DESCENT WITH MODIFICATION 165 



at almost amthing that may happen in individual cases, from 

 which we infer that we shall never be able to predict from the 

 parents what a particular offspring will be, but that we can tell 

 very close as to what they will be in the long run and on the 

 average ; and that the more uniform the ancestry, the more 

 accurate will the prediction be, and the more uniformity will 

 there be among individuals. We see, too, by the principle of 

 progression, that under selection the correspondence between 

 parent and offspring becomes rapidly closer. 1 



Summary. The individual offspring is seldom like the parent. It may 

 be better (progression) or it may be worse (regression), but in general the 

 offspring is like the parentage as a whole. More exceptional offspring arise 

 from common parentage than from exceptional parentage because mediocrity 

 is the common lot, yet the proportion of exceptional offspring is higher from 

 the exceptional parent than from the mediocre. 



Reversion shows that as long as even a trace of old-time characters 

 exists, the gametes are not absolutely pure, and an occasional appearance of 

 such ancient characters is inevitable. Being ancient, they are correlated 

 with others, and are likely to creep upon the breeder unawares, like the high 

 shoulders and thin flanks of cattle, or the inferior hams of pigs. 



The offspring, therefore, is to be considered not co much the product of 

 his personal parents as of his parentage as a whole. 



* The reader should understand that this chapter is designed to treat of 

 heredity as operative in a fairly homogeneous population, having no reference 

 to its behavior in hybridization. Since the chapter was written, however, evi- 

 dence is accumulating, tending to show that most species, and even varieties, 

 are really made up of a number of similar but somewhat divergent strains, 

 overlapping and more or less obscuring each other, and that the principal 

 office of selection is to isolate the favored strain. 



