ORIGIN OF DESERT FLORAS. 119 
mammals and birds from the domain of the Desert Laboratory for a few 
years, it would undoubtedly include dense forests of sahuaro. 
Xerophilous types of vegetation are of comparatively recent origin, 
yet at the present time suitable conditions for them are furnished by 
deserts, the total area of which is equal to that of a large continent. The 
further desiccation of the earth’s surface, premised by many geologists, 
will give extended conditions for the origination and survival of addi- 
tional forms, so that the movement toward xerophily may be reckoned 
as one of the most important in evolutionary procedure at the present 
time. 
