etc.

As they frequent chiefly the most rapid and boisterous torrents, among rocks, waterfalls, and huge boulders, the water is never frozen over, and they are thus able to live during the severest winters. Only a very few species of dipper are known, all those of the old world being so closely allied to our British bird that some ornithologists consider them to be merely local races of one species; while in North America and the northern Andes there are two species.

other

Here then we have a bird, which, in its whole structure, shows a close affinity to the smaller typical perching birds, but which has departed from all its allies in its habits and mode of life, and has secured for itself a place in nature where it has few competitors and few enemies. We may well suppose, that, at some remote period, a bird which was perhaps the common and more generalised ancestor of most of our thrushes, warblers, wrens, etc., had spread widely over the great northern continent, and had given rise to numerous varieties adapted to special conditions of life. Among these some took to feeding on the borders of clear streams, picking out such larvæ and molluscs as they could reach in shallow water. When food became scarce they would attempt to pick them out of deeper and deeper water, and while doing this in cold weather many would become frozen and starved. But any which possessed denser and more hairy plumage than usual, which was able to keep out the water, would survive; and thus a race would be formed which would depend more and more on this kind of food. Then, following up the frozen streams into the mountains, they would be able to live there during the winter; and as such places afforded them much protection from enemies and ample shelter for their nests and young, further adaptations would occur, till the wonderful power of diving and flying under water was acquired by a true land-bird.

That such habits might be acquired under stress of need is rendered highly probable by the facts stated by the wellknown American naturalist, Dr. Abbott. He says that "the water-thrushes (Seiurus sp.) all wade in water, and often, seeing minute mollusca on the bottom of the stream, plunge both head and neck beneath the surface, so that often, for

Now

several seconds, a large part of the body is submerged. these birds still have the plumage pervious to water, and so are liable to be drenched and sodden; but they have also the faculty of giving these drenched feathers such a good shaking that flight is practicable a moment after leaving the water. Certainly the water-thrushes (Seiurus ludovicianus, S. auricapillus, and S. noveboracensis) have taken many preliminary steps to becoming as aquatic as the dipper; and the winterwren, and even the Maryland yellow-throat are not far behind." "1

Another curious example of the way in which species have been modified to occupy new places in nature, is afforded by the various animals which inhabit the water-vessels formed by the leaves of many epiphytal species of Bromelia. Fritz Müller has described a caddis-fly larva which lives among these leaves, and which has been modified in the pupa state in accordance with its surroundings. The pupe of caddis-flies inhabiting streams have fringes of hair on the tarsi to enable them to reach the surface on leaving their cases. But in the species inhabiting bromelia leaves there is no need for swimming, and accordingly we find the tarsi entirely bare. In the same plants are found curious little Entomostraca, very abundant there but found nowhere else. These form a new genus, but are most nearly allied to Cythere, a marine type. It is believed that the transmission of this species from one tree to another must be effected by the young crustacea, which are very minute, clinging to beetles, many of which, both terrestrial and aquatic, also inhabit the bromelia leaves; and as some waterbeetles are known to frequent the sea, it is perhaps by these means that the first emigrants established themselves in this strange new abode. Bromelia are often very abundant on trees growing on the water's edge, and this would facilitate the transition from a marine to an arboreal habitat. Fritz Müller has also found, among the bromelia leaves, a small frog bearing its eggs on its back, and having some other peculiarities of structure. Several beautiful little aquatic plants of the genus Utricularia or bladder-wort also inhabit bromelia leaves; and these send runners out to neighbouring plants and thus spread themselves with great rapidity.

1 Nature, vol. xxx. p. 30.

The Importance of Isolation.

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 reptiletailed 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 paleozoic 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