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 con

ceive 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. For 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

times greater by the latter mode than by the former.1 We 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.

APPENDIX

Note on the Mode of Dispersal of Lizards.

MR. C. M. WOODFORD (author of A Naturalist among the Head Hunters) writes to me on this subject, that he has frequently found the eggs of lizards concealed between the bark and trunks of fallen timber, and should these trees be carried out to sea by a flood they might drift to great distances with the eggs uninjured. Again, when fruit, fire-wood, etc., are put on board the large canoes, lizards will often be among them unobserved. He has thus found them on board ship, and twice when bringing cases of orchids from the Solomon Islands to Sydney he has found lizards in the cases on arrival.

Note on Insects in Mid-ocean.

In addition to the cases given at p. 359, Mr. C. M. Woodford informs me that he saw a dragon-fly once in the Indian Ocean when 500 miles from the nearest land.

The following passage from the Narrative of the Cruise of the Challenger (p. 564) is also interesting as affording another clue to the dispersal of insects :—

"At the summit of the mountain [Banda Volcano] were numerous flying insects of various kinds, although there was nothing for them to feed upon, and large numbers of them lay dead in the cracks, killed by the poisonous vapours.

Similarly, large numbers of insects were noticed at the summit of the volcano of Ternate, at an altitude of more than 5000 feet. Insects are commonly to be seen being carried along by the wind in successive efforts of flight, and no doubt they are blown up to the tops of these mountains, there being no vegetation towards the summits for them to hold on to. The winds pressing against the mountains form currents up their slopes, and in the case of volcanoes there is no doubt a constant upward draught towards their tops caused by the ascending column of hot air. The accumulation of insects at the top of these mountains is interesting, because, when blown off into the free air from these elevations by heavy winds, as no doubt they often are, the insects are likely to fly and drift before the wind to very long distances, and thus become the means of colonising far-off islands."