Isolation is no doubt an important aid to natural selection, as shown by the fact that islands so often present a number of peculiar species; and the same thing is seen on the two sides of a great mountain range or on opposite coasts of a continent. The importance of isolation is twofold. In the first place, it leads to a body of individuals of each species being limited in their range and thus subjected to uniform conditions for long spaces of time. Both the direct action of the environment and the natural selection of such varieties only as are suited to the conditions, will, therefore, be able to produce their full effect. In the second place, the process of change will not be interfered with by intercrossing with other individuals which are becoming adapted to somewhat different conditions in an adjacent area. But this question of the swamping effects of intercrossing will be considered in another chapter.
Mr. Darwin was of opinion that, on the whole, the largeness of the area occupied by a species was of more importance than isolation, as a factor in the production of new species, and in this I quite agree with him. It must, too, be remembered, that isolation will often be produced in a continuous area whenever a species becomes modified in accordance with varied conditions or diverging habits. For example, a wide-ranging species may in the northern and colder part of its area become modified in one direction, and in the southern part in another direction; and though for a long time an intermediate form may continue to exist in the intervening area, this will be likely soon to die out, both because its numbers will be small, and it will be more or less pressed upon in varying seasons by the modified varieties, each better able to endure extremes of climate. So, when one portion of a terrestrial species takes to a more arboreal or to a more aquatic mode of life, the change of habit itself leads to the isolation of each portion. Again, as will be more fully explained in a future chapter, any difference of habits or of haunts usually leads to some modification of colour or marking, as a means of concealment from enemies; and there is reason to believe that this difference will be intensified by natural selection as a means of identification and recognition by members of the same variety or incipient species. It has also been observed that each differently coloured variety of wild animals, or of domesticated animals which have run wild, keep together, and refuse to pair with individuals of the other colours; and this must of itself act to keep the races separate as completely as physical isolation.
On the Advance of Organisation by Natural Selection.
As natural selection acts solely by the preservation of useful variations, or those which are beneficial to the organism under the conditions to which it is exposed, the result must necessarily be that each species or group tends to become more and more improved in relation to its conditions. Hence we should expect that the larger groups in each class of animals and plants—those which have persisted and have been abundant throughout geological ages—would, almost necessarily, have arrived at a high degree of organisation, both physical and mental. Illustrations of this are to be seen everywhere. Among mammalia we have the carnivora, which from Eocene times have been becoming more and more specialised, till they have culminated in the cat and dog tribes, which have reached a degree of perfection both in structure and intelligence fully equal to that of any other animals. In another line of development, the herbivora have been specialised for living solely on vegetable food till they have culminated in the sheep, the cattle, the deer, and the antelopes. The horse tribe, commencing with an early four-toed ancestor in the Eocene age, has increased in size and in perfect adaptation of feet and teeth to a life on open plains, and has reached its highest perfection in the horse, the ass, and the zebra. In birds, also, we see an advance from the imperfect tooth-billed and reptile-tailed birds of the secondary epoch, to the wonderfully developed falcons, crows, and swallows of our time. So, the ferns, lycopods, conifers, and monocotyledons of the palaeozoic and mesozoic rocks, have developed into the marvellous wealth of forms of the higher dicotyledons that now adorn the earth.
But this remarkable advance in the higher and larger groups does not imply any universal law of progress in organisation, because we have at the same time numerous examples (as has been already pointed out) of the persistence of lowly organised forms, and also of absolute degradation or degeneration. Serpents, for example, have been developed from some lizard-like type which has lost its limbs; and though this loss has enabled them to occupy fresh places in nature and to increase and flourish to a marvellous extent, yet it must be considered to be a retrogression rather than an advance in organisation. The same remark will apply to the whale tribe among mammals; to the blind amphibia and insects of the great caverns; and among plants to the numerous cases in which flowers, once specially adapted to be fertilised by insects, have lost their gay corollas and their special adaptations, and have become degraded into wind-fertilised forms. Such are our plantains, our meadow burnet, and even, as some botanists maintain, our rushes, sedges, and grasses. The causes which have led to this degeneration will be discussed in a future chapter; but the facts are undisputed, and they show us that although variation and the struggle for existence may lead, on the whole, to a continued advance of organisation; yet they also lead in many cases to a retrogression, when such retrogression may aid in the preservation of any form under new conditions. They also lead to the persistence, with slight modifications, of numerous lowly organised forms which are suited to places which higher forms could not fully occupy, or to conditions under which they could not exist. Such are the ocean depths, the soil of the earth, the mud of rivers, deep caverns, subterranean waters, etc.; and it is in such places as these, as well as in some oceanic islands which competing higher forms have not been able to reach, that we find many curious relics of an earlier world, which, in the free air and sunlight and in the great continents, have long since been driven out or exterminated by higher types.
