Natural selection can also bring about changes at every age, for the elimination of individuals begins from the egg, and any kind of egg which is in some way better able to escape elimination will transmit its useful characters to its descendants, because the resulting young animals will thus more frequently reach full development than the young from other eggs. In the same way, at every succeeding stage of development, every character favourable to the preservation of the individual will be maintained and intensified.
We see from all this that natural selection is vastly more powerful than artificial selection by Man. In the latter, only one character at a time can be caused to change, while natural selection may influence a whole group of characters at the same time, as well as all the stages of development. Through the weeding out of the individuals which are annually exterminated, it is always on an average the 'fittest' which survive, that is to say, those which have the greatest number of bodily parts and rudiments of parts in the fittest possible condition of development at every stage. The longer this process of selection continues, the smaller will be the deviations of the individual from this standard, and the more minute will be the differences of fitness determining which is to be eliminated and which is to survive to reproduce its characteristics. In the immeasurable periods of time which are at the disposal of natural selection, and in the inestimable numbers of individuals on which it may operate, lie the essential causes of superiority of natural selection over the artificial selection of Man.
To sum up briefly: Natural selection depends essentially on the cumulative augmentation of the most minute useful variations in the direction of their utility; only the useful is developed and increased, and great effects are brought about slowly through the summing up of many very minute steps. Natural selection is a self-regulation of the species which secures its preservation; its result is the ceaseless adaptation of the species to its life-conditions. As soon as these vary, natural selection changes its mode of action, for what was previously the best is now no longer so; parts that before had to be large must now perhaps be small, or vice versa; muscle-groups which were weak must now become strong, and so on. The conditions of life are, so to speak, the mould into which natural selection is continually pouring the species anew.
But the philosophical significance of natural selection lies in the fact, that it shows us how to explain the origin of useful, well-adapted structures purely by mechanical forces and without having to fall back on a directive force. We are thus for the first time in a position to understand, in some degree, the marvellous adaptation of the organism to an end, without having to call to our aid any supernaturally intrusive force on the part of the Creator. We understand now how, in a purely mechanical way, through the forces always at work in nature, all forms of life must conform to, and adapt themselves precisely to the conditions of their life, since only the best possible is preserved, and everything less good is continually being rejected.
Before I go on to expound in detail the phenomena which we refer to natural selection, I must briefly state that Darwin did not ascribe to natural selection by any means all the changes which have taken place in organisms in the course of time. On the one hand, he ascribed a not inconsiderable importance to the correlated variations we have already mentioned; still more, however, he relied on the direct influence of altered conditions of life, whether these consist in climatic and other changes in the environment, or in the assumption of new habits, and the increased or diminished use of individual parts and organs thereby induced. He recognized the principle so strongly emphasized by Lamarck, of use and disuse as a cause of heritable increase or decrease of the exercised or neglected part, though he did so with a certain reserve. I shall return later to these factors of modification, and shall then attempt to show that these too are to be referred to processes of selection, which are, however, of a different order from the phenomena which the Darwin-Wallace principle of natural selection serves to interpret. But, in the first instance, it appears to me to be necessary to show how far the Darwin-Wallace interpretation will suffice, and in the next lectures we shall occupy ourselves with this question exclusively.
LECTURE IV
THE COLORATION OF ANIMALS AND ITS RELATION TO THE PROCESSES OF SELECTION