VARIATION 



431 



of the determinants in question, which gradually decreases in 

 course of time. The cause of the regression of a determinant 

 is to be looked for in insufficient nutrition, — which condition 

 may occur in a determinant quite as likely as that of more 

 abundant nutrition. If this occurs in the majority of the ids 

 either directly, or in consequence of the accumulation produced 

 by amphimixis, the character controlled by these determinants 

 becomes regressive in that particular individual. If, however, it 

 no longer has a physiological value, it becomes slowly but surely 

 suppressed by panmixia in an ever-increasing number of indi- 

 viduals until it disappears. Specific characters which have long 

 been unrecognisable externally, instead of disappearing entirely, 

 may still be retained in individual ids in the form of incompletely- 

 degenerated determinants ; and, as already mentioned, these 

 may cause the reappearance of a character under particularly 

 favourable circumstances. 



3. Siinufiajy of Sections i and 2, and Conclusions 



The above remarks may be briefly summarised as follows : — 

 The origin of a variation is equally independent of selection 

 and of amphimixis, and is due to the constant recurrence of 

 slight inequalities of nutrition in the germ-plasm which aftect 

 every determinant in one way or another, and differ even in 

 the same germ-plasm, — not only in different individuals but 

 also in different regions. These variations are at first infini- 

 tesimal, but may accumulate ; and, in fact, they must do so 

 when the modified conditions of nutrition which gave rise to 

 them have lasted for several generations. In this way deviations 

 may occur in the structure of single determinants or of groups of 

 them, — never, perhaps, in all ids at once, but at any rate in sev- 

 eral or even many of them. A doubling of certain determinants 

 of the germ-plasm may originate in the same way. The proc- 

 ess of amphimixis has an important share in the accumulation 

 of these modified determinants, for it may raise the minority pre- 

 viously existing in the two parents to a majority by combining 

 their halved germ-plasms. Then, and then only, does selection 

 begin to take place. 



The extreme importance of sexual reproduction in processes of 

 transformation only becomes evident, however, when we realise 

 that adaptations are usually concerned with several variations 

 at the same time, and rarely or never arise in connection 



