Note 57 — THE IMMUTABILITY OF THE SPECIES.[57] I.
Reading edition
The Origin of Species. By Charles Darwin, A.M., F.R.S., etc. Fourth edition.
The Variation of Animals and Plants under Domestication. By Charles Darwin, A.M., F.R.S., etc. Two volumes, 8vo. London: John Murray. 1868.
The Principles of Biology. Vol. I. By Herbert Spencer. London: Williams & Norgate. 1864.