472 ELEMENTARY BIOLOGY 



With our present knowledge of physiology and heredity, 

 most of Lamarck's theory is seen to be unsound. Notwith- 

 standing all the evidence to the contrary, however, many people 

 still believe that evolution (and human progress) takes place 

 through the accumulation of the results of experience in the 

 course of generations. 



496. Selection theory. The theory of natural selection is 

 associated with the name of Charles Darwin (1809-1882), 

 but it was also formulated independently by Alfred Russel 

 Wallace (1823-1913) and Herbert Spencer (1820-1903). 

 This theory is that animal and plant species evolve through 

 the selective action of the environment, which kills off those 

 individuals in each generation that are least adapted to the 

 conditions of life, a process . resulting in the "survival of 

 the fittest." 



The theory rests on the fact of variation (see pp. 437 ff.) 

 and of overpopulation that is, the fact that more individuals 

 (eggs, seeds, spores, etc.) are born than can possibly reach 

 maturity and reproduce themselves. It assumes that individual 

 differences may be inherited, whereas the evidence shows that 

 only certain kinds of differences are transmitted to offspring. 

 The theory assumes that the agencies which kill off so large 

 a proportion of each generation are selective, that is, really 

 act upon important individual differences, whereas we know 

 that at every stage of development plants and animals are 

 killed off by agencies and forces and accidents that make no 

 discrimination whatever between the fit and the unfit. 



Many studies and experiments in recent years have thrown 

 doubts upon the theory of natural selection. It does not 

 account for the origin of new characters, and we know that 

 there is a limit to the improvement that may be brought about 

 by artificial selection. There is at least this much truth in the 

 doctrine, however : plants and animals that are unsuited to the 

 conditions about them, for whatever causes, are not likely to 

 leave similar progeny. Fitness is a requisite for all life. 



