Johnson, Roswell H., "The Malthusian Principle and Natural Selection," American Naturalist, XLVI (1912), pp. 372-376.

Karl Pearson, The Groundwork of Eugenics, p. 25, London, 1912.

"Let p be the chance of death from a random, not a constitutional source, then 1-p is the chance of a selective death in a parent and 1-p again of a selective death in the case of an offspring, then

(1-p)² must equal about ⅓, = .36, more exactly ∴  1-p = .6 and p = .40. In other words, 60% of the deaths are selective."

Archiv f. Rassen-u. Gesellschafts Biologie, VI (1909), pp. 33-43.