THE ECOLOGY OF MAN—SEARS 379 
of our planet is, like the soil, a reservoir of materials needed by plants 
and animals. It is also the medium through which solar energy 
reaches vegetation. But more than this, it is in constant turmoil, due 
primarily to that same solar energy. Its dynamic activity provides us 
with our daily weather while its behavior pattern in time and space 
we designate as climate, so important to the distribution and activity 
of living organisms. 
Although atmospheric behavior is based upon the geometrically 
regular relations of sun and earth, the irregular pattern of land and 
water causes many interesting and important variations from place to 
place, from time to time. Great shifts have taken place over long 
periods of time as witnessed by the presence of coal in the polar re- 
gions and evidence of continental ice masses as far south as the Ohio 
River. Lesser fluctuations, for example the recurrence of dry years in 
groups, are known to be normal events and should be so regarded in 
planning regional economy, notably in the High Plains. 
Water (the hydrosphere).—Here we deal with three states of 
matter, liquid, solid, and gaseous. Although the prevailing tempera- 
tures on earth lie between the melting and boiling points of water— 
a very fortunate circumstance for life as we know it—of the utmost 
importance is the ability of water to form vapor at temperatures 
below its boiling point. Thus water rises from the oceans and moves 
inland as vapor, to fall upon the land as rain or snow, then flows back 
to sea. The rate of this return is of great ecological moment. In 
general the more gradual it is, the greater its opportunity to sustain 
life on land and the less it disturbs land forms through erosion and 
deposition. And here we encounter one of those many mutual rela- 
tions that abound in the living landscape, for it is vegetation which, 
once established, chiefly restrains the rush and destructiveness of flow- 
ing water. 
Truly, water, one of the most familiar of substances, is yet one of 
the most amazing; and its presence in atmosphere, lithosphere, and 
living organisms as well, serves to remind us that environment is a 
great interwoven complex which we can take apart only mentally in 
our effort to understand it better. 
Life (the living biosphere of plants, animals, and some simple 
forms that may be either, or both).—In the old days when physical 
scientists were called natural philosophers, and geologists and biolo- 
gists were not ashamed to be called naturalists, it used to be said that 
nature abhors a vacuum. It might be said with equal truth that 
nature abhors the absence of life on earth, in air or water. Life, 
whether visible to us or not, abounds everywhere, thanks to the vari- 
ation, reproductive vigor, and fitness through natural selection that 
Darwin pointed out to us. 
