[Footnote 163: See A. Agassiz, _Three Cruises of the Blake_ (Cambridge, Mass., 1888), vol. i. p. 127, footnote.]
[Footnote 164: Even the extremely fine Mississippi mud is nowhere found beyond a hundred miles from the mouths of the river in the Gulf of Mexico (A. Agassiz, _Three Cruises of the Blake_, vol. i. p. 128).]
[Footnote 165: I have given a full summary of the evidence for the permanence of oceanic and continental areas in my _Island Life_, chap. vi.]
[Footnote 166: For a full account of the peculiarities of the Madagascar fauna, see my _Island Life_, chap. xix.]
[Footnote 167: See _Island Life_, p. 446, and the whole of chaps. xxi. xxii. More recent soundings have shown that the Map at p. 443, as well as that of the Madagascar group at p. 387, are erroneous, the ocean around Norfolk Island and in the Straits of Mozambique being more than 1000 fathoms deep. The general argument is, however, unaffected.]
[Footnote 168: For some details of these migrations, see the author's _Geographical Distribution of Animals_, vol. i. p. 140; also Heilprin's _Geographical and Geological Distribution of Animals_.]
[Footnote 169: For a full discussion of this question, see _Island Life_, pp. 390-420.]
[Footnote 170: _Géographie Botanique_, p. 798.]
[Footnote 171: _Nature_, 1st April 1886.]
[Footnote 172: Report of the Brit. Assoc. Committee on Migration of Birds during 1886.]
[Footnote 173: _Trans. Ent. Soc._, 1871, p. 184.]
[Footnote 174: _Nature_ (1875), vol. xii. pp. 279, 298.]
[Footnote 175: I am indebted to Professor R. Meldola of the Finsbury Technical Institute, and Rev. T.D. Titmas of Charterhouse for furnishing me with the weights required.]
[Footnote 176: See _Nature_, vol. vi. p. 164, for a summary of Kerner's paper.]
[Footnote 177: It seems quite possible that the absence of pappus in this case is a recent adaptation, and that it has been brought about by causes similar to those which have reduced or aborted the wings of insects in oceanic islands. For when a plant has once reached one of the storm-swept islands of the southern ocean, the pappus will be injurious for the same reason that the wings of insects are injurious, since it will lead to the seeds being blown out to sea and destroyed. The seeds which are heaviest and have least pappus will have the best chance of falling on the ground and remaining there to germinate, and this process of selection might rapidly lead to the entire disappearance of the pappus.]
[Footnote 178: See _Island Life_, p. 251.]
[Footnote 179: Mr. Hemsley suggests that it is not so much the difficulty of transmission by floating, as the bad conditions the seeds are usually exposed to when they reach land. Many, even if they germinate, are destroyed by the waves, as Burchell noticed at St. Helena; while even a flat and sheltered shore would be an unsuitable position for many inland plants. Air-borne seeds, on the other hand, may be carried far inland, and so scattered that some of them are likely to reach suitable stations.]
[Footnote 180: For fuller particulars, see Sir J. Hooker's _Introduction to Floras of New Zealand and Australia_, and a summary in my _Island Life_, chaps. xxii. xxiii.]
[Footnote 181: For a fuller discussion of this subject, see my _Island Life_, chap. xxiii.]
[Footnote 182: 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.]
