118 THE EVOLUTION TlIE(MtY 



eour.so of tlu'. iimltipliciitiou oi' tlio n'l-nii-cclls, Iho less vigoroUHly 

 working (letenniimnt, A, will griuhitilly, liut \vvy slowly, Ih'i'oiiic 

 weaker, that is, of diiuinishod power of ussiniilation, presupposing 

 of eouT'si' that the intra-gerniinal food-stream does not become 

 sti'ongt'r again at tlu' same plaec — a possiliility to whieh I shall 

 suliseqnently refer. Bnt while one determinant may he slowly 

 beeoming weaker, its neighbour, on the other hand, may bo varying 

 on an aseending seale, just lu'eaiise the former is, on ncixnnd. of its 

 dinunished pow(!r ol' as.similation, no longer able to exhausts 

 completely tlu^ food-stream which flows to it. 



The deti'rminants are thus in constant motion, lu'iv aseen<ling, 

 there deseentling, and it is in these tluctxiations of the ('([uilibrium 

 of the determinant-system that I see the roots of all heri'ditary varia- 

 tion, while in the fact that the variation-dii-eetions of jiart-icular 

 determinants nnist continue the same without limit as long as they 

 meet with no obstacle lies the possibility of the adaptation of tlu^ 

 organism to changing conditions, the ino'ease and transformation 

 of one part, the (h'generation and disappearance of another, in short, 

 tln' processes of natural iselection. Tlie reason why such variation 

 mo\ements must continue until they nu'et some resistanci^ is that 

 every chanct' upward or downwanl movement — due, that is, to mer(^ 

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

 or weakens the deternnnant, 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 moi'e 

 and more of the available food-supply will be withdrawn from it 

 by its neighbour-dc^terininants on all .sides; in the former Uw. 

 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 disapjiears altogether. To tlu^ ascending 

 progression, as is evident-, then^ are lindts set, not oidy by tlu^ 

 amount of food which can cii-cidat-e through the whole id, but also 

 by the neighbour determinants, which will sooner oi' latt-r resist 

 the withdrawal of nourishment fi'om them; bnt for the descA'ndiruf 

 progi-ession there are no lindts except total disappearance, and this 

 is actually I'eached in all cases in which the determinants are r(^late(i 

 to a part which has beconu; useless. But both these moveUKnits, 

 tlu^ upward and tlu^ downwai-d alike, are (juite ind(^])en(lent of natural 

 selection, i.e. of personal selection; they are ])roeesses of a nnicpie kind 

 which run their course j)urely in accordance with int -ra-gerndnal laws. 

 Wheth(!i' a detei-minant ' ascends ' or ' descends ' depc^nds isolely ni)()ii 

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



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