384 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1958 
jumpers around a gold strike. They encroach on each other’s terri- 
tory, and between fighting over the abundance of food and squabbling 
over nesting space they fail to raise the normal broods to maturity and 
their population starts to decline before food runs out. 
It seems reasonable to suppose that man’s progress over the earth 
was a matter of families or small groups living together for protection 
and cooperation, but spreading out and away from too close competi- 
tion with other groups. If so, it affords a beautiful illustration of 
the principle that “competition with other species has a centripetal 
effect, driving each back into its own territory, while competition 
within a species is a centrifugal force, causing the species to scatter.” 
Man has always had to reckon with both kinds of competition. In 
the beginning, and until fire and weapons were available, the now 
extinct Pleistocene mammals doubtless kept him in bounds, never far 
from caves, perhaps treetops, and other citadels. The effect was 
clearly centripetal. But the recent finding of extinct mammal re- 
mains in association with stone weapons shows clearly that he had 
discovered another refuge—the devices of his culture. He needed no 
longer to defend himself within a certain stronghold, but within a 
way of life. Modern man, having pretty well disposed of his larger 
competitors, now finds himself pitted against rodents, insects, and 
fungi for control of food and other organic materials. He has also 
long been engaged in a confused competition for space with forests, 
of which more later. 
It is a safe guess that as he overcame pressure from other species, 
that from his own increased, thus setting the weaker and more venture- 
some both on the road. What he accomplished, on foot, with only the 
dog, fire, and stone tools to aid him, is one of the most remarkable of 
human achievements. It happened during the accordionhke action of 
the glacial age, when climate compressed the habitable regions as the 
ice advanced, releasing them to expand as it retreated. Through this 
experience man learned to adjust himself to new vicissitudes of 
temperature and to utilize new forms of animal life from colder 
regions. The warm interglacials opened his path to the far north. 
Our observations in Greenland show us that glaciers can form only 
where both cold and moisture are present.. There are and were many 
dry cold areas where early man, by now accustomed to the cold, could 
hang on in spite of the relentless advance of the continental ice masses. 
This had spectacular consequences. As the ice grew from the 
moisture that fed it, the sea level was correspondingly lowered, until 
it was some 300 feet below where it is now. Then, of course, sea 
bottom was exposed in places, creating migration pathways. These 
served man as they had served various Old World animals for an 
entry into the New World and allowed the horse, evolved in the 
Americas, to move westward into Asia. Just how and when these 
