ORIGINATION OF SELF-GENERATING MATTER 611 
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 
be 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 
