year, in this case the vegetation goes through a generation at 

 each period. In the separation of Tropical plants into annuals, 

 biennials, perennials, shrubs and trees, the causative internal 

 factor is power to resist dryness, and the same external factor 

 i? humidity, since the temperature does not vary enough to be 

 considered. Since the changes of Tropical plants must be due 

 to the humidity and the struggle between species so far as the 

 climate of today is concerned, and since we can see no other 

 factor in past time except the lessening amount of carbonic 

 acid in the air whose effect would be jiroportional throughout 

 the world, we find less change in its flora than in that of the 

 other zones ; we assume that the origin of life was in the Trop- 

 ics and that from its species have sprung the plants of the 

 Temperate Zones. 



In the Temperate Zones we find an added factor to produce 

 seasons, namely, cold. As we pass out of the Tropics dryness 

 as a seasonal factor gradually disappears and cold takes its 

 place. This cold need be only of such a temperature as to 

 stop the life processes in order to cause a season of rest, and 

 the continuance of this condition will measure the relative 

 lengths of the growing and resting periods. On the border 

 land of the Tropics we find both cold and dryness as rest fac- 

 tors and thus there are frequently two seasons in a year, the 

 spring growth and the fall growth, there being a rest period 

 from dryness in the summer and another from cold in the win- 

 ter. An effect which we find in this region is shown by plants 

 which are annuals farther north become perennials here. 



The annual temperature in the Temperate Zones has a 

 marked effect on the vegetation, for the flora gradually changes 

 as the cold increases. Near the Tropics the variety of forms is 

 endless but as we advance poleward the variety decreases and 

 the number of individuals increases till near the poles we have 

 very few forms, nearly the same the world around, but myriads 

 of individuals. It is not always easy to see why a decreasing 

 temperature alone produces such a monotony in vegetation. 

 In the Tropics the heat is such that the species will be changed 

 throughout the entire gamut of life by a varying humidity 

 which is caused by topography chiefly, while in the North the 

 cold is great and 'the relative humidity therefore always high 

 in the growing season, because of the storage of moisture in 

 ice and snow during the long winter, and the inability of the 



