Blakeslee: Corn and Men 



513 



terest which has been aroused by the 

 application of scientific knowledge to 

 plant and animal breeding. The most 

 interesting study of mankind may be 

 man, but in biology at least this study 

 must make large use of the indirect 

 method, employing plants and rapidly 

 breeding animals for experimental pur- 

 poses. The present article is to call at- 

 tention to the fundamental differences 

 between heredity and environment and 

 to illustrate the subject by a com- 

 parison between the growth of corn and 

 of man. The discussion will deal with a 

 single comparatively simple characteris- 

 tic — height. Something will be said 

 about the manner in which a single 

 character of this kind may vary among 

 a group of individuals, and the influ- 

 ence of environment and heredity in 

 bringing about these variations will be 

 discussed. 



Variation is universal among living 

 organisms. It applies to individuals 

 and parts of a single individual. A 

 class of students after examining hun- 

 dreds of apple leaves in the attempt to 

 find two that are identical is willing to 

 admit that no two leaves exist exactly 

 alike. We are grateftil indeed that in 

 man no two persons have the same men- 

 tal make-up. Differences in man's 

 physical features as well as in his mental 

 equipment are easily recognized and 

 more readily analyzed. The relatively 

 simple and obvious character of height 

 may well serve for analysis. Each of 

 the 175 students shown in Figure 12 

 was measured in his stocking feet and 

 placed in the rank to which his height 

 most nearly corresponded. Only one 

 student was shorter than 5 ft. and only 

 one could be placed higher than the 

 6 ft. 1 in. rank. The great majority are 

 in the middle ranks 5 ft. 6 in. to 5 ft. 

 8 in. The extremes are rare, the aver- 

 age is common. Most of us are just 

 average people, neither extremely tall 

 nor short, fat nor slim, fast nor slow, 

 rich nor poor. 



Teachers know that differences in 

 mental ability follow the same laws as 

 differences in height. The majority of 

 a class are average students. Few are 

 unteachable and few gain the highest 

 ranks that may be obtained. 



If the ends of the ranks in the photo- 

 graph were connected by a line we 

 should have a typical curve of vari- 

 ability. Such a curve shows the dis- 

 tribution of the different heights in the 

 company — the manner in which the 

 individuals vary — but gives no definite 

 clue to the causes of the differences 

 which are found to exist. 



WHY MEN GROW TALL. 



Two general causes have been assigned 

 to account for the differences in in- 

 dividuals brought to view by a vari- 

 ability curve, namely, environment and 

 heredity. The first woiild assimie that 

 differences in height were due to differ- 

 ences in surroundings during growth; 

 the tall boy is tall, it would be claimed, 

 because he has been well nourished and 

 cared for; the small boy is small be- 

 cause his growth has been checked by 

 disease, accident, or insufficient nour- 

 ishment. The second suggested cause, 

 heredity, would assume that the dif- 

 ferences in height are due to differences 

 in the gifts of inheritance with which 

 each individual began life ; a tall boy is 

 tall, it would be claimed, because tall- 

 ness ran in his family, a short boy is 

 short because his parents or less im- 

 mediate ancestors were small of stature. 



Differences in environment or in 

 heredity, acting either alone or in 

 combination, will be able to cause vari- 

 ation in a group of individuals. If these 

 individuals are arranged in ranks ac- 

 cording to the variations which they 

 show in a single characteristic, they will 

 present such a variability curve as is 

 shown in Figure 12. Mere inspection 

 of the curve, however, will not enable 

 one to discover which factor is the pre- 

 dominant or exclusive cause of the va- 

 riations. For this a study of the ances- 

 try of the individuals and of the effect 

 upon them of the various environmental 

 conditions to which they are exposed 

 will be necessary. A discussion of the 

 com plant will be appropriate in this 

 connection. 



In the Agricultural Botanic Garden 

 at Storrs, Conn., a section is devoted to 

 plots illustrating the laws of genetics. 

 Differences in height and general vigor 

 of plants are classified as due either to 



