VARIATION. 331 



homogeneous mixture of the traits of the two parents. A 

 little consideration shows that the reverse is inferable. If, 

 throughout the process of development, the physiological 

 units derived from each parent preserved the same ratio in 

 all parts of the growing organism, each organ would show as 

 much as every other, the influence of either parent. But no 

 such uniform distribution is possible. It has been shown 

 (First Principles, § 163), that in any aggregate of mixed 

 units segregation must inevitably go on. Incident forces 

 will tend ever to cause separation of the two orders of units 

 from each other — will tend to 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 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 mingled in their offspring, 

 they must in all of them 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. Evidently, 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 physiological, units 

 which are exactly alike. But in no case can such a homo- 

 geneity exist. Each parent had parents who 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 

 physiological units bequeathed to it, the reproductive cells it 



