For centuries, man has been successfully engaged in influencing the characteristics of productive livestock by selective breeding. The most spectacular results are those obtained regarding those characteristics which directly determine the yields of milk, meat, eggs, wool, etc. Unduly one-sided selection may result in untoward side-effects. As regards genetic influencing of fertility, constitution and resistance, the results obtained are less significant, though there would also appear to be possibilities in these cases. Genetic influencing may produce permanent improvement in a large number of cases. Efforts are occasionally made to achieve temporary improvement by biotechnical procedures. Moreover, there is a risk in the latter case of the hereditary disposition being masked by these procedures, which is bound to interfere with the selective breeding.