This is not about the -- ehem -- Good Folk and associated folklore, though there's quite a bit about them. (Such as vampiric fairy ladies, with a goblin child, who asked to warm their hands at a peasant woman's fire, and being invited in, would feed the goblin with the youngest peasant child's blood, fatally for the child, or the danger of telling strangers your name.)
Nor is it about Victorian thought, about evolution or divorce or race, or many other topics. It's about the collision of the two.
Folklorists who were quite certain that changelings sprung from an older race putting their wizened and unhealthy children in place of a plump and rosy Saxon baby, and others who regarded the deformed or ailing children to be in actuality, racial throwbacks. Folklorists who, agreeing that swan-maiden brides were a folk memory from prehistoric time, disagreed about whether the swan-maiden represented a superior or inferior race -- and others who disputed whether the magical brides who left on the violation of a prohibition were justified.
Quite a bit about ambiguous figures that draw on fairy lore. (Not always convincing argued, in my opinion.)
Discussions of the retreat of fairies and folklore about that.