The structure of the eye has often been described as an evidence of design. There is an extremely interesting comparison of it with the photographic camera in Le Conte's 'Religion and Science,' pp. 20-33.
The whole reading public knows the masterly chapter on "The Machinery of Flight" in the Duke of Argyll's 'Reign of Law.'
The following writers treat at considerable length of the evidences of design to be traced in the constitution of the mind: Sir Matthew Hale in his 'Primitive Origination of Mankind;' Barrow in the seventh of his 'Sermons on the Creed;' Bentley in the second sermon of his 'Boyle Lecture;' Crombie in the second volume of his 'Natural Theology;' Lord Brougham in his 'Discourse on Natural Theology,' sect. iii., pp. 52-80; Turton's 'Natural Theology Considered,' pp. 65-160; Chalmers's 'Natural Theology,' Book III.; Buchanan's 'Faith in God,' pp. 213-231; Tulloch's 'Theism,' pp. 182-247; and Ulrici's 'Gott und Mensch.'
The phenomena of animal instinct are of themselves an inexhaustible source of instruction as to the Divine wisdom and goodness. "The spinning machinery which is provided in the body of a spider is not more accurately adjusted to the viscid secretion which is provided for it, than the instinct of the spider is adjusted both to the construction of its web and also to the selection of likely places for the capture of its prey. Those birds and insects whose young are hatched by the heat of fermentation, have an intuitive impulse to select the proper materials, and to gather them for the purpose. All creatures, guided sometimes apparently by senses of which we know nothing, are under like impulses to provide effectually for the nourishing of their young; and it is most curious and instructive to observe that the extent of provision which is involved in the process, and in the securing of the result, seems very often to be greater as we descend in the scale of nature, and in proportion as the parents are dissociated from the actual feeding or personal care of their offspring. The mammalia have nothing to provide except food for themselves, and have at first, and for a long time, no duty to perform beyond the discharge of a purely physical function. Birds have more to do--in the building of nests, in the choice of sites for these, and after incubation in the choice of food adapted to the period of growth. Insects, much lower in the scale of organisation, and subject to the wonderful processes of metamorphosis, have to provide very often for a distant future, and for successive stages of development not only in the young but in the _nidus_ which surrounds them. Bees, if we are to believe the evidence of observers, have an intuitive guidance in the selection of food which has the power of producing organic changes in the bodies of the young, even to the determination and development of sex, so that, by the administration of it, under what may be called artificial conditions, certain selected individuals can be made the mothers and queens of future hives. These are but a few examples of facts of which the whole animal world is full, presenting, as it does, one vast series of adjustments between bodily organs and corresponding instincts. But this adjustment would be useless unless it were part of another adjustment--between the instincts and perceptions of animals and those facts and forces of surrounding nature which are related to them, and to the whole cycle of things of which they form a part. In those instinctive actions of the lower animals which involve the most distant and the most complicated anticipations, it is certain that the prevision involved is a prevision which is not in the animals themselves. They appear to be, and beyond all doubt really are, guided by some simple appetite, by an odour or a taste, and, in all probability, they have generally as little consciousness of the ends to be subserved as the suckling has of the processes of nutrition. The path along which they walk is a path which they did not engineer. It is a path made for them, and they simply follow it. But the propensities and tastes and feelings which make them follow it, and the rightness of its direction towards the ends to be attained, do constitute an adjustment which may correctly be called mechanical, and is part of a unity which binds together the whole world of life, and the whole inorganic world on which living things depend."--Duke of Argyll on Animal Instinct (Cont. Rev., July 1875).