VARIATION. 267 



units derived from each, parent, preserved the same ratio to 

 each, other in all parts of the growing organism, each organ 

 would show as much, as every other, the influence of either 

 parent. But we know, a priori, that no such uniform dis- 

 tribution is possible. It has been shown (First Principles, 

 § 123), that in any mixed aggregate of units, segregation 

 must inevitably go on. Incident forces will tend ever to 

 cause separation of the two orders of units from each other — 

 will integrate groups of the one order in one place, and 

 groups of the other order in another place. Hence there 

 must arise, not a homogeneous mean between the two 

 parents ; but a mixture of organs, some of which mainly 

 follow the one parent and some the other. And this is the 

 kind of mixture which observation shows us. 



Still it may be fairly objected, that however the attributes 

 of the two parents are variously mixed in their several 

 offspring, they must in all the offspring fall between the 

 extremes displayed in the parents. In no characteristic 

 could one of the young exceed both parents, were there no 

 cause of " spontaneous variation " but the one alleged. Evi- 

 dently, then, there is a cause yet unfound. 



§ 89. Thus far we have contemplated the process under 

 its simplest aspect. While we have assumed the two parents 

 to be somewhat unlike, we have assumed that each parent 

 has a homogeneous constitution — is built up of physiologi- 

 cal units that are exactly alike. But in no case can such a 

 homogeneity exist. Each parent had parents that were more 

 or less contrasted — each parent inherited at least two orders 

 of physiological units, not quite identical. Here then we 

 have a further cause of variation. The sperm-cells or germ- 

 cells which any organism produces, will differ from each 

 other not quantitatively only, but qualitatively. Of the 

 slightly-unlike ph}< siological units bequeathed to an organism, 

 its reproductive cells cannot habitually contain the same pro- 

 portions ; and we may expect the proportions to vary not 



