Investigations currently reported by Dr. L. T. Webster1 from the Rockefeller Institute indicate that selective breeding of animals resistant to one microbic disease may inadvertently produce animals hypersusceptible to other infections. The geneticists have pointed out many examples of similar "linkage" between hereditary functions. They would merely assume that hereditary microbic resistance involves a multiplicity of unit factors carried by different chromosomes. The observations have, however, immediate practical interest in their bearing on problems of animal husbandry.
Webster's studies were made on mice reared and maintained for nearly two decades under the most scrupulous hygienic conditions. During this time the herds of mice had never been exposed to microbic infections. The most painstaking examinations had failed to reveal "carrier" conditions. On routine exposure of these herds to standard doses of mouse typhoid bacillus, the mice showed on an average 37.4 per cent herd mortality. The general mouse population, therefore,