NATURAL SELECTION AND EVOLUTION 565 



tion to the next, and, if the environment remains the same, the 

 modifications become so pronounced as to characterize new 

 species. 



Like the preceding theory, Lamarck's explanation assumes the 

 inheritance of acquired characters and the results of more recent 

 investigations do not support this assumption. Since his expla- 

 nation was proposed, it has been demonstrated that modifications 

 due to environment are seldom if ever inheritable. This means 

 that it does not matter how much a wading fowl stretches its 

 legs or a giraffe stretches its neck as none of this added length is 

 transmitted to the next generation. The modifications of each 

 generation disappear with that generation. Consequently there 

 is no accumulation of effects through successive generations as 

 Lamarck assumed. 



Natural selection and evolution. Evolution by natural selec- 

 tion is the explanation proposed by Charles Darwin. Among 

 both plants and animals in nature there is competition between 

 individuals for space, food, light, etc., and in this struggle many 

 individuals perish. The stronger plants shade and crowd out 

 the weaker ones, and the strongest and fleetest animals are not 

 so likely to perish as those less able to take care of themselves. 

 As a result of the intense struggle between individuals for exist- 

 ence, the individuals poorly equipped to meet the conditions 

 imposed upon them by their surroundings are rapidly eliminated, 

 and through their destruction the better equipped individuals 

 have more opportunity to thrive and multiply. Thus in nature 

 there is a sifting process which tends to preserve only those 

 individuals that are so favorably constituted as to win out in the 

 struggle. This sifting process in nature is what Darwin meant 

 by natural selection. The idea of natural selection involves a 

 number of subordinate ideas variation, struggle for existence, 

 survival of the fittest, and heredity. 



Variation. Variations, that is the differences in structure 

 and function between the individuals of any group of plants or 

 animals, afford a starting place for natural selection. Variations 

 in certain directions equip individuals to win in the struggle, 

 while variations in other directions doom individuals to perish. 

 If plants vary so as to have a deep root system and an epidermis 

 that protects against excessive transpiration, they can endure 

 drought. If plants vary so as to have a shallow root system and 



