518 EVOLUTION 
son. Thus if one should select a certain height, a certain num- 
ber of flowers per plant, etc., as typical of a given species and as 
standards of comparison, deviations of individuals from these 
standards are variations. Darwin was a careful student of 
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I leat 
Fic. 465.— Heads of Timothy selected from a field of Timothy to 
show variation in form and size of heads. After Clark. 
Fic. 466. — Variation in length of ears selected from a field of Black 
Mexican Sweet Corn. After East. 
variations and observed that variations in structure and func- 
tion are so common that no two individual organisms are ex- 
actly alike. In a field of grain or in a group of any of the culti- 
vated or wild plants no two individuals can be found that are 
exactly alike (Figs. 465 and 466). They differ in height, shape, 
