84 INTROGRESSIVE HYBRIDIZATION 



We can get around this handicap by letting the shape of the 

 dot represent a third character, and the color or intensity of 

 the dot a fourth. These pictorialized scatter diagrams are 

 of very general usefulness in analyzing for oneself some of 

 the main relationships in a population that one is just be- 

 ginning to study. In studying variation in fields of North 

 American maize, kernel width was diagrammed (Fig. 18) 

 on the horizontal axis, and number of rows of kernels on the 

 vertical axis; the shape of the dot represented the degree to 

 which the kernel was pointed at its apex, and the intensity 

 of the dot was proportional to the amount of soft starch in 

 the kernels. 



In making a population analysis by this method one takes 

 a random sample of 25 ears from each corn field and records 

 for each ear the kernel width, row number, amount of soft 

 starch, and shape of the kernel. In the resulting diagram, 

 each dot represents 1 ear. From the diagram as a whole, one 

 can tell at a glance the range of variation and the average 

 for each of these characters, as well as the relationships 

 among all 4. 



It is possible to demonstrate the reliability of the above 

 method, though not in a quantitative way. If repeated 

 samples of 25 are drawn from the same population, one can 

 see at a glance that the diagrams are essentially similar. At 

 the top of Fig. 18 are 2 samples from the same variety, with 

 and without the addition of artificial fertilizer. At the base 

 of the figure are 2 other varieties grown in the same Guate- 

 malan town. It will be seen that these pictorialized scatter 

 diagrams distinguish between varieties but give consistent 

 results for the same variety even under somewhat different 

 environmental conditions. This is not just a happy circum- 

 stance ; 5 years of preliminary studies of many kinds of maize 

 under various conditions of growth had been carried on be- 

 fore these 4 characters were finally chosen as the most re- 

 liable. 



These pictorialized scatter diagrams are particularly use- 

 ful because they also lend themselves to summarization. In 

 Fig. 18 each dot represents a single ear. It is possible to cal- 



