There is an echo of 19th century biology's recapitulation theory in the last paragraph of Hesse's own preface to Demian.

For further information on the theory read Stephen Jay Gould: Ontogeny and Phylogeny, Harvard U Press 1977. A folksier version by the same author in  Natural History (Dec 87): "Freud's Phylogenetic Fantasy". Another essay by Gould, touching on the same subject, appeared in the March 2000 issue  of Natural History: "Abscheulich! (Atrocious!)"

References to evolutionary theory, particularly "adaptation", are scattered throughout Demian, most prominently in the Pistorius chapters, 5 & 6.

"Beatrice". For a glimpse of what she might look like see Dante Gabriel Rosetti's famous painting Beata Beatrix of 1863 (Tate Gallery, London).

A comprehensive and enlightening treatment of "the woman on a pedestal" can be found in
Marina Warner, Alone of all her Sex: The Myth and the Cult of the Virgin Mary.
I can't resist adding a German title here:
Eva Schirmer, Eva - Maria: Rollenbilder von Maennern fuer Frauen.

For a background on the Christian tradition of repressed sexuality, a topic that surfaces in every major work of Hesse's, consult
Peter Brown, The Body and Society: Men, Women and Sexual Renunciation in Early Christianity.
Also: Elaine Pagels, Adam, Eve, and the Serpent.
For my own tongue-in-cheek condensed history of human sexuality see "Why Sex?" in The Cornell Daily Sun of 5-2-88.

And, more broadly, for a renewed emphasis on the importance of childhood in human development see the pioneering work of one of Hesse's contemporaries, the Swedish feminist and pedagogue

Ellen Key, The Century of the Child (1900). It draws heavily on Rousseau and Goethe and devotes a chapter to "The School of the Future".

There are multiple echoes of the renewed concern over the fate of children in the literature of Hesse's contemporaries. We will read one of them, Rilke's radical re-interpretation of the parable of the Prodigal Son at the end of his only novel, Malte Laurids Brigge. It begins with the lapidary  statement: "It will be difficult to persuade me that the story of the prodigal son is not the legend of one who did not want to be loved."

Interested in "Child Care among the Insects?" Read all about it in the January 1999 issue of Scientific American.

There is a striking parallel to Sinclair's story of the stolen apples, fictitious of course, in St. Augustine's Confessions. In Book  2 he deals at length with the theft (real in his case) of pears that were subsequently dumped before pigs. Why? There seemed to be no motive, it appeared utterly gratuitous. Augustine offers in the end the same rationale that motivated Sinclair: need of fellowship and group bonding.

Read Garry Wills, Saint Augustine (Viking Penguin, 1999) p. 10 ff. for some interesting speculations on the subject.