It is very interesting to separate the two arguments which occur interwoven in the “Origin”--the argument for evolution and the argument for natural selection. The paramount importance of Darwin’s contributions to the evidences of organic evolution are often forgotten in the brilliant theory which he believed to supply the motive cause of descent with modification. Organic evolution had been held to be true by certain thinkers during many centuries; but not only were its adherents entirely without a sufficient motive cause, but their evidences of the process itself were erroneous or extremely scanty. It was Darwin who first brought together a great body of scientific evidence which placed the process of evolution beyond dispute, whatever the causes of evolution may have been. And accordingly we find that, even at first, natural selection was attacked far more generally than the doctrine of descent with modification.
In Chapter I., Variation under Domestication and man’s power of selection in forming breeds of animals and plants are discussed; in Chapter II., Variation under Nature; in Chapter III., the Struggle for Existence; in Chapter IV., which Darwin, in writing to his publisher, called “the Keystone of my Arch,” the three preceding chapters are carried to their conclusion, and the operation of natural selection is explained and discussed. Hence, these four chapters deal almost exclusively with this process.
Chapter V. has for its subject the Laws of Variation, and explains causes of modification (external conditions, use and disuse, correlation, reversion, etc.) other than natural selection and the relation of the latter to the former.
In Chapter VI. difficulties are considered--partly those in the way of a belief in evolution and partly those which, at first sight, seem to be incapable of explanation on the theory of natural selection. Chapter VII. deals with a special difficulty of the latter kind, viz. Instinct, and shows how it can be accounted for by natural selection acting upon variation, although allowing some weight to the inheritance of habit. Chapter VIII. deals with Hybridism, the sterility of first crosses and of hybrids being considered as an objection to the doctrine of Descent with Modification. Chapter IX. treats of the Imperfection of the Geological Record as the explanation of the apparently insufficient evidence of evolution during past ages. Chapter X., on the Geological succession of Organic Beings, shows that, allowing for this Imperfection of Record, the facts brought to light by Geology support a belief in evolution and in some cases even in natural selection. Hence these five chapters deal partly with difficulties in the way of an acceptance of organic evolution and partly with those encountered by natural selection.