Two Types of Variation 71 



In his criticism of miscellaneous objections brought forward 

 against the theory of natural selection after the publication of the 

 first edition of The Origin of Species, Darwin stated his view on 

 this point very clearly: — "The doctrine of natural selection or tin- 

 survival of the fittest, which implies that when variations or individual 

 differences of a beneficial nature happen to arise, these will be 

 preserved 1 ." In this sentence the words "hapjjen to arise" appear 

 to me of prominent significance. They are evidently due to the 

 same general conception which prevailed in Darwin's Pangenesis 

 hypothesis 2 . 



A distinction is indicated between ordinary fluctuations which are 

 always present, and such variations as " happen to arise " from time 

 to time 3 . The latter afford the material for natural selection to act 

 upon on the broad lines of organic development, but the first do 

 not Fortuitous variations are the species-producing kind, which the 

 theory requires; continuous fluctuations constitute, in this respect, 

 a useless type. 



Of late, the study of variability has returned to the recognition 

 of this distinction. Darwin's variations, which from time to time 

 happen to arise, are mutations, the opposite type being commonly 

 designed fluctuations. A large mass of facts, collected during the 

 last few decades, has confirmed this view, which in Darwin's 

 time could only be expressed with much reserve, and everyone 



1 Origin of Species (6th edit.), p. 169, 1882. 



2 Cf. de Vries, Intracellulare Pangenesis, p. 73, Jena, 1889, and Die Miitationsthcorie, 

 i. p. 63. Leipzig, 1901. 



3 [I think it right to point out that the interpretation of this passage from the Origin 

 by Professor de Vries is not accepted as correct either by Mr Francis Darwin or by myself. 

 We do not believe that Darwin intended to draw any distinction between two typ?* of 

 variation; the words "when variations or individual differences of a beneficial nature 

 happen to arise" are not in our opinion meant to imply a distinction between ordinary 

 fluctuations and variations which "happen to arise," but we believe that "or" is here 

 used in the sense of alias. With the permission of Professor de Vries, the following 

 extract is quoted from a letter in which he replied to the objection raised to his reading 

 of the passage in question : 



"As to your remarks on the passage on page 6, I agree that it is now impossible to 

 see clearly how far Darwin went in his distinction of the different kinds of variability. 

 Distinctions were only dimly guessed at by him. But in our endeavour to arrive at a true 

 conception of his view I think that the chapter on Pangenesis should be our leading guide, 

 and that we should try to interpret the more difficult passages by that chapter. A careful 

 and often repeated study of the Pangenesis hypothesis has convinced me that Darwin, 

 when he wrote that chapter, was well aware that ordinary variability has nothing to do 

 with evolution, but that other kinds of variation were necessary. In some chapters he 

 comes nearer to a clear distinction than in others. To my mind the expression 'happen to 

 arise' is the sharpest indication of his inclining in this direction. I am quite convinced 

 that numerous expressions in his book become much clearer when looked at in this way." 



The statement in this passage that "Darwin was well aware that ordinary variability 

 has nothing to do with evolution, but that other kinds of variation were necessary " is 

 contradicted by many passages in the Origin. A. C. 8.] 



