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 



FIG. 465. Heads of Timothy selected from a field of Timothy to 

 show variation in form and size of heads. After Clark. 



J9 20 2? 



FIG. 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, 



