The breeding changes in salmon have attracted much attention from scientists and a considerable number of papers have been published on this subject (see Appendix), but the factors upon which the degree of development of these changes depends have not been studied.

One of these factors is the size of the breeding fish. The influence of this factor is great, and no clear understanding of the breeding characters is possible without such a study. Apparently all the various changes in the breeding salmon, growth of bones, cartilages, and teeth, changes in scales, thickening of the skin, changes in muscles and in colouring, etc., depend upon the size of the breeding fish.

A preliminary study could not detect any difference between a ripe salmon parr (male 87 mm. long) and other parr salmon (with juvenile genitals) of the same age, locality, and size. In a male grilse 532 mm. long the breeding characters are obvious; in a male 1003 mm. long these characters are developed to an extreme degree.

The present study is very limited. Only the skulls of salmon were studied from a small series of breeding specimens, eleven males from 532 mm. to 1003 mm. long and six females from 602 mm. to 910 mm. long. (Details of the material are given above.)

The results of this study can be thus summarized.