PART V 

 HEREDITY AND EVOLUTION 



CHAPTER LXXIX 



VARIATION 



463. No two alike. In some 

 respects all the members of a 

 species are alike; that is why 

 we classify them as of the 

 same species. But in some re- 

 spects every individual is unique. 

 If a person should get the tips 

 of his fingers inky and place 

 them on a sheet of paper, he 

 would make a mark that could 

 not be duplicated by anyone 

 else. Ordinarily we have no 

 difficulty in distinguishing from 

 each other human beings that 

 look very much alike, although 

 occasionally there is difficulty 

 in identifying a person beyond 

 every doubt. 



All species present this fact 

 of variation. And variation is 

 found with respect to every char- 

 acter. There is variation in size 

 and in proportions (Fig. 23 1), in 



437 



10 11 12 13 14 15 16 17 18 19 20 21 22 



FIG. 232. Individual variation in the 

 number of repeated parts 



The principal veins on each side of the 

 midrib on a beech leaf vary from 10 to 22. 

 The number most frequently found is 16. 

 The vertical columns correspond in height 

 to the frequencies with which the various 

 numbers of veins occur. Broken line a, a 

 tree in which the number of veins varied 

 from 13 to 17, leaves with 15 veins being 

 most frequent. Dotted line b, another 

 tree, in which the veins varied from 1 5 to 

 20, 18 veins being most frequent. Each 

 tree has its individuality, and each leaf 

 has its individuality 



