A 1908 Stanford University monograph reporting Vernon Kellogg's breeding experiments on inheritance in the mulberry silkworm (Bombyx mori), done with Ruby Green Smith. It covers Mendelian inheritance of larval color patterns and cocoon colors, strain idiosyncrasies, double mating, fluctuating variations such as wing pattern and venation, double cocooning, sports, and fertility. A section on the economic value of the findings for silk culture and an appendix of abstracts of the author's earlier silkworm papers follow. Plates show larvae, moths and cocoons of different races.