200 THE EVOLUTION THEORY 



in the course of geiieratioiiti tliey are uni"ailiiii;-ly eliminated, ospeciaHy 

 when a large nTiiulicr of unfavourably ^lll•y^ncJ; determinants are 

 coincident in the goriu-plasm. Then the indi\'idna-l.s which ari.se 

 from a germ-plasm tluiH composed must perisli in the struggle for 

 existence, and thus the id-combinations with I'xcessive determinants 

 are eliminated I'rom the germinal constitution of the species. As 

 this is I'cpcated as often as excesses of the ids occur, the species 

 is kept pui'c. 



It might be objected that, throxigh sucli a continual weeding- 

 oyt of rebellious determinants, the germ-plasm would become so 

 constant in its constitution that it would ultimately be secure from 

 all such aberrations of it on the part of its determinants, and therefore 

 would in time become quite incapable of diverging from its proper 

 path at all, and wo\ild thus no longer r(H|uire this continual correction 

 through amphimixis. 



I do not wish to contradict this conclusion ; indeed, 1 believe 

 that the constitution of the spc^cies becomes more and more constant 

 in the way I have indicated, and that an exer more ])erfect and stable 

 equilibrium of the whole determinant system is thus lirought about. 

 It follows that in the course of g(.'nerations tlu^ dix'erse determinants 

 of the germ-plasm will vary within a progressiN-ely shortened radius, 

 and will thus more and more I'arely overstep tlie limits of the 

 'variation-playground' — and yet I still belii've that this justifiable 

 conclusion tells in favour of my interpretation of tlie utility of the 

 persistence of amphigony once introduced. 



Let it be remarked, in the first jdace, tliat it is by no means 

 essential to the pifservation of a useful institution that it should 

 practically justify its utility in every generation. Although, for 

 instance, the warm winter coat of a species of mannnal may be 

 necessary to its survival, it does not disappear at once when a winter 

 happens to occur which is so warm that evcii individuals with poor 

 pellage can survive. In(k'.ed, several such mild winters might occur 

 in succession, in whicli thc^re was no wceding-out of the individuals 

 with poor fur, and yet the thickness of the winter fur of the species 

 would not become less fixed, just because this character no longei' 

 varies perceptibly in an old-established spc^cies which has long been 

 perfectly adapted, and it could only be brought into a static of marked 

 fluetuation very slowly through direct infhience on tlie germ-plasm, 

 or through panmixia. But exactly the same thing is true in regard 

 to the determinants of the reproductive cells, in respect of their 

 adaptation to amphimixis, only very much more c^mphatically. 



Before going further, I should like to show that the conclusion 



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