Charles R. Darwin. Three books: The Origins of Species, 1936 (WorldCat); Animals and Plants Under Domestication, Vol. 1, 1900 (WorldCat); Animals and Plants Under Domestication, Vol. 2, 1868 (WorldCat).
"It has been shown from many facts that the largest amount of life can be supported on each area, by great diversification or divergence in the structure and constitution of its inhabitants."
"In scientific investigations it is permitted to invent any hypothesis, and if it explains various large and independent classes of facts it rises to the rank of a well-grounded theory."
"How inexplicable is the similar pattern of the hand of a man, the foot of a dog, the wing of a bat, the flipper of a seal on the doctrine of independent acts of creation!"
"...I believe in the truth of the theory {of Natural Selection} because it collects, under one point of view, and gives a rational explanation of, many apparently independent classes of facts."
"From the strong principle of inheritance, any selected variety will tend to propagate its new and modified form."
"Furthermore, I am convinced that Natural Selection has been the most important, but not exclusive, means of modification."
"Even when we are quite alone, how often do we think with pleasure or pain of what others think of us?"
"At the moment of action, man will no doubt be apt to follow the stronger impulse; and though this may occasionally prompt him to the noblest deeds, it will more commonly lead him to gratify his own desires at the expense of other men."
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