GERMINAL SELECTION 117 



If each kind of determinant always secured the same quantity 

 of nourishment, all would grow in the same degree, that is, in exact 

 proportion to their power of assimilation. But we know that in 

 less minute conditions which we can observe more directly, there 

 is nowhere absolute equality, that all vital processes are subject to 

 fluctuations ; any little obstacles in the current of the nutritive fluid, 

 or in its composition, may cause poorer nutrition of one part, better 

 of another. We may therefore assume that there are similar 

 irregularities and difterences in the minute and unobservable 

 conditions of the germ-plasm likewise, and the result must be 

 a slight shifting of the position of equilibrium as regards size and 

 strength in the determinant system ; for the less well-nourished 

 determinants will grow more slowly, will fail to attain to the size and 

 strength of their neighbours, and will multiply more slowly. 



But the vigour of growth does not depend only on the influence 

 of nourishment ; one cell grows quickly, another slowly in the same 

 nutritive fluid ; it depends in great part on the cell's power of 

 assimilation. In the same way the assimilating power of the 

 determinants and their affinity for nourishment will varj^ with 

 their constitution, and a weaker determinant will remain smaller 

 than a stronger one, even when the stream of nourishment is the 

 same. 



It seems to me that it is upon the unequal nutrition of the 

 determinants conditioned by the chances of the food-supply that 

 individual hereditary variability ultimately depends. If, for instance, 

 the determinant A receives poorer nourishment at a particular time 

 than the determinant B, it will grow more slowly, remain weaker, and 

 then, when the germ-cells develop into an animal, the part to which 

 it gives rise will be weaker than it usually is in other individuals. 



These primary inequalities in the equipment of the determinants 

 which are caused by a passing inequality in the food-stream are, 

 of course, so slight that we are unable to observe their consequences. 

 They must persist for a considerable time before they become 

 observable, but they may persist for a long time, and their eft'ect 

 must then mount up, because every diminution in the strength of 

 the determinant also signifies a lessened power of assimilation, and 

 growth l»ecomes slower for the twofold reason that passive and 

 active nutrition decrease at the same time. In the less minute 

 conditions observable in the histological elements of the body we 

 know that function strengthens the organ, and that disuse weakens 

 it, and we are justified in applying this proposition also to these 

 more intimate conditions and minuter vital units. Thus, in the 



