1 8 ORGANIC EVOLUTION 



likelihood of leaving descendants than would one which 



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laid but few eggs, other things of course being equal. 

 Most animals and plants adopt both methods, being very 

 prolific and being well adapted to their environment. 



Now, evolution is brought about by the occurrence, 

 among the individuals of a species, of certain ones which 

 are better fitted for the life they are to live than are the 

 others of the species ; by the survival of these favored 

 ones; and by the transmission of their valuable qualities 

 from parent to offspring generation after generation. The 

 appearance of the desirable quality is an example of varia- 

 tion : the survival of those individuals which possess these 

 qualities is secured by natural selection : and the perpetua- 

 tion of the useful qualities is secured by heredity. It would 

 seem necessary that, given these three factors, variation, natu- 

 ral selection, and heredity, evolution should be the result. 

 Later we will take up some of the most frequently urged 

 objections and see if there is any flaw in this argument. 



Mutation. 



Recently De Vries has shown by a very careful and 

 very extensive series of observations of wild and cultivated 

 plants, chiefly of the species G^uot/icra laiuarckiana, that 

 there may be two somewhat different types of variation 



(1) "fluctuating variation," by which a species varies in 

 greater or less decree and in almost all directions, and 



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(2) " mutation," by which the whole character of the species 

 is changed and a new species established at one leap. The 

 new species thus established by mutation will show, as did 

 the former species, a series of fluctuating variations. Every 

 species of animal and plant with its numerous fluctuating 



