Selection for Small and Large Body Size in the House Mouse

H E animal breeder's main and typical problems concern economic char-T acters, such as meat or wool production, yield and fat content of milk, number and size of eggs.These are complex, quantitative characters, multifactorial and strongly influenced by environmental conditions.The great majority of the fundamental anatomical, physiological and taxonomic characters, and basic processes like growth and reproduction, are of this class.For man to change and guide improvements in such quantitative characters usually requires continuous, gradual selection of large numbers of individuals over a long series of generations.This is practically a return to DARWIN'S artificial selection by slow increase of small variations, which must be credited with the principal improvement of domestic animals over past centuries, and appears to be the chief hope of further improvements to come.The main object of this paper is to describe further the primary and secondary effects of a form of mass selection in an experiment designed to create for laboratory uses from one foundation stock both a small and large race of house mice.Progress reports (MACARTHUR 1944, MACARTHUR andCHIASSON 1945) gave the procedures and results for the first eight generations of the selections.The series is now extended to cover at least 21 generations, averaging three a year over seven years.The environment was kept similar for all the mice at any one time by having them share the same quarters, feed, care, etc., except that sibs were fed and reared by their own mothers.Maternal care itself varied greatly. SOURCES OF VARIATIONGenetic variations, the necessary raw materials for the changes in size, were presumably supplied by numerous multiple factors in nearly all chromosomes and segments, partly neutralizing each other's effects, some promoting, some retarding general growth.The field of variation was probably widened by intercrossing six inbred strains of mice of average size but of various colors.(GOODALE'S (1938) successful size increases in albino mice began without * Part of the cost of the accompanying figures is paid by the GALTON AND MENDEL MEMORIAL