SCIENCE IN MAN’S STRUGGLE ON ARID LANDS 29 
the more accurately these climatic types can be defined and shown 
on maps (5, 9). The interpretation of such maps in terms of plant 
growth and available water, even where highly accurate, depends 
upon a knowledge of the interactions between the local climatic 
type and the local land form and kind of soil. 
Within one climatic type, for example, a pervious soil of medium 
texture can take in water, even of sharp showers, and hold it for 
plant growth; whereas impervious soils refuse water, shallow ones 
cannot hold much, and loose, sandy soils allow the water to pass 
beyond the reach of plant roots. Yet water falling on thin soils 
over cracked limestone or basalt, finds its way into the deep 
substratum from which it appears again in springs or artesian 
wells, or may be tapped by pumping. 
Such basic relationships between soil and climate are sig- 
nificantly modified by variations in plant cover and soil use. 
Detailed studies of the interactions among climate, ground 
water, land form, soils, and plants are increasing in several parts of 
the world and are furnishing a continually better basis for assess- 
ing the potentialities that we have and the critical problems of use. 
Especially do these results give an essential part of the firm 
basis we need for transferring plant materials, techniques, and the 
results of research and experience from one place to another. But 
climate data alone are not enough. In fact, unless the other re- 
sources and the existing social patterns are well known also, the 
use of climatic analogues alone may give a sense of scientific se- 
curity in faulty conclusions less reliable than the direct impressions 
of an observant traveler. 
A challenging aspect of climate is the variation of its features 
from year to year and from century to century. There is abundant 
evidence in the rocks, in the soils, and in the cultural remains that 
climates of various parts of the world have changed drastically 
many times. Studies in some regions show that they have gone 
through several cycles of change. The recent studies of radioactive 
carbon in buried wood and soils have greatly foreshortened our 
geological time scale. Drastic differences must have existed in 
many areas less than 9,000 years ago; and there have been highly 
significant cycles within this recent period. 
