82 INTROGRESSIVE HYBRIDIZATION 



The chief disadvantage of these orthodox methods of hy- 

 brid analysis is that they can be appUed only when the 

 parental species are known, or at least strongly suspected. 

 They are useful largely in proving that certain hybridiza- 

 tions might have taken place. They cannot be used ana- 

 lytically as a basis for successful prediction. 



For the examination of hybrid populations or of popula- 

 tions in which hybridization is suspected, we need methods 

 that record precisely the extent to which variation in one 

 character is related to variation in other characters. 



The human mind is inefficient in judging variation in more 

 than one variable at a time. A good observer may examine 

 three different populations and note them efficiently for their 

 variation in pubescence, in leaf shape, or in flower color, but 

 careful tests have shown (Anderson, unpublished) that sci- 

 entists cannot look at three populations varying simul- 

 taneously in flower color and pubescence and leaf shape and 

 render an eflficient judgment of the comparative association 

 between these characters in the three different populations. 



WTiat is needed, therefore, in describing populations is 

 some means of recording simultaneously variation in several 

 different characters. Species characteristically differ by 

 slightly different proportions and trends in proportion for 

 several different characters (Anderson and Whitaker, 1934; 

 Anderson and Ownbey, 1939). We can differentiate most 

 effectively between interspecific and intraspecific variation 

 if we have some method for showing the relationships be- 

 tween the main variables in the population. 



For such a purpose the methods of conventional biometry 

 are laborious and inefficient. They were developed for other 

 types of problems, and though they are fairly good for an- 

 alyzing variation in any one character they are not efficient 

 for exploring relationships between groups of characters, 

 particularly when we do not know in advance the general 

 nature of that relationship. 



However, any methods with which we replace or precede 

 biometrical analysis must, like it, be exact, objective, and 



