SPECIAL TECHNIQUES 89^ 



population (or a random sample of them) are then scored 

 indi\ddually for all these characters. Attributes like sepal 

 length or petal length are measured; colors can be recorded 

 by comparison with a graded series as on the Alunsell and 

 Fischer color charts. Differences in shape can be scored as 

 essentially like one species, or like the other, or intermediate. 

 Raunkiaer (1925) had used and published such a method for 

 showing the great variety of character combinations to be 

 met with in Crataegus populations. By the simple addi- 

 tional step of throwing all these differences together into a 

 composite index, it was possible to extend the usefulness of 

 this method into the domain of analysis. One could then 

 employ it not merely to report the condition he had dis- 

 covered in a certain hybrid colony but also to inquire into 

 the forces that had produced the variation. 



In the simplest appHcation of this method each char- 

 acter (sepal length, petal color, height of plant, numbers of 

 nodes, etc.) was scored in three grades: (1) similar to one 

 species, (2) intermediate, and (3) similar to the other species. 

 One of the species was arbitrarily selected for the low end of 

 the scale, the other for the high end of the scale. Each char- 

 acter, therefore, was scored if it was like the former, 2 if it 

 was hke the latter, and 1 if it was intermediate. Supposing 

 6 characters had been chosen for study, we would then have 

 had a scale running from to 12. Plants exactly like the 

 first species would have scored in every character, and the 

 total score of each plant would have been 0. Plants exactly 

 hke the second would have scored 2 for each of the characters, 

 and their total score would have been 12. Plants that were 

 exactly intermediate would have scored 1 for each character, 

 and their total score would have been 6. In actual practice 

 it is usually advisable to give different score values to certain 

 characters, either because they can be more accurately 

 measured and therefore deserve more consideration as cri- 

 teria, or because they are known to rest upon a wider genie 

 basis and hence are representative of a large portion of the 

 germplasm. In Riley's study of introgression in Iris (1938),* 



See Chapter 1, pp. 2-11. 



* w". 



