"Letter 5140 – Wallace, A. R. to Darwin, C. R., 2 July 1866". Darwin Correspondence Project. Retrieved 12 January 2010. "Letter 5145 – Darwin, C. R. to Wallace, A. R., 5 July (1866)". Darwin Correspondence Project. Retrieved 12 January 2010.
^ "Herbert Spencer in his Principles of Biology of 1864, vol. 1, p. 444, wrote: 'This survival of the fittest, which I have here sought to express in mechanical terms, is that which Mr. Darwin has called "natural selection", or the preservation of favoured races in the struggle for life.'" Maurice E. Stucke, Better Competition Advocacy, retrieved 29 August 2007, citing HERBERT SPENCER, THE PRINCIPLES OF BIOLOGY 444 (Univ. Press of the Pac. 2002.)
darwinproject.ac.uk
"Letter 5140 – Wallace, A. R. to Darwin, C. R., 2 July 1866". Darwin Correspondence Project. Retrieved 12 January 2010. "Letter 5145 – Darwin, C. R. to Wallace, A. R., 5 July (1866)". Darwin Correspondence Project. Retrieved 12 January 2010.
^ "Herbert Spencer in his Principles of Biology of 1864, vol. 1, p. 444, wrote: 'This survival of the fittest, which I have here sought to express in mechanical terms, is that which Mr. Darwin has called "natural selection", or the preservation of favoured races in the struggle for life.'" Maurice E. Stucke, Better Competition Advocacy, retrieved 29 August 2007, citing HERBERT SPENCER, THE PRINCIPLES OF BIOLOGY 444 (Univ. Press of the Pac. 2002.)
