It is good thus to try in our imagination to give any form some advantage over another.  Probably in no single instance should we know what to do, so as to succeed.  It will convince us of our ignorance on the mutual relations of all organic beings; a conviction as necessary, as it seems to be difficult to acquire.  All that we can do, is to keep steadily in mind that each organic being is striving to increase at a geometrical ratio; that each at some period of its life, during some season of the year, during each generation or at intervals, has to struggle for life, and to suffer great destruction.  When we reflect on this struggle, we may console ourselves with the full belief, that the war of nature is not incessant, that no fear is felt, that death is generally prompt, and that the vigorous, the healthy, and the happy survive and multiply.

CHAPTER 4.

Natural selection.

Natural Selection:  its power compared with man’s selection, its power on characters of trifling importance, its power at all ages and on both sexes.  Sexual Selection.  On the generality of intercrosses between individuals of the same species.  Circumstances favourable and unfavourable to Natural Selection, namely, intercrossing, isolation, number of individuals.  Slow action.  Extinction caused by Natural Selection.  Divergence of Character, related to the diversity of inhabitants of any small area, and to naturalisation.  Action of Natural Selection, through Divergence of Character and Extinction, on the descendants from a common parent.  Explains the Grouping of all organic beings.

How will the struggle for existence, discussed too briefly in the last chapter, act in regard to variation?  Can the principle of selection, which we have seen is so potent in the hands of man, apply in nature?  I think we shall see that it can act most effectually.  Let it be borne in mind in what an endless number of strange peculiarities our domestic productions, and, in a lesser degree, those under nature, vary; and how strong the hereditary tendency is.  Under domestication, it may be truly said that the whole organisation becomes in some degree plastic.  Let it be borne in mind how infinitely complex and close-fitting are the mutual relations of all organic beings to each other and to their physical conditions of life.  Can it, then, be thought improbable, seeing that variations useful to man have undoubtedly occurred, that other variations useful in some way to each being in the great and complex battle of life, should sometimes occur in the course of thousands of generations?  If such do occur, can we doubt (remembering that many more individuals are born than can possibly survive) that individuals having any advantage, however slight, over others, would have the best chance of surviving and of procreating their kind?  On the other hand, we may feel sure that any variation in the least degree injurious would be rigidly destroyed.  This preservation of favourable variations and the rejection of injurious variations, I call Natural Selection.  Variations neither useful nor injurious would not be affected by natural selection, and would be left a fluctuating element, as perhaps we see in the species called polymorphic.