106 MASSACHUSETTS HORTICULTURAL SOCIETY. 



happens in parts of northwestern Enrope and of the Canadian 

 northwest. 



The distribution of moisture depends of course chiefly upon 

 rainfall, which in turn depends upon meteorological conditions of 

 which the discussion is not in place here. It is enough for our 

 present purpose to note that in general the tropics hare the greatest 

 rainfall and that there it is extremely regular. All readers of Mr. 

 Wallace's remarkable book, "Tropical Nature," will remember his 

 most graphic account of tropical rains and their effect upon tropical 

 vegetation. "With an abundance of evenly distributed solar heat 

 and an abundance of evenly distributed moisture, it is no wonder 

 that tropical forests are so luxuriant. Just outside of the trop- 

 ics comes a great rainless belt which includes nearly all of the 

 desert regions of the earth, and beyond this again, both north and 

 south of the equator, we come to the regions of varialile rainfall, 

 with which purely local conditions have so much to do, and passing 

 which we come to true Arctic conditions. Amongst the important 

 local causes influencing rainfall in the temperate regions are the 

 proximit}' of the sea, direction of the prevailing winds, and the 

 presence or absence of mountain ranges. The latter always tend 

 to cause precipitation on themselves, and between themselves and 

 the sea, and to shut off from the blessings of the rain the region on 

 the side away from the sea. It is, for instance, owing to the 

 presence of the Sierra Nevada and Cascade ranges that the region 

 to the west has so much more abundant a rainfall than has the re- 

 gion on the east of them. Upon this side of the continent, the Alle- 

 ghanies are not high enough to produce more than a partial effect 

 in this direction. 



And secondly among the controlling" agencies in distribution comes 

 the geological history of plants : geological historj' in this connec- 

 tion means simply the story of the changes in the distribution of 

 land and water in past times as compared with the present — and the 

 consequeut migrations plants have been forced to make, not always 

 into regions the most favorable to them. Hence it has come to 

 pass that plants toda}' are not all placed on the earth's surface just 

 where the conditions are most favorable to them, and many of therri 

 when introduced into new lands, often find there conditions more 

 congenial than in their old homes. But geological histor}' is of 

 more importance as a distributing agency, and we shall in a few 

 moments consider it in that connection. 



Now these two, I must ask von to notice asain, are the limitins: 



