There is one thinking sensible principle, intimately allied to one kind of organic matter—have & which thinking principle seems to be given a assumed according to a more extended relations of the individuals, whereby choice with memory or reason? is necessary—which is modified into endless forms bearing a close relation in degree & kind to the endless forms of the living beings.
" Notebook C http://darwin-online.org.uk/EditorialIntroductions/vanWyhe_notebooks.html" (1838) page 210e http://darwin-online.org.uk/content/frameset?pageseq=186&itemID=CUL-DAR122.-&viewtype=side
quoted in [Creativity, Psychology and the History of Science, 2005, Howard E., Gruber, Katja, Bödeker, Springer, 9781402034916, 142, http://books.google.com/books?id=MDbruQKIu-wC&pg=PA142]
also quoted in [The Cambridge Companion to Darwin, 2003, Robert J., Richards, Darwin on mind, morals, and emotions, Johnathan, Hodge, Gregory, Radick, Cambridge University Press, 9780521777308, 95-96, http://books.google.com/books?id=uj_by_Sg3LkC&pg=PA95]
Other letters, notebooks, journal articles, recollected statements