"Zoology has a principle of reasoning which is peculiar to it, and which it employs with advantage on many occasions: this is the principle of the conditions of existence, vulgarly called the principle of final causes. As nothing can exist if it do not combine all the conditions which render its existence possible, the different parts of each being must be co-ordinated in such a manner as to render the total being possible, not only in itself, but in its relations to those which surround it; and the analysis of these conditions often leads to general laws, as clearly demonstrated as those which result from calculation or from experience."
 "The disciples of the former of the two schools express their tenets by the phrases unity of plan, unity of composition; and the more detailed developement of these doctrines has been termed the Theory of Analogues, by Geoffroy Saint-Hilaire, who claims this theory as his own creation. According to this theory, the structure and functions of animals are to be studied by the guidance of their analogy only; our attention is to be turned, not to the fitness of the organization for any end of life or action, but to its resemblance to other organizations by which it is gradually derived from the original type."
 "It is generally acknowledged that all organic beingss have been formed on two great laws--Unity of Type, and the Conditions of Existence. By unity of type is meant that fundamental agreement in structure which we see in organic beings of the same class, and which is quite independent of their habits of life. On my theory, unity of type is explained by unity of descent. The expression of conditions of existence, so often insisted on by the illustrious Cuvier, is fully embraced by the principle of natural selection. For natural selection acts by either now adapting the varying parts of each being to its organic and inorganic conditions of life; or by having adapted them during past periods of time: the adaptations being aided in many cases by the increased use or disuse of parts, being affected by the direct action of the external conditions of life, and subjected in all cases to the several laws of growth and variation. Hence, in fact, the law of the Conditions of Existence is the higher law; as it includes, through the inheritance of former variations and adaptations, that of Unity of Type."
 "Perhaps the most remarkable service to the philosophy of Biology rendered by Mr. Darwin is the reconciliation of Teleology and Morphology, and the explanation of the facts of both, which his views offer. The teleology which supposes that the eye, such as we see it in man, or one of the higher vertebrata, was made with the precise structure it exhibits, for the purpose of enabling the animal which possesses it to see, has undoubtedly received its death-blow. Nevertheless, it is necessary to remember that there is a wider teleology which is not touched by the doctrine of Evolution, but is actually based upon the fundamental proposition of Evolution."
 "Let us recognize Darwin's great service to Natural Science in bringing back to it Teleology; so that instead of Morphology versus Teleology, we shall have Morphology wedded to Teleology."
 "What you say about Teleology pleases me especially, and I do not think any one else has ever noticed the point. I have always said you were the man to hit the nail on the head."
 Georges Cuvier, Règne Animal; quoted in William Whewell, History of the Inductive Sciences Bk. 17, ch. 8, sect. 3.
 William Whewell, History of the Inductive Sciences Bk. 17, ch. 8, sect. 1.
 Charles Darwin, The Origin of Species, ch. 6.
 T. H. Huxley, "Genealogy of Animals"; quoted in The Autobiography of Charles Darwin and Selected Letters, Francis Darwin, ed., ch. xvi.
 Asa Gray, "Charles Darwin"; quoted in The Autobiography of Charles Darwin and Selected Letters, Francis Darwin, ed., ch. xv.
 Charles Darwin, letter to Asa Gray; quoted in The Autobiography of Charles Darwin and Selected Letters, Francis Darwin, ed., ch. xv.