Page:Evolution of Life (Henry Cadwalader Chapman, 1873).djvu/209

lions of years would have passed away in the formation of the Aqueous Rocks. So that, while admitting the looseness of the data, we feel that we are less likely to err in assuming an amount of time practically unlimited for the development of life than if we attempt to fix a definite limit. It is often asked, How could the instincts of animals and man arise through a process like Natural Selection? The manner in which the young uneducated Pointer acquired the instinct of pointing explains the origin of all instincts. The original Pointer was taught to point, and in the course of generations, this peculiarity being inherited, the pointing became instinctive. All of our ideas have arisen in the same way, mind being the impressions of the brain derived from the external world through the medium of the senses. If there really be what metaphysicians call "a priori ideas," originally they have been derived a posteriori; that is, these ideas were originally derived by the parent organism, and later inherited by posterity. Finally, to many persons, complex structures like the eye and ear are insuperable objections to the theory of Natural Selection, it seeming incredible to those who are unacquainted with Comparative Anatomy that such organs could have arisen through the Survival of the Fittest caused by the action of a blind, objectless, working Nature. The eye is usually studied in a most developed state, as in man, for example; but the visual organ of some of the lower animals is only a pigment spot, more or less sensitive to the rays of light, but incapable of forming the image of an external object. As we ascend in the scale of life, we notice there is added to this pigment spot a sensitive nerve, and as we gradually progress there appears the lens, a light-refracting organ, which, collecting the rays of light in a focus, delineates the image of an external object. Still more highly organized animals exhibit additional media of service in transmitting the light, a complex retina for receiving the