4 Pages in Charles Darwin's Library associated with "mental states - concentration"

Naturgeschichte der Saeugethiere von Paraguay
By: Rengger, Johann Rudolph.
Publication info: Basel, Schweighauser, 1830
Pages: Page 165 Page 173
Wissenschaftliches System der Mimik und Physiognomik
By: Piderit, T.
Publication info: Detmold, Klingenberg, 1867
Pages: Page 46 Page 48