392 EVOLUTION AND GENETICS 



Accidental Destruction. Belief in the positive or negative value 

 of variation does not mean that they are absolute determiners 

 of individual success in all cases. Undoubtedly many well endowed 

 organisms are destroyed by chance, both through contact with 

 other organisms and through natural catastrophes. A volcanic 

 eruption or a great flood is an extreme condition which cannot be 

 met directly, but must be avoided if possible, and to many organ- 

 isms the chance for safety is not given. In general, however, 

 conditions of the environment are subject only to gradual fluctua- 

 tion with which normal variations may be an adequate basis of 

 adjustment. 



Change of Habits and Migration. It is necessary to recognize 

 the fact that overproduction and consequent crowding, in addition 

 to determining the survivors in a given region, may result in 

 individuals utilizing their fitness for other conditions of the imme- 

 diate environment. If a species depends normally upon one kind 

 of plant for food, as is often the case among the insects, crowding 

 may result in the shifting to a new food plant of the individuals 

 which are less successful in securing a share of the available supply. 

 Shortage of food arising from any other cause might have a similar 

 effect. This response would depend, of course, upon the ability 

 of the animal to thrive upon the new diet. On the other hand, 

 such conditions might result in the species spreading through 

 gradual migration into adjacent regions, whereupon the factor of 

 spatial isolation might play a part in its development. During a 

 severe winter in the vicinity of Sioux City, Iowa, the writer once 

 witnessed a minor case of migration induced by hunger, although 

 it was, of course, temporary. Heavy snowfall, beginning early 

 in the winter, had kept the prairie chickens from their normal 

 supply of food. Under ordinary conditions it was unusual to see 

 more than two or three of these birds in a season, but during 

 that winter they had left their range, and on one day sixty-seven 

 were seen within a distance of a few miles. Hunters also reported 

 their abundance, but said that the birds shot were very thin. 



Summary of Natural Selection. Natural selection makes use 

 of all of the foregoing factors. The theory may be summarized 

 as follows: 



1. All organisms produce more young than can survive. 



2. Struggle for existence results from overproduction. 



