No. 543] FACTORS AND METHODS OF EVOLUTION 133 



than we had dreamed. Sometimes a prominent charac- 

 ter is represented by a single determiner like (perhaps) 

 roseness of the comb of the fowl ; but in most cases there 

 is a multiplicity of factors, as in human hair and skin 

 pigments, in the yellow of mice, in shank feathering of 

 fowls and in seed coat-color of oats. In consequence of 

 the fact of this multiplicity of factors and of the fact 

 that a variable number may be present in different cases 

 the adult character appears in numerous grades of de- 

 velopment. 



Indeed, the gradation of characters is, in these cases, 

 such that one has to recognize that discontinuous varia- 

 tion passes over into continuous variation, in the sense 

 that 40, 41, 42 form a continuous series, if not in the 

 sense that x, x + dx, x -f 2dx, etc., do. If a desire for 

 uniformity leads us to conclude that all variations in the 

 germ-plasm are discontinuous at least we see in many of 

 these variations sufficient justification for the continuity 

 hypothesis of the old-fashioned selectionist. The new 

 light that has been thrown on the subject is the certainty 

 of discontinuity in most cases and apparent continuity 

 only in the limiting case. The reason why the old con- 

 tinuity hypothesis was for so long a time accepted was 

 that we had underestimated the fineness and the multi- 

 plicity of the units of inheritance. 



Third, experimental work has thrown a new light on 

 the process of selection. It is clear that Darwin confused 

 under this term two ideas that we now sharply separate ; 

 namely, the selection of the most favorable individuals 

 and the selection of the most favorable blood, race, strain 

 or pure line (biotype, Johannsen). In so far as not the 

 soma but the germ-plasm is the proper basis of selection 

 it is clear that the favorable biotype is what we should 

 seek for to make most rapid advance. By this means 

 Pearl has increased the fecundity of his poultry: thus, 

 probably Castle has extended step by step the color 

 pattern of rats ; thus poultry fanciers have improved the 

 color pattern of Barred piymouth Kocks; thus I have 

 gained a syndactyl race of fowl. 



