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AN INTRODUCTION TO ZOOLOGY 



one species originates from another has not been adequately 

 explained. The current belief assumes that species are slowly 

 changed into new types. In contradiction to this conception the 

 theory of mutation assumes that new species and varieties are 

 produced from existing forms by sudden leaps. These may arise 

 simultaneously and in groups, or separately at more or less widely 

 distributed periods " (272). 



While directly opposed to Darwin's theory of the origin of spe- 

 cies in regard to the kinds of variations selected, de Vries gives 

 Darwin credit for the discovery of the " great principle which 

 rules the evolution of organisms. It is the principle of natural 

 selection. It is the sifting out of all organisms of minor worth 

 through the struggle for life. It is only a sieve, and not a force 

 of nature, no direct cause of improvement, as many of Dar- 

 win's adversaries, and unfortunately many of his followers also, 

 have so often asserted. It is only a sieve, which decides which 

 is to live, and what is to die. . . . By this means natural 

 selection is the one directing cause of the broad lines of evolution " 

 (272, p. 6). De Vries claims, however, that " species and varie- 

 ties have originated by mutation, and are, at present, not known 

 to originate in another way." 



Two sorts of species are recognized by de Vries, the system- 

 atic species of the systematist, and elementary species, which are 

 more numerous and are described as " any form which remains 

 constant and distinct from its allies " (272, p. 10). Elementary 

 species have unit characters which are indivisible and distinct 

 in inheritance. All organisms are not mutable to the same degree, 

 and to observe the origin of species, "It is only necessary to 

 have a plant in a mutable condition " (272, p. 26). 



Since the publication of de Vries' results biologists have exam- 

 ined the data from a large number of experiments, but only a 

 comparatively small amount of proof is available to support the 

 theory from the zoological side. 



Perhaps the best attitude to take toward the three theories 

 outlined in the last few pages is that of Professor Whitman, who 



