114 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 
must have been subject to periods of comparative aridity. Attempts 
to recognize xerophilous characters in the fossil plant remains have met 
with but little success, however. Some of the plants of the Carbonifer- 
ous period had fleshy bodies, with small, central woody cylinders, great 
development of pulpy ground-tissue and scaly appendages, suggestive 
of the reduced foliar organs of desert plants, but currently accepted 
interpretations place these ancient Lycopods and Calamites as residents 
of regions with abundant supplies of moisture, or perhaps as actually 
living in swampy areas. 
There seems to be no escape from the conclusion, therefore, that if 
desert conditions did occur in previous epochs, the only types of vege- 
tation found in them must have been those which were active during 
the seasons of maximum precipitation as described above. The con- 
sideration of a still wider range of facts leads to the inevitable conclusion 
that the form-characters, moisture conserving capacities and resistance 
to desiccation distinctive of xerophytic species must have made their 
appearance within comparatively recent geologic time. 
Our investigations of this matter may therefore be restricted to a 
comparatively modern phase of the history of the earth. ‘The desert 
conditions to be encountered on the earth’s surface at the present time 
are not to be taken as having come about by a simple, direct, and con- 
tinuously increasing aridity. An accumulation of evidence from the great 
central basin of Asia, from southern Australia, northern and southern 
Africa, and from the basins and plains of western America tends to estab- 
lish the conclusion that the climate has undergone oscillations between 
periods of lower temperature and maximum precipitation and of maxi- 
mum aridity with increased variations in temperature, the swing from 
one maximum to another occupying periods of a thousand, two thousand, 
or many thousands of years. 
Furthermore, the main movement in the change of climatal condi- 
tions may be complicated by minor fluctuations of such pronounced 
character as to obscure the total movement for hundreds of years. Thus 
the climate of the Great Basin in western America is supposed to be 
undergoing a change toward increased humidity, but it can not be said 
whether this may be part of a major change which will finally carry the 
climate to a condition of great precipitation and equable temperatures, 
or whether it may be simply a minor variation of the change toward 
a period of still more marked aridity. 
The extremes reached in such variations are so wide apart as to pred- 
icate an almost total change in the character of the floras of the regions 
concerned, and it may be safely assumed that but few Species now inhabit 
the great basin of Nevada which grew along the shores or in the tribu- 
tary valleys of Lake Lahontain. A large proportion of the flora doubtless 
originated during the present arid cycle, although of course some dis- 
