92 INTROGRESSIVE HYBRIDIZATION 



the edge if need be, so that it can be cut out if the photo- 

 graph serves as a published illustration. The background 

 should be neutral, identical for each series, if possible, and 

 the scale should be photographed in each picture. Two ex- 

 amples will show the ways in which this technique may be 

 adapted to population problems. (1) As worked out by Dr. 

 W. L. Brown (Bro^\^l and Anderson, 1947) for Zea Mays: 

 A 10-foot white board (hinged in the middle for more ready 

 storage) is securely fastened to the north side of a field 

 laboratory. At 25-centimeter intervals, lines of black ad- 

 hesive lantern slide tape are stretched across it to provide a 

 scale. Down the center of the board a series of nails driven 

 part way in and with their heads filed off provide a rack by 

 which the corn plants can be quickly affixed to the board. 

 Labels give the year and the record number of each plant. 

 The leaf above the ear (usually on a sister plant) is traced 

 on wrapping paper and photographed in a standardized posi- 

 tion at the left of the photograph. (2) In studying Nicotiana 

 hybrids the calyx and corolla and the dissected limb of the 

 corolla were photographed in a standardized fashion against 

 a frame just one half natural size. By printing these pictures 

 on an enlarger equipped with a frame of natural size, it is a 

 simple matter to produce a large number of exact, standard- 

 ized records all of them just twice natural size. 



This is one of those simple techniques that are more im- 

 portant than they seem. Everyone who has tried it has 

 learned unexpected things about the material he was study- 

 ing. When one sits down afterwards with a set of stand- 

 ardized photographs of variable populations, it is possible to 

 see slight trends in variation or regional differences, which 

 had completely escaped one in the field. 



THE METHOD OF EXTRAPOLATED CORRELATES 



The methods described above have been used in the field, 

 in the experimental plot, and in actual plant breeding with 

 a great variety of hybrid material. At first in a very tent a- 



