November 8, 1912] 



SCIENCE 



635 



rather than a text; but perhaps nowhere has 

 he stated more clearly his position with re- 

 spect to the methods of evolution. In this, as 

 rather opposed to the present tendency, he 

 stands with Darwin in his belief that the se- 

 lection of small fluctuating variations has been 

 a more potent factor in evolution than dis- 

 tinct genetic mutations. In other words, he 

 believes in the changeability or modifiability 

 of " unit-characters "■ — we wonder if it is with 

 intent aforethought that he does not say 

 " unit-factors " ? From a purely logical 

 standpoint, however, it is diiSicult to see why 

 those cases which Castle adduces in support 

 of the changeability idea — the polydactyloua 

 race of guinea pigs and the color-pattern 

 series in rats (pp. 56-61) — are not capable of 

 the same explanation as the size of the maize 

 ear (pp. 54^56), in which case Castle accepts 

 East's explanation of a number of indepen- 

 dent factors. Castle states his position suc- 

 cinctly in the last paragraph of Chapter III. 

 (p. 61), which is accordingly worth quoting 

 in full: 



" Accordingly we conclude that unit-char- 

 acters are not unchangeable. They can be 

 modified, and these modifications come about 

 in more than a single way. Occasionally a 

 unit-character is lost altogether or profoundly 

 modified at a single step. This is mutation. 

 But more frequent and more important, prob- 

 ably, are slight, scarcely noticeable modifica- 

 tions of unit-characters that afford a basis for 

 a slow alteration of the race by selection. Mu- 

 tation, then, is true, but it is a haK-truth; 

 selection is the other and equally important 

 half of the truth of evolution, as Darwin saw 

 it and as we see it." 



Leaying aside the restricted use of " selec- 

 tion " in the above paragraph — for the strictest 

 mutationist could scarcely dispense with se- 

 lection as an operative force in evolution — 

 interest centers on the question of the modifi- 

 ability of unit-characters. In Chapter V., 

 " Inheritance in the Higher Plants," Professor 

 East takes up this point, and maintains that 

 the instability of unit-characters does not 

 affect " the truth of the genotype conception 



as a help toward an idea of the process of 

 heredity." If Castle means by "unit-char- 

 acters " the " personal qualities " of Johann- 

 sen, it would seem that East has justification 

 in his opinion that their views are not incom- 

 patible. 



One hears much criticism of Mendelists on 

 the ground that they are too ready to think of 

 "factors" as material things, to regard 

 genetic formulae as representing actualities, 

 and to juggle with increasingly complicated 

 theories which have no secure foundation. 

 While it is true of all theorizing that there is 

 danger in the joy of construction of forget- 

 ting flaws in the basic premises, this is really 

 a criticism of individual method. Every 

 worker should make his own reservations, 

 however much he may try to fit his facts to 

 this or that theory. East, who is perhaps as 

 dyed-in-the-wool a Mendelist as there is in 

 this country, shows commendable caution 

 when he sums up the essentials of Mendelism 

 in the following words (pp. 89, 90) : 



" Stated in fewer words, the essential fea- 

 ture of Mendelianism is the segregation of 

 potential characters in the gamete in a state 

 of apparent purity, and their recombination 

 by the law of chance through random mating. 

 The term ' Mendelian notation ' was there- 

 fore used adrisedly. Mendelian notation is a 

 simple interpretation of certain facts of hered- 

 ity obtained in pedigree cultures. It is a con- 

 venient notation and is used much as the ele- 

 ment symbols are used in chemistry. It makes 

 no difference to analytical chemistry whether 

 or not an atom is a reality, for the law of 

 'Definite and Multiple Proportions' upon 

 which analytical chemistry is hased is still 

 valid. In the same way it makes no differ- 

 ence whether one regards unit-characters as 

 actual units and their segregation as com- 

 plete, or whether one sees in organisms a mu- 

 tual dependence between characters and a 

 quantitative or partial segregation among 

 gametic factors, the notation is useful either 

 way to make clear the facts of heredity as 

 shown, by actual experiment." ' 



Chapter V., from which quotation has just 



' Italics not in original. 



