organ to the system which needs and has produced it for ends of maintenance. In the degree that the organic parts have special activities imposed upon them, in that degree do they become modified by those activities, and therefore adapted to the doing of those activities. An incipient leg, tail, fin, or eye, or any other organ, impelled to a particular thing, to act in a particular way, will do that thing more perfectly, will act in that way more completely and efficiently, with every repetition of the acting, for the reason that the parts of the organ and of the organism become with every such repetition, up to a certain natural limit, more and more adapted to the doing of that particular thing, to acting in that particular way; and this is why use is said to improve organs. The parts of such a system rearrange themselves in such a way as in every case continually to lessen the resistance offered within the system to the acting needed for each particular end. Just as from the simple foot of the snail to the leg of the vertebrates, so from the membrane of the worm sensitive to light, from the ocelli of insects and marine organisms to the highly developed eye of mammals, or from the incipient forms of internal organs to the more perfect and efficient forms of such organs, there have been progressive stages of ascent in the economy of energy with which given ends have been reached, as well as improvement of the ends themselves. In the case of organs, as in that of tools, the improvement has been made possible by a finer sense on the parts of the organism acting of the direction of least resistance, a finer self-adaptation by that organism to the environment, and a more perfect reaching of more perfect ends as the result of that adaptation.
We now see that the advantage gained by the perfection of any given organ or appliance necessary to maintenance is the advantage which, given the end to be reached, is gained by the saving of energy in the reaching of that end—that, in other words, the inducement to the improvement of any given organ is the saving of the energy spent in reaching, with the aid of that organ, the general end of maintenance. The more perfect are the appliances of the organic system, the more easily and completely does that system reach its end of maintenance; hence the gradual improvement of the organs with which maintenance is accomplished is so much movement in the direction of the least resistance. Thus the eye is gradually perfected in successive organisms, not because there is anywhere any foreknowledge that a given configuration of parts will lead to so highly useful an appliance as the organ of vision, but because, given the impulsion to maintenance and the general conditions of organic life, all structural changes leading away from the development of an organ like the eye would involve loss of energy to the organism in the reaching
