90 INTROGRESSIVE HYBRIDIZATION 



tube color, sepal length, petal shape, stamen exsertion, size 

 of style appendages, and presence of a crest were all scored as 

 like Fulva, like HGC, or intermediate. The color of the 

 sepal was scored in five grades from to 4, and the length 

 of the sepal in four. This gave an index running from for 

 plants like Iris fulva to 17 for plants like 7m giganti-caerulea. 

 Riley has given a meticulous description of the way in which 

 the hybrid index was constructed in this particular study 

 (loc. cit., pp. 727-734), to which the interested reader is re- 

 ferred for further details. 



In such cases as hybridization between the Louisiana 

 irises, in which the differences between the species are con- 

 spicuous and many of them are easily measured, this method 

 is simple to apply and yields satisfactory results. When the 

 contributing parental species are closely similar or only 

 vaguely different, it is much less satisfactory. Hubbs and 

 Hubbs (1943) have replaced it in their studies of hybridiza- 

 tion in fishes with a similar but statistically more elegant 

 method that is superior for their material. At the present 

 time, at least for plant material, the Hybrid Index Method is 

 a powerful means of analysis. It is efficient in exploring a 

 complex situation and pointing out the general overall 

 picture. In my own estimation its main application is in 

 digging into such a problem. When the main facts have been 

 secured, one can then work out a more precise technique 

 adapted to any particular case. From a statistical point of 

 view it is a crude device, and although it could easily be 

 turned into something more respectable mathematically, 

 for the higher plants at least, the time is premature. When 

 we know more about hybridizing populations than we now 

 do — when, in other words, the general problem has been 

 more thoroughly explored on a biological level — we shall 

 then be ready to work out more precise and elegant methods 

 for dealing with such phenomena. 



To understand the value of methods as mathematically 

 crude as the Hybrid Index, one needs to keep in mind the 

 general principle behind the doctrine of significant figures: 



