44 GENERAL PRINCIPLES OF ZOOLOGY 



evening primrose, CEnothera lamarckiana, besides plants of the true 

 lamarckiana type there also arise a not inconsiderable number of others 

 which are distinguished from the mother plant in noticeable ways and 

 these may be arranged in sharply defined groups of forms which de Vries 

 has named Oe. gigas, Oe. nanella, Oe. scintillans, etc. These groups of 

 forms resemble 'small species' to the extent that, from the first, inter- 

 mediate forms are lacking and, prevented from crossing, they produce only 

 individuals with the same characteristics. These suddenly appearing 

 and sharply marked and hereditary variations de Vries calls Mutations. 

 They have long been known in English as 'sports' and are Darwin's 

 'single variations.' While formerly these were regarded as merely special 

 instances of general variation, de Vries regards them as different. The 

 slight variations with which Darwinism had previously dealt, oscillate- 

 like a pendulum about its point of rest around a central point of greatest 

 frequency and.result in no permanent modifications. Even in domestica- 

 tion it is not possible to advance by the continued selection of the slight 

 differences of these 'fluctuating variations/ and to fix them as inheritances. 

 On the other hand stable forms are produced by mutations and these per- 

 sist if the mutants are better adapted to the conditions of life than is the 

 parent stock. Just so far as the struggle for existence here plays a 

 deciding role the mutation theory is a basis for the selection theory. It 

 differs from Darwinism to the extent of being an 'explosive' method of 

 species formation, by which several kinds, and these relatively constant, 

 suddenly come into existence. For the proper valuation of the mutation 

 theory it will be necessary in the future to separate two questions: (i) 

 Whether the sharp distinctions postulated by de Vries between mutation 

 and variation actually exist; (2) W 7 hether the mutation theory is able to 

 explain the numerous adaptations of organisms to their -environment. 



The mutation theory has shown anew how necessary it is to subject the 

 phenomena of heredity and variation to exact examination. Thus there has 

 developed in the field of botany an experimental direction which has contributed 

 much to the solution of the problems and promises rich results for the future. 

 Here come the studies of the statistics of variation, established by the mathe- 

 maticians Galton and Pearson upon the old foundation of Quetelet, which more 

 recently had received considerable modifications at the hands of the botanist 

 Johannsen. This strives to show by the statistical method whether the charac- 

 ters arising by fluctuating variations can be made hereditary by selection. 



If a study be made of the modifications of a single character in a 'population' 

 (that is a large number of men, animals or plants living under similar conditions, 

 but springing from different ancestors), it is seen especially clearly if char- 

 acters such as length, breadth, weight, number, capable of accurate quantitative 

 statement be chosen that the majority of individuals assume a middle point 

 with regard to the development of this character, and that the variations from 

 this mean, on either the positive or negative side (plus and minus variants) are 



