THE ECOLOGY OF MAN—SEARS 385 
various shifts occurred is one of the intriguing problems of science, 
but we can say they did happen. 
At any rate man had spread his wide-meshed net of sparse popula- 
tions into every continent before the next revolutionary step, his do- 
mestication of the cereals. We are pretty sure of our ground here 
because we can, by various methods, trace the use of cultivated plants 
outward from the centers of first use, and we know the general location 
of three such independent centers. These are Asia Minor (wheat), 
southeastern Asia (rice), and Mesoamerica (maize). In each in- 
stance a grass of weedlike readiness to grow in open bare ground and 
with fruits of good size and nutritious character played the star role. 
Around each was developed a complex of other plants (in America, 
squash, beans, peppers, tomatoes, etc.). It has been suggested, with 
good arguments, that the rich bare surface of household dumps, where 
bones, scraps, seeds, and doubtless manure accumulated, were man’s 
first gardens. The readiness with which tomato and melon vines 
spring up today out of garbage heaps makes this seem reasonable. 
So, too, does the fact that certain weeds of the amaranth and goosefoot 
family not only appear at the beginnings of agriculture in the Old 
and New Worlds both, but are still used as food in places. And, un- 
fortunately, the average single-crop field happens to be the ecological 
equivalent of the pioneer, or weed stage in community development. 
If you will recall that at this stage of succession living organisms have, 
unlike the mature forest or prairie, a minimum stabilizing effect on 
the habitat, you will see why unskillful agriculture has often been its 
own undoing. 
But the important thing is that by domesticating food plants, man 
reduced enormously the space required for sustaining each individual 
by a factor of the order of 500 at least. This meant that people could 
live closer together and in larger groups. It also meant that time was 
available for something beside the constant search for food. Cities 
and leisure for the arts were born, the offspring of agriculture. And 
yet, by a wry twist of fate, we see in history that repeatedly the status 
of the farmer has been lowered and with it the quality of his work 
and the response of the soil. Bad as it is for the offspring to look 
down on his parent, it is far worse for him to destroy that parent. 
Today, as a result of the present population explosion and resultant 
urban spread, choice garden, orchard, and farm land is being con- 
verted into suburbs and lost to the production of food and fiber. But 
we are ahead of our story. 
The human record becomes less vague—more continuous and clear— 
following the invention of agriculture and cities. Technical im- 
provements in the use of minerals, fabrication of tools and utensils, 
the arts of writing, sculpture, and architecture, and civic and military 
organization came on in succession, either by independent invention 
