Charles Darwin (1809-1882) who wrote the book titled, 'The Origin of Species by Means of Natural Selection or the Preservation of Favoured Races in the Struggle for Life' published in 1859, had doubts about his own theory.
Darwin said before his book was published, "You will be greatly disappointed (by the forth coming book), it will be grievously too hypothetical. It will very likely be of no other service than collocating some facts; though I myself think I see my way approximately on the origin of the species. But alas, how frequent, how almost universal it is in an author to persuade himself of the truth of his own dogmas" (Charles Darwin, 1858, in a letter to a colleague regarding the concluding chapters of his 'Origin of Species', As quoted in 'John Lofton's Journal', The Washington Times 8 February 1984).
Quotes by Charles Darwin after his book was published in 1859 :
"For I am well aware that scarcely a single point is discussed in this volume on which facts cannot be adduced, often apparently leading to conclusions directly opposite to these at which I have arrived. A fair result can be obtained only by fully stating and balancing the facts and arguments on both sides of each question; and this is here impossible" (Charles Darwin , 1859, Introduction to Origin of Species, p.2, Also quoted in 'John Lofton's Journal', The Washington Times, 8 February 1984).
"To suppose that the eye with all its inimitable contrivances for adjusting the focus to different distances for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest degree" (Charles Darwin in the Origin of Species, J. M. Dent & Sons Ltd., London , 1976, p. 167).
Darwin frequently commented in private letters that he recognized that there was no evidence for his theory, and that it could destroy the morality of the human race.
"Long before the reader has arrived at this part of my work, a crowd of difficulties will have occurred to him. Some of them are so serious that to this day I can hardly reflect on them without some degree becoming staggered" (Charles Darwin, Origin of Species, 1860, p. 178; quoted from Harvard Classics, 1909 edition , vol. 11).
"Often a cold shutter has run through me, and I have asked myself whether I may have not devoted myself to a phantasy" (Charles Darwin, Life and Letters, 1887, vol. 2, p. 229).
"Not one change of species into another is on the record we cannot prove that a single species has been changed" (Charles Darwin, life and Letters).
Darwin needed to read these passages of Scripture : "For the invisible things of him from the creation of the world are clearly seen, being understood by the things that are made, even his eternal power and Godhead; so that they are without excuse: because that, when they knew God, they glorified him not as God, neither were thankful; but became vain in their imaginations, and their foolish heart was darkened. Professing themselves to be wise, they became fools" (Romans 1: 20-22).