552 THE CAUSES OF EVOLUTION 



like wolves hunting herbivora like deer. In certain species, like 

 the fur seal, there may be a literal struggle among the males at the 

 breeding season for possession of the females. Finally, there is the 

 struggle with the physical factors of environment, hke heat 

 and cold, moisture and dryness, changing chmate, and changing 

 conditions upon the land surface or an ocean bottom swept by 

 varying currents. Hence, J. Arthur Thomson has characterized 

 the struggle as one that is three-fold, for food, for mates, and 

 against fate. Such a struggle, in one or more of its aspects, is 

 ever-recurring for all hfe, although it is intermittent and may not 

 act for considerable periods in the hfe of any individual. Because 

 it thus seems to be a necessary consequence of the limitation of 

 numbers in the face of great overproduction, the struggle for 

 existence may be placed in the column of proved facts as the 

 chart indicates. 



Variation and Heredihj. — The modern concept of variation 

 and heredity was discussed in the chapter on Genetics. It is 

 sufficient to repeat that variations are the differences between 

 individuals of a species, and that heredity is the resemblance in 

 these characters as they occur from generation to generation. 

 Some variations are inherited, others are not. The theory of 

 selection, of course, deals only with such variations as may be 

 inherited, since only these can play a part in evolutionary changes. 

 It is important for us, therefore, to discover the way in which the 

 struggle for existence acts upon variations that are inherited. 



Survival of the Fittest. — Giv en inherited variations of many 

 sorts, some will be of value to the individual in its struggle for life. 

 If the members of a species of plants differ in their ability to resist 

 frost, those that are most resistant will survive low temperatures. 

 If this variation is inherited, the next generation will be composed 

 of more resistant individuals. If such a process of natural selec- 

 tion is repeated generation after generation, sufficient resistance to 

 meet the exigencies of hfe will become a part of the permanent 

 inheritance of the species. Thus, evolution might occur by change 

 of this particular feature of the organism, in such a manner as to 

 suit a changing climate or to enable the species to extend its range 

 northward. If rabbits differ in quickness of start and in sustained 

 speed, and if these differences are important, the quickest and 

 speediest in each generation will tend to survive. If these varia- 

 tions are inherited, a race having greater speed will gradually come 



