NATURAL SELECTION 7 



Variation. 



Yet however clearly we see that offspring tend to re- 

 semble their parents, it is no less evident that this resem- 

 blance is not an exact one. Among human kind we find 

 excellent illustrations of this principle. However strong 

 may be the family resemblance between the different mem- 

 bers of a family, still each has his or her own individual 

 peculiarities. No two are exactly alike. The children do 

 not exactly resemble each other or their parents. These 

 facts of individual differences we group under the one term, 

 variation. We say that while, under the influence of he- 

 redity, the young tend to resemble their parents, because of 

 variation this resemblance is more or less imperfect. 



No one doubts the existence of variation. All about us 

 we constantly see illustrations of the principle. Yet few but 

 trained biologists realize how universal and how extensive is 

 variation. All species of organisms are always varying in 

 every characteristic and in almost all directions, and the 

 extent of the variation is very considerable in most species. 

 The individual plants of any species vary in size, in size of 

 the several parts, in shape of stem and roots and leaves, in 

 number of leaves and of blossoms, in color of petals, in num- 

 ber of seeds, and in hardiness, that is, in ability to resist 

 adverse conditions of heat or cold, of drouth or flood, and of 

 unfavorable soil. In all features, both structural and physio- 

 logical, we find the individuals of any species of plant will 

 differ from one another. Absolute uniformity is not found 

 in organic nature (Plate 2). 



Study a thousand individuals of any species with regard 

 to any single character, and you will see how true this is. 

 Take the common trailing arbutus as an example. You will 



