BIOLOGICAL EVOLUTION 7 1 



the novel character of the mutant, if we may use the word, is independently 

 heritable and does not blend; it can be grafted intact onto another stock, 

 or it can be dropped out as such. Again, mutations are what may be called 

 qualitative, as contrasted with fluctuations which are quantitative.^ 



Having established the principle of mutation or discontinuous 

 variation, which Darwin denied, De Vries raises several questions 

 which have not as yet been answered: Is mutability a temporary 

 or permanent condition ? If temporary what is its cause and 

 how is the quality lost ? How may mutations be induced or 

 controlled ? ^ By way of answer he suggests several working 

 hypotheses: Mutability may be a periodic phenomenon; It may 

 be permanent in the main line of development with loss of muta- 

 bility in lines branching from the main trimk of the genealogic 

 tree; One primary mutation giving rise to one or more unit 

 characters, initiates a multitude of minor changes.^ He thinks 

 that besides these periodic seasons of mutation there are stray 

 mutations that are potent also in the evolutionary process. 



Our author discusses at length the possibilities and limitations 

 of artificial selection,* shows how inconstant are races improved 

 thus and how prone they are to revert to the species-type,^ also 

 how uncertain is selection based on visible qualities,^ holding that 

 the study of pedigree is of first importance. 



JOHANN GrEGOR MeNDEL (1822-1884) 



Independent Unit Characters 



Although Mendel's great work antedated both that of Weis- 

 mann and De Vries, it was entirely lost to the scientific world for 

 nearly forty years and not brought to Hght till 1900 when within 

 a few months De Vries, Correns and Tschermak working inde- 

 pendently pubHshed papers setting forth the substance of his 

 discoveries. 



1 Darwinism and Human Life, p. 107. 2 species and Varieties, pp. 690 f. 



' " At the beginning of each series of analogous mutations there must have been 

 one greater and more intrinsic mutation, which opened the possibility to all its 

 successors. This was the origination of the new character itself, and it is easily 

 seen that this incipient change is to be considered as the real one. AU others are 

 only its visible expressions," ibid., p. 703. 



* Ihid., pp. 805 f. ^ Ihid., pp. 770 f. ^ Ihid., pp. 810 £. 



