Page:Scientia - Vol. X.djvu/126

118 within the genotype — within a group of individuals where the offspring breed absolutely true generation after generation? Merely to state the question shows the absurdity of the current biometrical treatment of inheritance which regards all variations in the same correlation table as of equal hereditary significance. What a correlation coefficient deduced from a parent-offspring correlation table which includes a random sample of parents in general, or offspring in general, really measures is what may be called the orderly heterogeneity of the material which goes into the table. One gets out of the table by way of the correlation coefficient merely a measure of what was put into the table as raw data. If the table includes individuals belonging to several distinct genotypes we may expect to get from it in many cases at any rate a sensible correlation coefficient between parent and offspring. But this coefficient does not measure the « intensity of inheritance » between parent and offspring with reference to the character considered. It simply measures the mutual interrelation (as to range and distribution of variation) of the several genotypes which went into the table. The coefficient indicates, in other words, that there are certain groups of individuals within the table which are differentiated, in respect to both parent and offspring, from certain other groups in the same table. It does not tell us what the basis of this differentiation is. It may be inheritance, as it is assumed to be in the illustration here under discussion; it may be local environmental differences or it may be anything whatever so far as the correlation method per se helps us. The only way to determine whether the « differences » indicated by the correlation method are really heritable is to apply the method of individual pedigree analysis, to the complex, heterogeneous material of the table. If it is possible to isolate and propogate distinct genotypes from the material then it may be concluded that the primary basis of the differentiation or heterogeneity detected by the correlation coefficient was inheritance.

To summarize this discussion it may be said that to attempt to draw conclusions in regard to inheritance from studies involving the « correlation method » alone is futile, because the coefficient of correlation in such studies can only tell us of the existence and degree of an orderly differentiation or heterogeneity in the material collected together in