Blackwell continued her career until domestic responsibilities and her disagreement with many aspects of the women's rights movement caused her to discontinue lecturing. Writing became her new outlet for asserting social change for women; in her works she encouraged women to seek out masculine professions and men to share in household duties, yet she retained the belief that women's primary role was care of the home and family. Inspired by yet critical of the writings of Charles Darwin and Herbert Spencer, who she considered to be the most influential men of her day, Blackwell published several works in the fields of theology, science and philosophy. She believed both Darwin and Spencer employed a tainted version of the scientific method, one that embraced a solely masculine vantage point. Blackwell instead asserted that in order to understand women in society, women themselves ought to conduct the study of women, which Blackwell termed the "science of Feminine Humanity." Perhaps her most notable work was published in 1875, "The Sexes Throughout Nature", in which she presented a quasi-scientific theory arguing that the sexes are different but equal by way of natural evolution. She knew she would be considered presumptuous for criticizing evolutionary theory, but wrote that "However great the disadvantages under which we are placed, these will never be lessened by waiting." Darwin had written a letter to her in 1869, thanking her for a copy of her book, "Studies in General Science". She also wrote a novel, "The Island Neighbors", in 1871, and a collection of poetry, "Sea Drift", in 1902.