33 8 POPULAR SCIENCE MONTHLY. 



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 com- 

 pletely 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 act- 

 ing in that particular way ; and this is why use is said to im- 

 prove 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 particu- 

 lar 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 econ- 

 omy 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 appli- 

 ances of the organic system, the more easily and completely does 

 that system reach its end of maintenance ; hence the gradual im- 

 provement 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 con- 

 figuration of parts will lead to so highly useful an appliance as 

 the organ of vision, but because, given the impulsion to mainte- 

 nance 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 



