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SCIENCE 



[N. S. Vol. XXXI. No. 802 



and his followers quite another. Therefore, 

 it is of prime importance to keep in mind the 

 fact that with the species of the systematist 

 the mutation theory has primarily nothing to 

 do; and this fact is specifically stated. Thus, 

 on page 165 : 



In order to be qualified to discuss this question 

 we must first of all make quite sure what we 

 understand by the term "species" and, more 

 important still, we must form a clear idea as to 

 which forms we are going to regard as the units 

 of the natural system. For it is only in the case 

 of the real units of the system that we can hope 

 to obtain experimental proof of their common 

 descent: the theory of Descent as applied to 

 groups of these units is, and will probably always 

 remain, a comparative science. 



And again, on page 168, it is insisted that : 

 The ordinary Linnean species of the systematist 

 . . . are artificial groups whose limits can be 

 altered by the personal taste of any systematist 

 and are indeed, as a matter of fact, much too often 

 so altered. The origin of such a species, like 

 that of a genus, is a historical occurrence and it 

 can neither he repeated experimentally, nor can 

 the whole process be observed. (Italics mine.) 

 The object of an experimental treatment of these 

 phenomena must assuredly be to make the origin 

 of the units which really exist in nature the 

 subject of experiment and observation. We must 

 deal not ivith the origin of the groups made by 

 the systematist, but with those which are pre- 

 sented by nature. (Italics mine.) 



Thus the long-standing argument against 

 organic evolution, that no one ever observed 

 the origin of a species (of the systematist), is 

 frankly acknowledged, but clearly shown to 

 have no special significance for the theory of 

 Descent. The elementary species, "those 

 which are presented by nature," " do arise in 

 the garden and in agricultural practise" (p. 

 169). This is no longer a debatable question. 

 It is absolutely essential clearly to under- 

 stand the above points in order to discuss the 

 mutation theory, or to undertake investiga- 

 tions in experimental evolution. It is worth 

 repeating that, " The solution of this prob- 

 lem must ... be sought among the facts them- 

 selves " (p. 462). As to whether mutations are 

 realities or figments of imagination, no one is 

 competent to hold an opinion who has never 



carried through a series of pedigreed cultures, 

 or observed the results of such work. 



Contrary to the implication of so many of 

 his adverse critics, the author has tried to 

 keep as close to Darwinian theory as the facts 

 would permit. Throughout the book (cf., 

 e. g., pp. 51, 87, 198, 205) there has been a 

 constant endeavor to give fuU credit to the 

 great master, and to present the mutation 

 theory, not as an alternative to natural selec- 

 tion, but as a supplementary hypothesis. Not 

 Darwinism as a whole, but only the formerly 

 baffling and embarrassing difficulties of Dar- 

 winism are explained away. 



A perusal of the book before us recalls a list 

 of many important and positive contributions 

 rendered by the author through this and his 

 numerous other related writings. 



1. The application of the experimental 

 method to the question of the origin of specific 

 characters. This is justly regarded by de 

 Vries as " the most important general result " 

 of his work (p. 497). 



2. The development of the method of pedi- 

 gree-culture. 



3. Making clear the fundamental distinc- 

 tion between fluctuation and saltation (muta- 

 tion), and showing its prime importance. 

 Just as Darwin was not the first to suggest 

 natural selection, so de Vries was not, by any 

 means, the first to draw the distinction be- 

 tween continuous and discontinuous varia- 

 tions (cf. p. 63) ; but, as was the case with 

 Darwin, he stated the distinction so clearly, 

 demonstrated it so convincingly, established it 

 so firmly upon a wide range of facts, as to 

 bring it into the focus of attention of all biol- 

 ogists, and compel them to reckon with it in 

 all subsequent work. 



4. Eecognition of character units and of 

 unit characters, and their significance; a 

 principle fully developed in his " Intracellu- 

 lare Pangenesis." 



5. Actual observation of the origin of new 

 plant-forms of the value of elementary species. 



6. A resurvey of the vast literature of horti- 

 culture and experimental breeding, with a new 

 interpretation of the facts in the light of a 

 new working hypothesis (mutation). 



