i] INTRODUCTORY 3 



plants and animals be supposed to owe their existence 

 to the gradual operation of this factor working upon 

 small variations ? 



Of recent years there has arisen a school of biolo- 

 gists to whom the terms mutationist and Mendelian are 

 frequently applied. Influenced by the writings of 

 Bateson and de Vries, and by the experimental results 

 that have flowed from Mendel's discovery in heredity, 

 they have come to regard the process of evolution as 

 a discontinuous one. The new character that differ- 

 entiates one variety from another arises suddenly as 

 a sport or mutation, not by the gradual accretion of 

 a vast number of intermediate forms. The white 

 flowered plant has arisen suddenly from the blue, or 

 the dwarf plant from the tall, and intermediates 

 between them need never have existed. The ultimate 

 fate of the new form that has arisen through causes 

 yet unknown may depend upon natural selection. 

 If better endowed than the parent form in the struggle 

 for existence it may through natural selection come to 

 supplant it. If worse endowed natural selection will 

 probably see to its elimination. But if, as may quite 

 possibly happen, it is neither better nor worse adapted 

 than the form from which it sprang, then there would 

 seem to be no reason for natural selection having 

 anything to do with the relation of the new form 

 to its parent. 



Between the older and the newer or mutationist 

 point of view an outstanding difference is the role 

 ascribed to natural selection. On the one view it 



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