Charles Darwin 
Origin of Species 
By Means of Natural Selection, or the Preservation of Favored Races in the Struggle for Life

Whilst the greatest effort has been made to ensure the quality of this text, due to the historical nature of this content, in some rare cases there may be minor issues with legibility. 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.