PARENT AND OFFSPRING 295 



of their coats of fur. They vary also in their fleetness in pur- 

 suit of food, in their ability to escape enemies, and in their 

 physical endurance of the rigor of their environment. Those 

 whose variations best fit them for their environment are usually 

 the ones that live. The others die of hunger or of physical 

 hardships, or by being caught and killed by their enemies. In 

 succeeding generations, year after year, variations occur, and 

 the animals that can endure the conditions of their environment 

 are the ones that live. 



321. Changes in environment. Any change in the environ- 

 ment affects the living things within it. Indeed, the changes 

 in plant and animal life are in themselves fundamental changes 

 in the environment, for plants and animals constitute a most 

 important part of the environment. For example, when wolves, 

 foxes, and rabbits occur in undisturbed nature in their usual 

 numbers, since the wolves and foxes may feed upon rabbits 

 when they can catch them, only those rabbits can live which 

 can escape from, the wolves and foxes (and other enemies). 

 If we suppose that there is an epidemic of disease which kills 

 off all the wolves and foxes over a territory of 100 square 

 miles, the environment for rabbits is greatly altered. Under 

 such conditions many rabbits can live which could not live if 

 the wolves and foxes were present in their usual numbers. 

 It is evident that, with all the rabbits that are produced, 

 with their many slight individual variations, normally only 

 those can live that can escape from the wolves and foxes; 

 but with wolves and foxes absent there is a much greater 

 breadth of variation among rabbits that may now persist. 

 Thus, the changed environment, so far as these factors 

 are concerned, is favorable to the life of a larger number 

 of rabbits. 



But what do rabbits live upon ? In one region in which 

 rabbits live they eat in winter large quantities of buds from 

 low shrubs (as young trees of sassafras, hawthorn, etc.). An 

 unusual increase of rabbits thus greatly affects the environment 



