May 13, 1910] 



SCIENCE 



741 



The general outlines of the mutation-theory 

 are now so familiar to biologists that a state- 

 ment of it here would be superfluous ;' and yet 

 the literature on the subject since the appear- 

 ance of the first part of the German edition, 

 in 1900, has so often shown a lack of clear 

 understanding of the details and scope and 

 claims of the theory, and especially, as the 

 translators state (p. vi), of " a detailed knowl- 

 edge of the contents of ' Die Mutations- 

 theorie,' " that the English translation is most 

 timely and most welcome. Many attempts 

 have evidently been made to debate the ques- 

 tions involved without familiarity with the 

 original work, and hence it may not seem out 

 of place to emphasize here a few cardinal 

 points which are daily becoming more gener- 

 ally correctly understood. 



In the first place, " The special problem 

 which the mutation theory seeks to explain is 

 the manifold diversity of specific forms " (p. 

 45). It has long been recognized that natural 

 selection really explains, not the origin of 

 species, nor even the origin of adaptations, 

 but the elimination of the unfit, and the per- 

 sistence of adaptations; the fact that char- 

 acters, both adaptive and non-adaptive, specific 

 or not specific, must exist before they can be 

 selected was previously well nigh lost sight of. 

 The mutation-theory, then, seeks to account 

 for " the origin of specific characters " (p. 

 211). 



In the second place, " Spontaneous varia- 

 tions are the facts on which this explanation 

 is based " (p. 45), or, " We may express . . . 

 the essence of the mutation theory in the 

 words : ' Species have arisen after the manner 

 of so-called spontaneous variations' " (p. 165) . 

 This marks the fundamental distinction be- 

 tween Darwinism and de Vriesism. For Dar- 

 win, specific characters originated, chiefly if 

 not entirely, by the selection of fluctuating or 

 continuous variations; for de Vries by discon- 

 tinuous (i. e., non-fluctuating) variation only. 

 " In order that species may engage in compe- 



^ Such a statement has previously been given in 

 a review of de Vries's " Species and Varieties : 

 their Origin by Mutation," Plant World. 8: 86, 

 110, 13.5, 159. 1905. 



tition with one another it is evidently an es- 

 sential condition that they should already be 

 in existence; the struggle only decides which 

 of them shall survive and which shall disap- 

 pear (p. 212). 



The struggle which is significant in descent 

 takes place, not between the individuals of the 

 same elementary species, but between the 

 various elementary species themselves (p. 

 211). The former results in acclimatization 

 and the formation of local races (pp. 92- 

 and 211) ; the latter in the elimination of unfit 

 elementary species. "It is moreover evident 

 that this ' elimination of species ' must have 

 weeded out many more than it has preserved. 

 In a word, from the standpoint of the theory 

 of mutation it is clear that the role played by 

 natural selection in the origin of species is a 

 destructive, and not a constructive one." 



One of the commonest misconceptions of 

 saltation is that the difPerence between muta- 

 tion and fluctuating variation is a quantita- 

 tive one; that mutations are large variations. 

 Nothing could be more erroneous. The- 

 amount of the change has nothing to do with 

 the question. " Many mutations are smaller 

 than the differences between extreme vari- 

 ants " (of fluctuating variation) (p. 55). 

 Mutations are characterized first, by being 

 entirely new features, " In contradistinction 

 to fluctuating variations which are merely of 

 a plus or minus character (p. 21-3) ; second, by 

 the abruptness with which they appear, and 

 third, by being transmitted by inheritance 

 without selection. " They arise suddenly and 

 without any obvious cause; they increase and 

 multiply because the new characters are in- 

 herited " (p. 212). "According to the theory 

 of mutation species have not arisen gradually 

 as the result of selection operating for hun- 

 dreds, or thousands, of years, but discontinu- 

 ously by sudden, however small changes " (p. 

 213; italics mine). 



Moreover, de Vries has carefully defined the 

 term species as used by him. This was never 

 done by Darwin. There is evident need to em- 

 phasize this, for in many controversial papers 

 it has been entirely overlooked, the critics 

 meaning one thing by the term, de Vries 



