88 INTROGRESSIVE HYBRIDIZATION 



arate measurements and ratios for each ideograph. There 

 are first of all the four original measurements — sepal length, 

 sepal width, petal length, and petal width ; then there are the 

 six proportions between these four, taken two at a time (the 

 length of the petal in proportion to its width, the width of 

 the petal in proportion to the width of the sepal, etc.) ; then 

 there are four three-way relationships (such as the length- 

 width of the petal in relation to the length of the sepal); 

 and finally there is the relationship of all four measurements 

 taken at once. 



RADIATE INDICATORS 



This type of polygraphic analysis has been used by several 

 students of populations, notably by Norman Fassett (1941) 

 and by Carson and Stalker (1947), but apparently has never 

 yet been dignified with a name. Radiate indicators are use- 

 ful in presenting for a number of different populations the 

 occurrence of certain different traits or subtypes. 



HYBRID INDICES 



One of the most difficult types of population to analyze is 

 one in which two or more species have hybridized freely and 

 produced second-generation hybrids and backcrosses. Sup- 

 pose, for instance, that the two species differ principally in 

 flower color, in petal shape, and in plant height. In the 

 second generation of -hybrids and in backcrosses there will 

 be various and multitudinous recombinations of flower 

 colors, shapes, and heights, and no two plants will look very 

 much alike. If we are to make an efficient comparison of 

 two such populations, or a series of them, we must have some 

 means of getting an overall picture of each population so 

 that, roughly at least, we can equate one to another. 



For such situations there was evolved (Anderson, 1936c/) 

 a method so crude that it was published only after its general 

 usefulness had been demonstrated in a number of different 

 problems. It consists in drawing up a list of differences be- 

 tween the hybridizing entities. All the plants in the hybrid 



