4 SOCIAL EVOLUTION 



The resemblance of parent and offspring is, however, 

 not exact enough to be duplication. The family likeness 

 IS such that parents and progeny are cpiite distinguish- 

 able. "Tom" has his own individuality, and "Molly" 

 has her peculiarities. Thus we see that there are indi- 

 vidual differences which indicate how much the offspring 

 vary from their parents and among thetnselves. The 

 facts of individual difference we call variation. Our 

 knowledge of variation permits us to say "that while, 

 under the influence of heredity, the young tend to resem- 

 ble their parents, because of variation this resenil)lance 

 is more or less imperfect."" 



Ti) be convinced of this fact of variation one has only 

 to take a few liundred indiviihials of any species and 

 compare tliem witli refei-enee to any single trait. If one 

 measure the lengths of a thousand oak leaves taken from 

 the same tree, he will find that some are considerably 

 longer than others, but that within certain limits most 

 of the leaves have approximately the same length. So 

 it is with any trait of any plant or animal, — there is much 

 variation. The winter birds of east Florida show a vari- 

 ation in size of from fifteen to twenty per cent, among 

 si)ecimens of the same species and sex when taken in the 

 same locality. 



Thus the relation l)etween parents and offspring is of 

 such a nature that like tends to beget like, yet at the same 

 time opportunity is allowed for the individual differen- 

 ces which we have called variations. But how does it 

 happen that like tends to beget like? Why is it that the 

 young of a horse will always be a horse and not a zebra? 

 How is it that nature keeps things in order? For some 

 time biologists have known that "when the parent's 

 body is developing from the fertilized ovum, a resi- 



'■i/hia., p. 7. 



