528 Cytogenetics and Evolution 



has been termed introgressive hybridization by Anderson and 

 Hubricht, who have illustrated it by a diagram reproduced as Fig. 

 150. In this figure, the solid black indicates species A, species 

 B, or the Fi hybrids, according to position. The stippled area 

 illustrates the hybrid swarm exclusive of the Fi's. The arrows 

 indicate the approximate limits of the terms "species" and "hy- 

 brids" and show that many of the extreme variants included 

 under species A are actually of more or less remote hybrid origin. 



Species A Species B 



>■ Hybrids "* ^ 



Fig. 150. Diagram showing the relation of two species, A and B (solid 

 black), the first generation hybrids (also sohd black), and later hybrid 

 generations and backcrosses to species A (dotted). The arrows at the base 

 of the figure delimit the "species" and "hybrids" in the populations in 

 which introgressive hybridization is taking place. (Redrawn from Ander- 

 son and Hubricht in the American Journal of Botany.) 



Studies of populations of Tradescantia, as examined and evalu- 

 ated from herbarium material, show that there has been a strong 

 introgression of T. canaliculata into T. occidentalis and also into 

 T. bracteata. However, there is no evidence to show any intro- 

 gression of T. subaspera into T. canaliculata, even though these 

 species can be crossed to produce fertile offspring. 



Other Indices. The method we have described of arbitrarily 

 assigning index values to each of a number of characters of two 

 species and their intermediate forms in order to assign a single 

 value to individual plants as a means of comparing putative 

 hybrids with both species is not the only method that has been 

 used for that purpose. Hubbs and Whitlock introduced the char- 

 acter index for studying fish, and Anderson and Whitaker intro- 

 duced a similar expression under the term general index in a 

 study of Uvularia. 



Hubbs has devised a valuable hybrid index method which has 

 been applied successfully to groups of fish. The hybrid index is 



