THE SURVIVAL OF THE FITTEST 425 



plants. For the first time the biologist has reached the 

 point where the mechanism of heredity is fairly well 

 understood, and the causes and methods of evolution 

 stand out in a clearer light. 



The Survival of the Fittest. — This term signifies 

 that in the struggle for existence among animals and 

 plants those individuals with the greatest vigor, the 

 keenest senses, the highest powers of speed, or those 

 otherwise best adapted to their surroundings, are the ones 

 which tend to outlast their companions. They accord- 

 ingly are the ones most likely to produce offspring. And 

 these individuals are likely to transmit the same heritable 

 variations to some of their progeny, while the others less 

 fit will die in the struggle. From time to time mutations 

 occur in one or more individuals. Where these are bene- 

 ficial the possessor is more certain to be preserved and to 

 transmit this new character to some of its descendants. 

 If the mutation renders an organism less fit, less able to 

 withstand the rigors of its surroundings, it perishes. 

 Nature thus continually puts a premium on the fittest 

 individuals, while the imperfect ill-adapted ones are van- 

 quished in the combat. The species is thus gradually 

 changed or in other words it evolves "by the incorpo- 

 ration into the race of those mutations that are beneficial 

 to the life and reproduction of the organism.'' 



Application of the Theory. — There is conclusive 

 evidence that the excessively long-necked giraffe camels 

 evolved from a relatively short-necked ancestor (see page 

 411). It is also highly probable that the modern giraffe 

 likewise descended from comparatively short-necked deer- 

 like progenitors. According to the theory of Natural 

 Selection some of the individuals of this ancient stock 

 possessed somewhat longer necks than their fellows. 

 They were thus enabled to browse in their semi-arid sur- 

 roundings on foliage out of reach of the others. This 

 advantage, seemingly insignificant, nevertheless enabled 

 more long-necked than short-necked individuals to come 



