Friedrich Engels (1820-1895) was a German political theorist and developer of Marxist theory, who financially supported Marx for many years. After Marx's death Engels edited the second and third volumes of "Das Kapital."
This book (originally published in 1884) deals with many topics about which Marx never wrote. It was strongly influenced by Lewis Henry Morgan's famous book, Ancient society (1877).
Modern feminists may not agree with his assertion that matriarchy, "in the sense of power held by women over men comparable to that later held by men over women," had never existed (pg. 35). However, he enthusiastically agrees with Morgan that matrilineal clans, rather than the "family," was the precursor of later patriarchal clans, and that a "conspiracy of silence" is the cause of this not being more widely known. (Pg. 83-84)
He states that "The overthrow of mother right was the world historical defeat of the female sex." Woman became degraded and "reduced to servitude." (Pg. 120-121) By contrast, the modern industrial family is founded on the "open or concealed domestic slavery of the wife, and modern society is a mass composed of these individual families as its molecules." (Pg. 137)
This book in particular was very influential on the "Second Stage" women's movement in the late 1960s and 1970s.