BIOLOGICAL EVOLUTION 71 
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 trunk 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. 
Jouann 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 tc light till 1900 when within 
a few months De Vries, Correns and Tschermak working inde- 
pendently published papers setting forth the substance of his 
discoveries. 
1 Darwinism and Human Life, p. 107. 2 Species and Varieties, pp. 690 f. 
3 “ 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. All others are 
only its visible expressions,”’ ibid., p. 703. 
4 Ibid., pp. 805 f. 5 Ibid., pp. 770. 6 Tbid., pp. 810 £. 
