262 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



more clear-sighted than many of his followers. 1^11 that Darwin 

 needed for his purpose was proof of variations that are heritable, and 

 these are found in mutations, be they large or small) 



(Just as Mendelism has to do with the mechanism and not the fact 

 of heredity, so the mutation theory deals with the nature and not the 

 fact of variations. / Neither, in my opinion, has any implication that 

 is antagonistic to the theory of natural selection. 



The statement has been made that natural selection "originates 

 nothing" because it does not explain the origin of variations. I 

 must confess scant patience with this point of view. As well say 

 that the sculptor does not make the statue because he does not 

 manufacture the marble or his chisel; or that the worker in mosaic 

 originates nothing because he does not make the bits of stone which 

 he assembles in his design! 



(The material corresponding to the bits of stone in the mosaic is 

 furnished by heredity and variation, and its quantity by geometrical 

 ratio of increase^ [Natural selection acts in selecting and putting 

 together this material in the formation of new species. Thus, in a 

 true sense, it seems evident that something new has appeared — 

 something that is, but was not!) 



Another favorite figure, introduced I believe by De Vries, is 

 "Natural selection acts only as a sieve" determining which forms 

 shall be retained and which shall be discarded. This also seems to 

 me to fall short of a complete statement of the truth. If the material 

 subjected to the sifting process be regarded as changing with each 

 generation by the addition of variations, or mutations if you prefer, 

 some of which are favorable to a nicer adjustment of the species to its 

 environment, the figure would be more nearly correct. To make it 

 complete, however, the mesh of the sieve must change from generation 

 to generation so that a quantitative variation which would be preserved 

 in one generation would be discarded in a later one. But in this case 

 natural selection would do more than a sieve could do. It would 

 combine a number of favorable variations in the production of 

 something new, a new species! 



4n conclusion it seems to me that we are justified in maintaining 

 that Mendelism and the mutation theory, while forming the basis of 

 the most brilliant and important advances in biological knowledge of 

 the last half-century, have neither weakened nor supplanted the 

 Darwinian conception of the "Origin of species by means of Natural 

 Selection.^ 



