76 evolution: the ages and tomorrow 



steady population. At irregular intervals the rate increases to 

 three or four litters during the summer with sometimes ten 

 or more to the litter, and the individuals of the first litter 

 are reproducing before fall. The whole population explodes. 

 There even seems to be at these times a greater immunity to 

 disease. Migration starts and many move out of their mead- 

 ows and into the surrounding forests. The high reproduc- 

 tivity is repeated a second summer. Then the great hordes 

 which have been produced respond to an irresistible urge and 

 begin to move westward, even though this carries them over 

 barren wastes, exposed to death from all sorts of predatory 

 birds, to drowning in the rivers and fiords, and to final ex- 

 tinction in the sea. They travel in parallel rows, and in some 

 of the greatest migrations on record have literally choked 

 the Norwegian streams with their bodies. 



This case of explosive reproduction is not unique in the 

 kingdom of life. The tendency toward overpopulation is a 

 characteristic of all organisms, but it is ordinarily checked by 

 physical and biotic factors before the disastrous consequences 

 observed in the lemming. In Chapter 14 the pressure of over- 

 population will be examined more fully, since, according to 

 most authorities, it constitutes a dire threat to the future of 

 man's societies. 



Although the advantages of grouping are sometimes 

 obscure, there are many instances in nature where the or- 

 ganism gains greatly by social organization— warmth, pro- 

 tection, food, companionship, security, enjoyment, and 

 understanding. Quails gain warmth on cold nights by their 

 habit of huddling together, sitting wing to wing in a circle, 

 some birds perched on the backs of others. Our western 

 pronghorn gains protection by grouping a dozen or more 

 individuals and standing against wolves, the males fighting 

 them oif . If there are too few in the group, they scatter and 

 one is usually killed. Fishes, birds, and mammals sometimes 

 show a very strong territorial sense, and gain control over 

 the food supply by this kind of social organization. All ani- 



