104 MASSACHUSETTS HORTICULTURAL SOCIETY. 



shores. Aud the extremes of these conditions are represented on 

 the one hand by the frigid barrenness of the Arctic zone, and on 

 the other by the rank luxuriance of the Middle Tropics, and there is 

 every gradation between. It is true that other influences are at 

 work also, — prevailing character of the soil, abundance and kind of 

 enemies, etc., but heat and moisture are undoubtedly the most impor- 

 tant of all those which belong to the physical surroundings. 



Now the distribution of heat over the earth's surface is deter- 

 mined by several conditions, amongst which the most important 

 are these : 



(1) Other things being equal, the greatest quantity of heat is 

 received from the sun by the earth at the equator, and the amount 

 diminishes regularly towards the poles, the whole being subject to 

 regular variations owing to the movement of the earth placing the 

 the sun alternately north and south of the equator. It is plain 

 that, were it not for the disturbing effects to lie mentioned below, 

 the earth would be divided into great latitudinal zones of tempera- 

 ture of uniform In-eadth, to which vegetation would tend to corres- 

 pond, and our problem from this point of view would be greatly 

 simplified. 



But (2) the relative distribution of land and water powerfully 

 affects these zones. This includes the influence of cold and warm 

 currents, whose effects are marked enough. I^verybody knows how 

 the Gulf Stream raises the mean annual temperature of North- 

 western I^urope, or how the great Japanese current raises that of 

 British Columbia and Alaska. And on the contrary we are very 

 sensible ourselves of the way in which the cold Labrador current 

 keeps our mean annual temperature reduced below that to which 

 our latitude entitles us. Again the principle that great masses of 

 water are equalizers of temperature, and that great masses of land 

 permit of extremes, is one of much importance ; and abundant 

 illustration will occur to all in the case of the extreme range of 

 temperature to which the cities of the central states are subjected 

 as compared with the much smaller range of the mercury in the 

 seaport towns.* And its importance becomes still more manifest 

 when we compare the well-known evenness of the temperature range 

 of oceanic islands, with the conditions prevailing at Yakutsk in 

 Siberia, which lies well within the greatest body of land in the world 



*The cliraate of Boston does not afford a fair illustration. It is so near the Cape 

 Cod Peninsula, which is the great natural boundary between the colder northern and 

 warmer southern waters, that it is exi)osed to a set of very unusual conditions. 



