4 SOCIAL EVOLUTION 



Although the offspring tends to resemble the parent, 

 it is evident that the resemblance is not an exact one. 

 The resemblance between the different members of the 

 family is such that brothers and sisters are quite dis- 

 tinguishable. Each has his or her own individual pecul- 

 iarities. No two are exactly alike. These facts of in- 

 dividual difference we group under one term. We call 

 this phenomenon variation. We can now change our 

 statement and say: under the influence of heredity the 

 young tend to resemble their parents, but this resem- 

 blance is never exact. It is more or less imperfect. 



If one doubts this fact of variation let him take a 

 thousand individuals of any species with regard to any 

 single character and he will be convinced. If one meas- 

 ure 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 character of any animal or plant, there is 

 much variation. The winter birds of east Florida show 

 a variation in size of from fifteen to twenty per cent, 

 among specimens of the same species and sex when taken 

 in the same locality. 2 



The relation then, between parents and offspring, is 

 such that like tends to beget like, while at the same time 

 opportunity is afforded for the individual new depar- 

 tures 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 it has been known by biologists that when the par- 

 ents ' body is developing from the fertilized ovum, a resi- 



2/6tU, p. 9. 



