The Genetical Interpretation of Statistics of the Third Degree in the Study of Quantitative Inheritance.

For the past 20 years genetical methods have gradually been made more and more familiar to the practical breeders of plants and animals, upon whom the improvement for human use of the domesticated animals and cultivated plants finally depends. During this period it has become increasingly clear that the hereditary mechanism is well represented by the Mendelian scheme, as extended mainly by the work of the Drosophilists. It has been equally clear, however, that in all the practical problems of animal or plant improvement we are invariably faced with quantitative characters, which have shown themselves to be entirely intractable by the familiar genetical methods. These methods rest primarily upon the recognition of the effects of different single factors, and when these can be recognized the study of their effects in combination follows as a matter of routine. When individual factors cannot be recognized the analytic method of genetic study cannot even be commenced, and the question arises as to whether genetics as a science has any further resource to offer. The successes of analytic genetics have been obtained mainly with the numerous deleterious recessives which are abundant in most species, with certain easily recognizable characters of practical importance to the plant or animal breeder and with fancy characters such as the crest, or silky plumage in the fowl, which, however attractive to fanciers, cannot be regarded as of general utility to mankind. The development of the quantitative characters on which practical utility is founded owes very little to genetic analysis except in so far as i t has been demonstrated that i t depends on a definite gene complex. This is implicit in the study of the individual effects of Mendelian factors without the means of evaluating the mass effects of a large number of minor factors severally influencing the utility character. We would stress, however, that the study of the metrical characters is not only of utilitarian interest. The nature of the heritable