[52] For the scientific basis of this spiritual law the following works may be consulted:--
"The Origin of Species." By Charles Darwin, F.R.S. London: John Murray. 1872.
"Degeneration." By E. Ray Lankester, F.R.S. London: Macmillan. 1880.
"Der Ursprung der Wirbelthiere und das Princip des Functions Wechsels." Dr. A. Dorhn. Leipzig: 1875.
"Lessons from Nature." By St. George Mivart, F.R.S. London: John Murray. 1876.
"The Natural Conditions of Existence as they Affect Animal Life." Karl Semper. London: C. Kegan Paul & Co. 1881.