EVOLUTION AND NATURAL SELECTION 15 



making deductions accordingly. In this way it has been shown 

 by Galton that certain individuals of a species, represented by 

 the mean measurements, survive in the struggle for existence, 

 while those individuals which depart somewhat radically from 

 this type, as represented by the maximum and minimum, are 

 eliminated in the struggle for existence. This law was earlier 

 known to Quetelet^ and now bears the latter's name. 



This law of trial by error has an important bearing on the 

 question of evolution, especially on the origin of species, for 

 it will be remembered that DeVries saj's the origin of species 

 has been brought about by the sudden though slight changes 

 that have been acquired and transmitted to the offspring. 

 According to Quetelet's law, mutations would seemingly not 

 be preserved in the struggle for existence (see under "Varia- 

 tion"). 



The question may be asked : If the tendency to variation is 

 so great, why is it that some simple animals from the first 

 apparently remain in the same condition? Why have not the 

 more highly developed forms supplanted and exterminated 

 the lower ones.' In answer to these questions Darwin says 

 that natural selection does not necessarily include progressive 

 development, it only takes advantage of such variations as 

 arise and are beneficial to each creature under its complex 

 relation of life. He adds: "What advantage would it be 

 to an earthworm or infusorium to be highly organized? If it 

 were no advantage these forms would be left by natural selection 

 unimproved, or but little improved, and might remain for 

 indefinite ages in their present lowly condition. In some cases 

 variations or individual differences of a favorable nature may 

 never have arisen for natural selection to act on and accumulate." 



There must arise something of a selective value, says Romanes, 

 for natural selection to act on. Darwin further says: "In no 

 case has time sufficed for the utmost possible amount of develop- 

 ment. Nevertheless, low and simple forms \\nll long endure 

 if well fitted for their environment, namely, simple condition 

 of life." Darwin's principle explains that selection takes place 

 between individuals, while the mutation theory decides between 

 whole species, and ultimately it is simply the ability for existence 

 - Discovered in 1846. 



