38 Mutability and Individual Variation. 



North British Review.^ This writer tried to prove, by 

 calculations, that the likelihood of single variations main- 

 taining themselves in the struggle for existence or of ulti- 

 mately being victorious in it was very faint. Darwin 

 allowed himself to be convinced by this and says forth- 

 with : / ahvays thought individual differences more im- 

 portant, but I was blind, and thought that single varia- 

 tions might be preserved much oftener than I now see 

 is possible. As the result of this criticism he made many 

 alterations in the subsequent editions of the Origin. 



Finally I shall refer to the conclusion which Darwin 

 derived from his theory of Pangenesis in its relation to 

 these two forms of variability.^ There are two abso- 

 lutely different groups of causes. First, the relative 

 number of the units, their activity, their inactivity, their 

 relative positions and the calling to life of those long in- 

 active. Such changes occur without the units themselves 

 being modified by them. Such changes zvill amply ac- 

 count for much fluctuating variability, that is for that 

 kind of variability which we now call individual, gradual 

 or fluctuating variability. 



The second group of causes includes the direct effect 

 of altered conditions on the organization of the indi- 

 vidual, in which case Darwin supposes the units them- 

 selves to be altered. If the new units have then suffi- 

 ciently multiplied, to be a match for the units already ex- 

 isting they will lead to the elaboration of new structures. 



These quotations convince me that Darwin believed 

 the main branches of his genealogical tree to have arisen 

 bv a modification of his oremniules and that he res^arded 



^ Origin, p. 71. Life and Letters, III, p. 108. 



Animals and Plants under Domestication. 2d ed., 1875, II, 

 P- 390. 



