VARIATION 417 



control of natural selection, -which eliminated every case of 

 deviation from the perfectly adapted form. If insignificant 

 individual characteristics which are of no use to the species can, 

 however, be preserved in the human race for several generations, 

 this must be owing to the fact that the corresponding de- 

 terminants have very little tendency during their growth and 

 multiplication to suffer any marked variation, and that, on the 

 contrary, they reproduce exact duplicates of themselves. I was 

 therefore quite justified in supposing that the germ-plasm pos- 

 sessed a great power of remaining constant.* 



We can none the less avoid assuming that tJie elements of the 

 gen)i-plasiii — /.^., the biophors and determinants — are subject 

 to continual changes of co7nposition during their almost uninter- 

 rupted growth, and that these very minute fluctuations^ which 

 are imperceptible to us, are the priinary cause of the greater 

 deviations in the determinants^ which we finally observe in the 

 for7n of individual variations. 



The assumption that such very minute fluctuations occur, 

 naturally follows from the impossibility of a complete uniformity 

 as regards nutrition existing during growth ; and in fact, though 

 underestimating its importance,! I formerly made this assump- 

 tion, in correctly supposing that the influences producing these 

 fluctuations ' are mostly changeable, and occur sometimes in 

 one and sometimes in another direction.'' I had not then taken 

 into consideration the fact that the fluctuations accumulate 

 in consequence of the process of amphimixis. If a single 

 determinant increases to a hundred thousand during the multi- 

 plication of the germ-cells of an individual, it is not likely that 

 the nutrition of all these determinants during the process will 

 be absolutely the same in strength and kind. If this is not the 

 case, minute differences could not fail to appear in the subsequent 

 determinants. These minute fluctuations may undoubtedly 

 again disappear, as I formerly assumed, provided that the modi- 

 fied determinant is exposed to counteracting influences, and alone 

 they are quite incapable of producing an individual variation of 

 any perceptible character ; but they may become cumulative. 

 For the germ-plasm always consists of a large number of ids, 

 each of which contains one of the homologous determinants in 



*(!/. my essay ' Die Bedeutung der sexuellen Fortpflanzung,^' Jena, 

 1886, p. 28. English edition, Oxford, 1889, p. 271. 

 t Loc, cit., p, 272. 