Summary of the first Five Chapters.
We have now passed in review, in more or less detail, the main facts on which the theory of "the origin of species by means of natural selection" is founded. In future chapters we shall have to deal mainly with the application of the theory to explain the varied and complex phenomena presented by the organic world; and, also, to discuss some of the theories put forth by modern writers, either as being more fundamental than that of Darwin or as supplementary to it. Before doing this, however, it will be well briefly to summarise the facts and arguments already set forth, because it is only by a clear comprehension of these that the full importance of the theory can be appreciated and its further applications understood.
The theory itself is exceedingly simple, and the facts on which it rests—though excessively numerous individually, and coextensive with the entire organic world—yet come under a few simple and easily understood classes. These facts are,—first, the enormous powers of increase in geometrical progression possessed by all organisms, and the inevitable struggle for existence among them; and, in the second place, the occurrence of much individual variation combined with the hereditary transmission of such variations. From these two great classes of facts, which are universal and indisputable, there necessarily arises, as Darwin termed it, the "preservation of favoured races in the struggle for life," the continuous action of which, under the ever-changing conditions both of the inorganic and organic universe, necessarily leads to the formation or development of new species.
But, although this general statement is complete and indisputable, yet to see its applications under all the complex conditions that actually occur in nature, it is necessary always to bear in mind the tremendous power and universality of the agencies at work. We must never for an instant lose sight of the fact of the enormously rapid increase of all organisms, which has been illustrated by actual cases, given in our second chapter, no less than by calculations of the results of unchecked increase for a few years. Then, never forgetting that the animal and plant population of any country is, on the whole, stationary, we must be always trying to realise the ever-recurring destruction of the enormous annual increase, and asking ourselves what determines, in each individual case, the death of the many, the survival of the few. We must think over all the causes of destruction to each organism,—to the seed, the young shoot, the growing plant, the full-grown tree, or shrub, or herb, and again the fruit and seed; and among animals, to the egg or new-born young, to the youthful, and to the adults. Then, we must always bear in mind that what goes on in the case of the individual or family group we may observe or think of, goes on also among the millions and scores of millions of individuals which are comprised in almost every species; and must get rid of the idea that chance determines which shall live and which die. For, although in many individual cases death may be due to chance rather than to any inferiority in those which die first, yet we cannot possibly believe that this can be the case on the large scale on which nature works. A plant, for instance, cannot be increased unless there are suitable vacant places its seeds can grow in, or stations where it can overcome other less vigorous and healthy plants. The seeds of all plants, by their varied modes of dispersal, may be said to be seeking out such places in which to grow; and we cannot doubt that, in the long run, those individuals whose seeds are the most numerous, have the greatest powers of dispersal, and the greatest vigour of growth, will leave more descendants than the individuals of the same species which are inferior in all these respects, although now and then some seed of an inferior individual may chance to be carried to a spot where it can grow and survive. The same rule will apply to every period of life and to every danger to which plants or animals are exposed. The best organised, or the most healthy, or the most active, or the best protected, or the most intelligent, will inevitably, in the long run, gain an advantage over those which are inferior in these qualities; that is, the fittest will survive, the fittest being, in each particular case, those which are superior in the special qualities on which safety depends. At one period of life, or to escape one kind of danger, concealment may be necessary; at another time, to escape another danger, swiftness; at another, intelligence or cunning; at another, the power to endure rain or cold or hunger; and those which possess all these faculties in the fullest perfection will generally survive.