GERMINAL SELECTION 465 



as far as that structure is concerned ; a state of panmixia, as 

 it is called, sets in ; and the structure in question tends to 

 dwindle. But this external selection is abetted by the germinal 

 selection, for when a determinant corresponding to the useless 

 structure becomes weaker throu^-h the intragerminal fluctuations 

 of nutrition, " it finds itself upon an inclined plane, along which 

 it glides very slowly but steadily downwards. The determinant 

 whose assimilative power is weakened by ever so little is con- 

 tinually being robbed by its neighbours of a part of the nourish- 

 ment which flows towards it, and must consequently become 

 further weakened." By hypothesis, personal selection cannot 

 help it to persist — i.e. cannot favour those individuals in whose 

 inheritance it is relatively stronger ; therefore, by an internal 

 struggle and selection, which may be quite real though quite 

 unverifiable, the determinants of a disused part dwindle away 

 in the course of many generations. On the other hand, when 

 personal selection favours the increase of a part — i.e. favours 

 individuals whose inheritance includes strong determinants of 

 that part, again the internal struggle will back up the external 

 sifting. In short, nothing succeeds like success. 



The theory helps us to understand the slow dwindling of 

 useless structures, but it is also applicable to the augmentation 

 of useful parts. Suppose it be important for humming-birds 

 to have a longer tongue, and that natural selection favours 

 variants with longer tongues. Corresponding to the tongue 

 there are, by hypothesis, in the germ-plasm, several sets of 

 homologous determinants. (We need not complicate the 

 argument by recognising that many different kinds of deter- 

 minants will be required for a complex structure like the tongue.) 

 There are fluctuations in the food-supply and some tongue- 

 determinants get the advantage ; they become stronger, they 

 exhibit a plus variation, and as they become stronger they 

 increase in assimilative capacity. They therefore tend to pre- 

 dominate more and more over other tongue-determinants which 



