THE ECOLOGY OF MAN—SEARS 381 
Any community has both structure and composition. Structure 
depends on the form of dominant plant life, as forest, grassland, or 
scrub. This tells us a good deal about local conditions such as mois- 
ture and length of growing season. Grass requires more moisture 
than scrub, trees more than grass, as a rule. 
To get further information we have to see what kinds of trees make 
up a forest, what kinds of grass and other herbs form a given prairie 
or steppe. Desert plants look much alike the world over but are very 
different botanically, cactuses being 100 percent American; or rather 
they were until misguided people took them to North Africa and Aus- 
tralia where they have become an unbounded nuisance. It has become 
necessary, in fact, to send back to America for insects that serve to 
keep cactuses in hand in Mexico and our own Southwest. As we have 
said earlier, plants and animals through occupying niches and perform- 
ing roles have a general tendency to maintain a balance among them- 
selves and with the physical conditions around them. 
This balance is not a perfect thing, nor is it achieved immediately. 
The living organisms that first occupy a particular site change it by 
shading, enriching, and in other ways, so that they actually write 
their own finis, being succeeded by other species. This process, ap- 
propriately known as ecological succession, continues up to the point 
where members of the community can perpetuate themselves as hem- 
lock does under hemlock, which for its part may have come in under 
fir or spruce. And perhaps the fir or spruce has followed pine after 
the latter had occupied bare ground. Thus it is that one can read 
climate, soil moisture and quality, and history to boot by observing 
living communities. It is important, not only to human enjoyment, 
but to human understanding that we do not destroy completely the 
natural communities which have preceded us and literally prepared 
the way for us. 
These communities have built up the soil to sustain far more life 
than could the original bare ground, virgin soil being notably rich. 
They have also regulated the flow of water and held the ground in 
place. They are the homes of plants and animals that are useful, in- 
teresting, and enjoyable. They serve, far better than any instruments 
man has devised, to give us a measure of the capacity of an environ- 
ment to sustain life. And, believe it or not, they afford us models for 
our own use of land. The most destructive practices, such as growing 
single crops in rows, are a far cry from what we find in nature. But 
grass and legume farming with livestock to consume the product and 
enrich the soil, winter cover crops to protect fields that otherwise 
would be bare, and forests on rough ground are far better for the 
land and, in the long run, for the man who lives on it. They accord 
better, too, with certain fundamental laws that govern the efficient 
use of energy and materials. 
