Distinction between the sterility of first crosses and of hybrids Sterility various in degree, not universal, affected by close in terbreeding, removed by domestication — Laws governing the sterility of hybrids — Sterility not a special endowment, but incidental on other difierences, not accumulated by natural selection — Causes of the sterility of first crosses and of hybrids — Parallelism between the effects of changed conditions of life and of crossing — Dimorphism and trimorphism — Fertility of varieties when crossed and of their mongrel ofispring not uni versal — Hybrids and mongrels compared independently of their fertility — Summary.