286 D. T. MACDOUGAL 



momentous change, and one of great importance in connection with 

 the general subject under consideration. 



Temperatures alone have been unduly drawn upon in the inter- 

 pretation of distributional features of ancient and existing floras, 

 a fact made more plainly apparent by recent observations at the 

 Desert Laboratory, in which it has been found that several species 

 range over a vertical mile. Such species endure cold of —35° C. and 

 have a growing season of less than a hundred days in the more boreal 

 or alpine portion of their ranges, while in the southern or lower locali- 

 ties inhabited by them, temperatures of 48° C. may be encountered; 

 the growing season extending over 300 days; the thermometer going 

 below the freezing-point not more than 12 hours during the entire 

 year. 



It is with no surprise, therefore, that it is learned that there is 

 no single feature in the structure and functionation of plants that 

 with perfect assurance may be connected with the influence of tem- 

 perature alone, although alpine and polar floras bear a distinct aspect 

 by reason of a combination of conditions of moisture, insolation, 

 duration of the seasons, and course of the humidity. 



While temperature is not in itself a direct factor in shaping the 

 general trend of evolutionary development in plants, yet it is indi- 

 rectly concerned by the influence exerted upon precipitation, and the 

 relation of the amount of the rainfall to the possible evaporation. The 

 great changes in the climatic pattern of the surface of the earth, 

 both in this and preceding periods, produced by whatever cause, may 

 De taken to have affected vegetation chiefly through the humidity 

 and desiccation effects, which not only determined the range and 

 habitats of the species, but also played a predominant part in shaping 

 the general development of the vegetal organism. 



It will be profitable therefore to analyze the changes accompanying 

 a modification of a climate toward or away from the desiccation of 

 a region and the response of the flora to such altered conditions of 

 environment. To do this most effectively let us suppose that the 

 rainfall in New York, Pennsylvania, Labrador, Iowa, or Florida 

 were reduced to one-fourth of the present amount by a gradual 

 decrease through a long term of years. In the lower levels of the 

 region affected, the total production of organic matter would be 



