1. The history of the Danish system of progeny testing swine is traced briefly.

2. Changes in the average characteristics of the Danish swine since this system began in 1907 are shown in graphs.

3. The variance of six of these characteristics is analyzed, largely by means of correlations between litter mates, between half-sibs and between progeny tests of sire and of son, to find the extent to which individual variance in each characteristic can be attributed to the additive effects of genes.

4. A little less than half the individual variance in body length, thickness of back fat and thickness of belly can be thus ascribed to additive gene effects. Differences in rate of gain, yield of export bacon and in economy of gain are less highly hereditary, yet there seems to be in them enough additive gene variance to permit selection still to make distinct changes in the population for at least a few more generations. (Summary in table 11.)

5. Although the actual basis of the selections which the Danish farmers practice is not completely demonstrated, the figures from these progeny tests must have played a considerable part.

6. The Danish plan of progeny testing has been developed in such close connection with the economic peculiarities of Danish cooperative organizations that its operating principles might need much revision before it could be used in other countries. The biological principles involved, however, are the same everywhere and any people wishing seriously to improve the real economic productivity of their livestock, especially in characteristics which cannot be seen or measured until the animals are slaughtered, will find useful suggestions in this Danish model.