SPECIAL TECHNIQUES 93 



live way, and later with increasing confidence, they have 

 been employed to determine the putative parentage of hy- 

 brid swarms. The general method, which is here formally 

 designated for the first time as the Method of Extrapolated 

 Correlates, has a sound theoretical basis (Anderson, 19396; 

 see particularly p. 692, where the theory's application to 

 criteria of hybridity was specifically pointed out). It was 

 presented pragmatically by Anderson and Turrill in 1938, 

 its application to a particular example being illustrated step 

 by step. 



The method of extrapolated correlates is based on the 

 demonstration (set forth in detail in Chapter 3) that in a 

 species cross all the multiple-factor characters are linked 

 with each other (Anderson, 19396). When well-differentiated 

 entities hybridize, we may expect their cohesive forces to 

 continue to operate for many successive generations in hy- 

 brid swarms. Certainly for scores, and perhaps for hun- 

 dreds, of generations, we may expect to find the characters 

 that went into the cross together still tending to stay together. 

 By a precise and detailed examination of such populations 

 we can discover the cohesive centers of variation still exist- 

 ing within them. By comparative, quantitative methods we 

 €an draw up descriptions of the original entities that must 

 have operated to produce these centers of variation. It is 

 possible, working with a single variable population of a 

 species previously unknown to the investigator, to draw up 

 a precise description of the other species which is intro- 

 gressing into that population. The subsequent discovery 

 that such a species does actually exist and could have oper- 

 ated in that area cannot be dismissed as a remarkable co- 

 incidence; when the prediction has been verified for a com- 

 plicated series of technical details, it then becomes proof. 

 It is even possible by this method to work with a hybrid 

 swarm and draw up detailed descriptions of both parents 

 when neither of them are known to the observer. Crude ex- 

 amples of such a prediction are given in Anderson and Tur- 

 rill (1938) and in Anderson and Hornback (1946). The 



