THE ECOLOGY OF MAN—SEARS 383 
binations. It is thus possible to have spoken symbols enough to com- 
municate our experiences and thoughts to others. In this way knowl- 
edge can be exchanged and accumulated, and handed down through 
the generations, an immense advantage over the old do-it-yourself, 
learn-the-hard-way system that prevails so widely in nature. The 
upshot was a true biological revolution, the birth of culture, carrying 
with it the awful gift of conscious responsibility. 
Man’s unique power to manipulate things and accumulate experience 
presently enabled him to break through the barriers of temperature, 
aridity, space, seas, and mountains that have always restricted other 
species to specific habitats within a limited range. With the cultural 
devices of fire, clothing, shelter, and tools he was able to do what no 
other organism could do without changing its original character. 
Cultural change was, for the first time, substituted for biological 
evolution as a means of adapting an organism to new habitats in a 
widening range that eventually came to include the whole earth. 
There is not much doubt that man is an Old World product and 
great interest centers just now on research in Africa where very 
early humanoid primate material keeps turning up. We reason that at 
first man was a gatherer, eating what he could find and digest. To 
judge by the thoroughness with which eatables, poisons, and drugs 
were known before the days of modern science, man must have tried 
nearly everythine—plant, animal, and inanimate—that he found. We 
can be fairly certain of one thing—early man had to have considerable 
space to pick over if he survived. Even when he had developed tools 
and weapons for hunting it is estimated that some 4 square miles per 
person were required to sustain a family. Until the domestication 
of cereals and other food plants mankind was spread thin. The State 
of Ohio, an area of about 40,000 square miles and now holding 7 to 9 
million people, is estimated to have been populated by only 12,000 to 
15,000 Indians even when some agriculture was combined with hunt- 
ing, gathering, and fishing in that very fertile land. 
Now spacing is a crucial and constant problem with living organ- 
isms, plants as well as animals. It has a twofold aspect. We might 
say that it involves both contact and elbowroom. The individual 
must not become completely out of touch with his own kind. No 
matter how unsocial the species, there must be communication for 
reproduction at the very least. Beyond that one finds all degrees 
up to the highly social and cooperative. 
On the other hand the most dangerous potential competition of a 
species is that with precisely the same needs, namely its own kind. 
The sweet song of the nesting robin is more often a No Trespass sign 
than a love call. The jaeger, an Alaskan bird of prey that feeds on 
rodents, gets along very well when food is scarce. But when the 
rodent population builds up, the jaegers behave like a lot of claim 
