388 ELEMENTS OF GENERAL SCIENCE 



and in their physical endurance of the rigor of their environ- 

 ment Those whose variations best fit them for their envi- 

 ronment 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. 



FIG. 180. Hybrids from a zebu and Hereford cattle 



The humps and horns in some cases show relationship to the zebu, but the coloring 



and general conformation of body are usually like those of the Hereford. Some 



of the hybrids, like the zebu, are free from the tick, which carries the germs 



that cause Texas fever. Photograph by Robert K. Naboure 



399. Changes in environment. Any change in the envi- 

 ronment affects the living things within it. 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, 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 one hundred 



