118 THE EVOLUTION THEORY 



course of the multiplication of the germ-cells, the less vigorously 

 working determinant, A, will gradually, but very slowly, become 

 weaker, that is, of diminished power of assimilation, jDresupposing 

 of course that the intra- germinal food-stream does not become 

 stronger again at the same place — a possibility to which I shall 

 subsequently refer. But while one determinant may be slowly 

 becoming weaker, its neighbour, on the other hand, may be varying 

 on an ascending scale, just because the former is, on account of its 

 diminished power of assimilation, no longer able to exhaust 

 completely the food-stream which flows to it. 



The determinants are thus in constant motion, here ascending, 

 there descending, and it is in these fluctuations of the equilibrium 

 of the determinant-system that I see the roots of all hereditary varia- 

 tion, while in the fact that the variation-directions of particular 

 determinants must continue the same without limit as long as they 

 meet with no obstacle lies the possihilit}" of the adaptation of the 

 organism to changing conditions, the increase and transformation 

 of one part, the degeneration and disappearance of another, in short, 

 the processes of natural selection. The reason why such variation 

 movements must continue until thev meet some resistance is that 

 every chance upward or downward movement — due, that is, to mere 

 passive fluctuation in the food-supply, at the same time strengthens 

 or weakens the determinant, and makes it either more or less capable 

 of attracting nourishment to itself ; in the former case an increasingly 

 strong stream of food will be directed towards it, in the latter more 

 and more of the available food-supply will he withdrawn from it 

 by its neighbour-determinants on all sides : in tlie former the 

 determinant will go on increasing in strength as long as it can go 

 on attracting more nourishment, in the latter it will continue to 

 become weaker until it disappears altogether. To the ascending 

 progression, as is evident, there are limits set, not only by the 

 amount of food which can circulate through the whole id, but also 

 l)y the neighbour determinants, which will sooner or later resist 

 the withdrawal of nourishment from them ; but for the descendino- 

 progression there are no limits except total disappearance, and this 

 is actually reached in all cases in which the determinants are related 

 to a part which has become useless. But both these movements, 

 the upward and the downward alike, are quite independent of natural 

 selection, i.e. of personal selection ; they are processes of a unique kind 

 which run their course purely in accordance with intra-germinal laws. 

 Whether a determinant ' ascends ' or ' descends ' depends solely upon 

 the play of forces within the germ-plasm, not upon whether the 



