40 CAUSES WHICH FAVOUR OR [pt. i 



occurring in both zones (average 16), and 21 genera with 484 

 species (largest 43, 42, 40, 38, average 23) have separate species 

 in each zone. 



There are many ecological changes which may be summed up 

 as climatic, and which, if they occur over a sufficient depth and 

 width of country, may offer very formidable checks or barriers 

 to dispersal. Such, for instance, are change of rainfall, of dis- 

 tribution of rainfall, of temperature, of dampness of air, of light, 

 of wind, etc. The combined effects of these form what may be 

 termed the cHmate of a place. In the existing conditions of the 

 world the climate is determined in broad outline chiefly by lati- 

 tude, position with regard to the sea, to prevailing winds, and 

 to mountain chains which are at no very great distance. The 

 lower the latitude, the Avarmer the climate; the nearer the sea, 

 and the more wind blows from it, the damper; the nearer the 

 lee side of a range crossing the ])revailing wind, the drier. 



Further, during at any rate the later periods of the world's 

 history, great ranges of mountains have sprung up in different 

 directions, especially from east to west in the Old AVorld, from 

 north to south in the New. These ranges are so lofty that apart 

 from the changes of climate due to them, they have acted as 

 very formidable barriers. And when to this is added the enor- 

 mous difference of climate on the two sides, it is clear that they 

 must have completely altered the distribution of species, and in 

 general rendered it more difficult for the greater number, though 

 on the other hand, species, chiefly herbaceous, which can live 

 at high levels in the mountains, have been enabled to travel 

 through and into regions otherwise impassable (cf. p. 37). It 

 is in this way, probably, that many herbaceous and shrubby 

 types of vegetation, including such genera as Caltha, Lignsticum, 

 and Veronica, characteristic of the north temperate regions, but 

 now also found in New Zealand, South America, etc., have been 

 enabled to reach those countries; and that the comparatively 

 young Compositae have spread so widely over the world. 



The effects of the mountain ranges on the two chief continents 

 may be seen by comparing the climates of North America and 

 of Europe, both in the zone of pre\ailing westerly winds from 

 the ocean. The west coast of the former is very wet, in latitudes 

 equal to those of northern Europe, and was originally covered 

 with forest; but as one comes to the east of the Cascades and 

 Rocky Mountains, which lie across the path of the westerly 

 winds, one reaches the land of prairie, which is especially dry 



