452 Journal of Agricultural Research Voi. vi, No. la 



By attempting to measure the extent to which types persist by means 

 of correlation coefficients, it is necessary to distinguish different kinds of 

 correlations. For this purpose correlations are here classified as physical, 

 physiological, and genetic. A method is also proposed by which physio- 

 logical and genetic correlations may be distinguished. 



The case studied was a hybrid between two extreme types that must 

 have been completely isolated from very remote times. The large num- 

 ber of well defined characters which differentiate the varieties rendered 

 this material exceptionally favorable for the study of coherence, by 

 which is meant the tendency for characters associated in one of the 

 parents of a hybrid to remain together in the later generation of the 

 hybrid. 



For the study of correlations 1 1 characters were selected in which the 

 parent varieties showed little or no overlapping. The correlation coeffi- 

 cients of all the combinations were calculated, and of the 55 possible 

 combinations 20 were found to exhibit significant correlations. In all 

 but 5 of these, however, the correlations are believed to be physiological 

 rather than genetic. In no instance was there a correlation between two 

 characters closer than 0.5, a fact which in itself offers an explanation of 

 the difficulty of recognizing types in maize. 



This lack of coherence of characters in maize, taken with the fact that 

 to maintain a satisfactory degree of vigor a diversified ancestry must be 

 maintained, would appear to make the method of isolating types inap- 

 plicable to this plant. As an offset to the limitation thus imposed, advan- 

 tage may be taken of the facility with which desirable characters derived 

 from different parents can be combined. 



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