Charles Darwin - Descent of Man, extract Vol. 2, Part 2, 325-326 lyrics

There can be little doubt that the greater size and strength of man, in comparison with woman, together with his broader shoulders, more developed muscles, rugged outline of body, his greater courage and pug- nacity are all due in chief part to inheritance from some early male progenitor, who, like the existing anthropoid apes, was thus characterised. These cha- racters will, however have been preserved or even augmented, during the long ages whilst man was still in a barbarous condition, by the strongest and boldest men having succeeded best in the general struggle for Life, as well as in securing wives, and thus having left a large number of offspring. It is not probable that the greater strength of man was primarily acquired through the inherited effects of his having worked harder than woman for his own subsistence and that of his family; for the women in all barbarous nations are compelled to work at least as hard as the men. With civilised people the arbitrament of battle for the possession of the women has long ceased; on the other hand, the men, as a general rule, have to work harder than the women for their mutual subsistence; and thus their greater strength will have been kept up.