the temperate zone 

 the forests are often made up of a gTeat variety of trees, 

 though never so completely intermingled as in the typical areas 

 of the tropics. 



Another, and perhaps more important character of the 

 tropical flora, is the large number of distinct types of vegeta- 

 tion which are almost or quite peculiar to ihe warmest and 



