Page - 120 - in The Origin of Species

120 ORIGIN OF SPECIES create new places, ready to be filled up by better adapted forms ; but all this will take place very slowly. Although all the individuals of the same species differ in some slight de- gree from each other, it would often be long before differ- ences of the right nature in various parts of the organisation might occur. The result would often be greatly retarded by free intercrossing. Many will exclaim that these several causes are amply sufficient to neutralise the power of nat- ural selection. I do not believe so. But I do believe that natural selection will generally act very slowly, only at long intervals of time, and only on a few of the inhabitants of the same region. I further believe that these slow, intermittent results accord well with what geology tells us of the rate and manner at which the inhabitants of the world have changed. Slow though the process of selection may be, if feeble man can do much by artificial selection, I can see no limit to the amount of change, to the beauty and complexity of the co- adaptations between all organic beings, one with another and with their physical conditions of life, which may have been affected in the long course of time through nature's power of selection, that is by the survival of the fittest. EXTINCTION CAUSED BY NATURAL SELECTION This subject will be more fully discussed in our chapter on Geology; but it must here be alluded to from being inti- mately connected with natural selection. Natural selection acts solely through the preservation of variations in some way advantageous, which consequently endure. Owing to the high geometrical rate of increase of all organic beings, each area is already fully stocked with inhabitants; and it follows from this, that as the favoured forms increase in number, so, generally, will the less favoured decrease and become rare. Rarity, as geology tells us, is the precursor to extinction. We can see that any form which is represented by few individuals will run a good chance of utter extinc- tion, during great fluctuations in the nature of the seasons, or from a temporary increase in the number of its enemies. But we may go further than this ; for, as new forms are pro- duced, unless we admit that specific forms can go on indefi-