or perish, and all which come within sight of an island will struggle to reach it as their only refuge. But, with mountain summits the case is altogether different, because, being surrounded by land instead of by sea, no bird would need to fly, or to be carried by the wind, for several hundred miles at a stretch to another mountain summit, but would find a refuge in the surrounding uplands, ridges, valleys, or plains. As a rule the birds that frequent lofty mountain tops are peculiar species, allied to those of the surrounding district; and there is no indication whatever of the passage of birds from one remote mountain to another in any way comparable with the flights of birds which are known to reach the Azores annually, or even with the few regular migrants from Australia to New Zealand. It is almost impossible to conceive that the seeds of the Himalayan primula should have been thus carried to Java; but, by means of gales of wind, and intermediate stations from fifty to a few hundred miles apart, where the seeds might vegetate for a year or two and produce fresh seed to be again carried on in the same manner, the transmission might, after many failures, be at last effected.

A very important consideration is the vastly larger scale. on which wind-carriage of seeds must act, as compared with bird-carriage. It can only be a few birds which carry seeds attached to their feathers or feet. A very small proportion of these would carry the seeds of Alpine plants; while an almost infinitesimal fraction of these latter would convey the few seeds attached to them safely to an oceanic island or remote mountain. But winds, in the form of whirlwinds or tornadoes, gales or hurricanes, are perpetually at work over large areas of land and sea. Insects and light particles of matter are often carried up to the tops of high mountains; and, from the very nature and origin of winds, they usually consist of ascending or descending currents, the former capable of suspending such small and light objects as are many seeds long enough for them to be carried enormous distances. each single seed carried away by external attachment to the feet or feathers of a bird, countless millions are probably carried away by violent winds; and the chance of conveyance to a great distance and in a definite direction must be many

For

times greater by the latter mode than by the former.1 have seen that inorganic particles of much greater specific gravity than seeds, and nearly as heavy as the smallest kinds, are carried to great distances through the air, and we can therefore hardly doubt that some seeds are carried as far. The direct agency of the wind, as a supplement to birdtransport, will help to explain the presence in oceanic islands of plants growing in dry or rocky places whose small seeds are not likely to become attached to birds; while it seems to be the only effective agency possible in the dispersal of those species of alpine or sub-alpine plants found on the summits of distant mountains, or still more widely separated in the temperate zones of the northern and southern hemispheres.

Concluding Remarks.

On the general principles that have been now laid down, it will be found that all the chief facts of the geographical distribution of animals and plants can be sufficiently understood. There will, of course, be many cases of difficulty and some seeming anomalies, but these can usually be seen to depend on our ignorance of some of the essential factors of the problem. Either we do not know the distribution of the group in recent geological times, or we are still ignorant of the special methods. by which the organisms are able to cross the sea. The latter difficulty applies especially to the lizard tribe, which are found

1 A very remarkable case of wind conveyance of seeds on a large scale is described in a letter from Mr. Thomas Hanbury to his brother, the late Daniel Hanbury, which has been kindly communicated to me by Mr. Hemsley of Kew. The letter is dated "Shanghai, 1st May 1856," and the passage referred to is as follows:

"For the past three days we have had very warm weather for this time of year, in fact almost as warm as the middle of summer. Last evening the wind suddenly changed round to the north and blew all night with considerable violence, making a great change in the atmosphere.

"This morning, myriads of small white particles are floating about in the air; there is not a single cloud and no mist, yet the sun is quite obscured by this substance, and it looks like a white fog in England. I enclose thee a sample, thinking it may interest. It is evidently a vegetable production; I think, apparently, some kind of seed.'

Mr. Hemsley adds, that this substance proves to be the plumose seeds of a poplar or willow. In order to produce the effects described-quite obscuring the sun like a white fog,-the seeds must have filled the air to a very great height; and they must have been brought from some district where there were extensive tracts covered with the tree which produced them.

in almost all the tropical oceanic islands; but the particular mode in which they are able to traverse a wide expanse of ocean, which is a perfect barrier to batrachia and almost so to snakes, has not yet been discovered. Lizards are found in all the larger Pacific Islands as far as Tahiti, while snakes do not extend beyond the Fiji Islands; and the latter are also absent from Mauritius and Bourbon, where lizards of seven or eight species abound. Naturalists resident in the Pacific Islands would make a valuable contribution to our science by studying the life-history of the native lizards, and endeavouring to ascertain the special facilities they possess for crossing over wide spaces of ocean.

CHAPTER XIII

THE GEOLOGICAL EVIDENCES OF EVOLUTION

What we may expect-The number of known species of extinct animalsCauses of the imperfection of the geological record-Geological evidences of evolution-Shells-Crocodiles-The rhinoceros tribeThe pedigree of the horse tribe-Development of deer's horns--Brain development Local relations of fossil and living animals-Cause of extinction of large animals-Indications of general progress in plants and animals—The progressive development of plants-Possible cause of sudden late appearance of exogens-Geological distribution of insects-Geological succession of vertebrata-Concluding remarks.

THE theory of evolution in the organic world necessarily implies that the forms of animals and plants have, broadly speaking, progressed from a more generalised to a more specialised structure, and from simpler to more complex forms. We know, however, that this progression has been by no means regular, but has been accompanied by repeated degradation and degeneration; while extinction on an enormous scale has again and again stopped all progress in certain directions, and has often compelled a fresh start in development from some comparatively low and imperfect type.

The enormous extension of geological research in recent times has made us acquainted with a vast number of extinct organisms, so vast that in some important groups-such as the mollusca-the fossil are more numerous than the living species; while in the mammalia they are not much less numerous, the preponderance of living species being chiefly in the smaller and in the arboreal forms which have not been so well preserved as the members of the larger groups. With such a wealth of material to illustrate the successive stages

through which animals have passed, it will naturally be expected that we should find important evidence of evolution. We should hope to learn the steps by which some isolated forms have been connected with their nearest allies, and in many cases to have the gaps filled up which now separate genus from genus, or species from species. In some cases these expectations are fulfilled, but in many other cases we seek in vain for evidence of the kind we desire; and this absence of evidence with such an apparent wealth of material is held by many persons to throw doubt on the theory of evolution itself. They urge, with much appearance of reason, that all the arguments we have hitherto adduced fall short of demonstration, and that the crucial test consists in being able to show, in a great number of cases, those connecting links which we say must have existed. Many of the gaps that still remain are so vast that it seems incredible to these writers that they could ever have been filled up by a close succession of species, since these must have spread over so many ages, and have existed in such numbers, that it seems impossible to account for their total absence from deposits in which great numbers of species belonging to other groups are preserved and have been discovered. In order to appreciate the force, or weakness, of these objections, we must inquire into the character and completeness of that record of the past life of the earth which geology has unfolded, and ascertain the nature and amount of the evidence which, under actual conditions, we may expect to find.

The Number of known Species of Extinct Animals.

If

When we state that the known fossil mollusca are considerably more numerous than those which now live on the earth, it appears at first sight that our knowledge is very complete, but this is far from being the case. The species have been continually changing throughout geological time, and at each period have probably been as numerous as they are now. we divide the fossiliferous strata into twelve great divisions -the Pliocene, Miocene, Eocene, Cretaceous, Oolite, Lias, Trias, Permian, Carboniferous, Devonian, Silurian, and Cambrian,—we find not only that each has a very distinct and characteristic molluscan fauna, but that the different sub