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.]

CHAPTER XIII

THE GEOLOGICAL EVIDENCES OF EVOLUTION

What we may expect--The number of known species of extinct
animals--Causes of the imperfection of the geological
record--Geological evidences of
evolution--Shells--Crocodiles--The rhinoceros tribe--The
pedigree of the horse tribe--Development of deer's horns--Br