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

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