1. Charles Darwin, On the Origin of Species, facsimile of first edition (Cambridge, Mass.: Harvard University Press, 1964), pp. 80?81.
2. Morse Peckham, ed., The Origin of Species, by Charles Darwin: A Variorum Text (Philadelphia: University of Pennsylvania Press, 1959), p. 22.
3. Charles Darwin, The Variation of Animals and Plants under Domestication (London: Murray, 1868), p. 17.
4. Morse Peckham, ed., The Origin of Species, by Charles Darwin: A Variorum Text (Philadelphia: University of Pennsylvania Press, 1959), p. 164.
5. Morse Peckham, ed., The Origin of Species, by Charles Darwin: A Variorum Text (Philadelphia: University of Pennsylvania Press, 1959), p. 145.