It is very necessary to understand exactly what the theory of natural selection is and is not; because champions of the Faith, even when believing in Evolution, base some of their arguments on the alleged collapse of the Darwinian theory. Thus, in Present-day Rationalism Critically Examined, the Rev. Professor George Henslow affirms that, while the theory of Evolution stands on an impregnable basis, Haeckel's Monism and Rationalistic agnosticism are based on Darwin's doctrine of natural selection, and he enters upon an elaborate argument--covering sixty pages of his book--to show that the origin of species by means of natural selection is false, and that the primary cause of Evolution is the definite action of the environment, combined with the adaptive powers of the living organism. Such arguments, coming from a clergyman having scientific attainments, are likely to impress the average Christian reader and confuse the main issue. Natural selection is "the action of the environment" (see The Origin of Species, chap. iv.), and even if it were not, and if natural selection (or elimination) were not the primary cause, the doctrine of the action of environment will suit the Monist just as well. Regarding the minor, but not unimportant, part played by sexual selection, Darwin writes: "For my own part, I conclude that of all the causes which have led to the differences in external appearance between the races of men, and to a certain extent between man and the lower animals, sexual selection has been by far the most efficient" (Descent of Man, ed. 1871, ii., 367). Scientists who are advocates of the Christian cause are not always as candid as one could wish. While the Church cited Sir Richard Owen "as an authority against the Darwinian theory, especially in its application to man's descent, there remained in the memory of his brother savants his lack of candour in never withdrawing the statement made by him, and demonstrated by Huxley as untrue, that the hippocampus minor in the human brain is absent from the brain of the ape." (See p. 172 of Mr. Clodd's Pioneers of Evolution. See also remarks by Sir Charles Lyell, pp. 485 and 486 of his work, Antiquity of Man. On p. 290 he further tells us that "we may consider the attempt to distinguish the brain of man from that of the ape on the ground of newly-discovered cerebral characters, presenting differences in kind, as virtually abandoned by its originator.") P. 205, lines 18-20.--That there are not more links missing is due principally to the discovery of fossil remains.