Let me now trace these principles as sources of action in Darwin’s work or theory.  Let us see how much there is of design acting to produce a foreseen end, and thus proving a reasoning and self-conscious Creator; and how much of mere blind power acting without rational design, or without a specific purpose or conscious foresight.  Mr. Darwin has specified in a most clear and unmistakable manner the operation of his three great powers, or rather, the three great laws by which the organic power of life acts in the formation of an eye. (See p. 169.) Following the method he has pointed out, we will take a number of animals of the same species, in which the eye is not developed.  They may have all the other senses, with the organs of nutrition, circulation, respiration, and locomotion.  They all have a brain and nerves, and some of these nerves may be sensitive to light; but have no combination of retina, membranes, humors, etc., by which the distinct image of an object may be formed and conveyed by the optic nerve to the cognizance of the internal perception, or the mind.  The animal in this case would be merely sensible of the difference between light and darkness.  He would have no power of discriminating form, size, shape, or color, the difference of objects, and to gain from these a knowledge of their being useful or hurtful, friends or enemies.  Up to this point there is no appearance of necessity upon the scene.  The billiard-balls have not yet struck together, and we will suppose that none of the arguments that may be used to prove, from this organism, thus existing, that it could not have come into form and being without a creator acting to this end with intelligence and design, are opposed by anything that can be found in Darwin’s theory; for, so far, Darwin’s laws are supposed not to have come into operation.  Give the animals, thus organized, food and room, and they may go on, from generation to generation, upon the same organic level.  Those individuals that, from natural variation, are born with light-nerves a little more sensitive to light than their parents, will cross or interbreed with those who have the same organs a little less sensitive, and thus the mean standard will be kept up without any advancement.  If our billiard-table were sufficiently extensive, i. e., infinite, the balls rolled from the corners would never meet, and the necessity which we have supposed to deflect them would never act.

The moment, however, that the want of space or food commences natural selection begins.  Here the balls meet, and all future action is governed by necessity.  The best forms, or those nerves most sensitive to light, connected with incipient membranes and humors for corneas and lenses, are picked out and preserved by natural selection, of necessity.  All cannot live and propagate, and it is a necessity, obvious to all, that the weaker must perish, if the theory be true.  Working on, in this way, through countless generations, the eye is at last