occasionally extends to the ship's position at that time of year. The
captain adds that "frequently, in that part of the ocean, he has had
moths and butterflies come on board." The position is 960 miles
south-west of the Cape Verde Islands, and about 440 north-east of the
South American coast. The specimen preserved is Deiopeia pulchella, a
very common species in dry localities in the Eastern tropics, and rarely
found in Britain, but, Mr. MacLachlan thinks, not found in South
America. They must have come, therefore, from the Cape Verde Islands, or
from some parts of the African coast, and must have traversed about a
thousand miles of ocean with the assistance, no doubt, of a strong
north-east trade wind for a great part of the distance. In the British
Museum collection there is a specimen of the same moth caught at sea
during the voyage of the _Rattlesnake_, in Lat. 6° N., Long. 22-1/2°
W., being between the former position and Sierra Leone, thus rendering
it probable that the moths came from that part of the African coast, in
which case the swarm encountered by the _Pleione_ must have travelled
more than 1200 miles.

A similar case was recorded by Mr. F.A. Lucas in the American periodical
_Science_ of 8th April 1887. He states that in 1870 he met with numerous
moths of many species while at sea in the South Atlantic (Lat. 25° S.,
Long. 24° W.), about 1000 miles from the coast of Brazil. As this
position is just beyond the south-east trades, the insects may have been
brought from the land by a westerly gale. In the _Zoologist_ (1864, p.
8920) is the record of a small longicorn beetle which flew on board a
ship 500 miles off the west coast of Africa. Numerous other cases are
recorded of insects at less distances from land, and, taken in
connection with those already given, they are sufficient to show that
great numbers must be continually carried out to sea, and that
occasionally they are able to reach enormous distances. But the
reproductive powers of insects are so great that all we require, in
order to stock a remote island, is that some few specimens shall reach
it even once in a century, or once in a thousand years.


_Insects at great Altitudes._

Equally important is the proof we possess that insects are often carried
to great altitudes by upward currents of air. Humboldt noticed them up
to heights of 15,000 and 18,000 feet in South America, and Mr. Albert
Müller has collected many interesting cases of the same character in
Europe.[173] A moth (Plusia gamma) has been found on the summit of Mont
Blanc; small hymenoptera and moths have been seen on the Pyrenees at a
height of 11,000 feet, while numerous flies and beetles, some of
considerable size, have been caught on the glaciers and snow-fields of
various parts of the Alps. Upward currents of air, whirlwinds and
tornadoes, occur in all parts of the world, and large numbers of insects
are thus carried up into the higher regions of the atmosphere, where
they are liable to be caught by strong winds, and thus conveyed enormous
distances over seas or continents. With such powerful means of
dispersal the distribution of insects over the entire globe, and their
presence in the most remote oceanic islands, offer no difficulties.


_The Dispersal of Plants._

The dispersal of seeds is effected in a greater variety of ways than are
available in the case of any animals. Some fruits or seed-vessels, and
some seeds, will float for many weeks, and after immersion in salt water
for that period the seeds will often germinate. Extreme cases are the
double cocoa-nut of the Seychelles, which has been found on the coast of
Sumatra, about 3000 miles distant; the fruits of the Sapindus saponaria
(soap-berry), which has been brought to Bermuda by the Gulf Stream from
the West Indies, and has grown after a journey in the sea of about 1500
miles; and the West Indian bean, Entada scandens, which reached the
Azores from the West Indies, a distance of full 3000 miles, and
afterwards germinated at Kew. By these means we can account for the
similarity in the shore flora of the Malay Archipelago and most of the
islands of the Pacific; and from an examination of the fruits and seeds,
collected among drift during the voyage of the _Challenger_, Mr. Hemsley
has compiled a list of 121 species which are probably widely dispersed
by this means.

A still larger number of species owe their dispersal to birds in several
distinct ways. An immense number of fruits in all parts of the world are
devoured by birds, and have been attractively coloured (as we have
seen), in order to be so devoured, because the seeds pass through the
birds' bodies and germinate where they fall. We have seen how frequently
birds are forced by gales of wind across a wide expanse of ocean, and
thus seeds must be occasionally carried. It is a very suggestive fact,
that all the trees and shrubs in the Azores bear berries or small fruits
which are eaten by birds; while all those which bear larger fruits, or
are eaten chiefly by mammals - such as oaks, beeches, hazels, crabs,
etc. - are entirely wanting. Game-birds and waders often have portions of
mud attached to their feet, and Mr. Darwin has proved by experiment that
such mud frequently contains seeds. One partridge had such a quantity of
mud attached to its foot as to contain seeds from which eighty-two
plants germinated; this proves that a very small portion of mud may
serve to convey seeds, and such an occurrence repeated even at long
intervals may greatly aid in stocking remote islands with vegetation.
Many seeds also adhere to the feathers of birds, and thus, again, may be
conveyed as far as birds are ever carried. Dr. Guppy found a small hard
seed in the gizzard of a Cape Petrel, taken about 550 miles east of
Tristan da Cunha.


_Dispersal of Seeds by the Wind._

In the preceding cases we have been able to obtain direct evidence of
transportal; but although we know that many seeds are specially adapted
to be dispersed by the wind, we cannot obtain direct proof that they are
so carried for hundreds or thousands of miles across the sea, owing to
the difficulty of detecting single objects which are so small and
inconspicuous. It is probable, however, that the wind as an agent of
dispersal is really more effective than any of those we have hitherto
considered, because a very large number of plants have seeds which are
very small and light, and are often of such a form as to facilitate
aerial carriage for enormous distances. It is evident that such seeds
are especially liable to be transported by violent winds, because they
become ripe in autumn at the time when storms are most prevalent, while
they either lie upon the surface of the ground, or are disposed in dry
capsules on the plant ready to be blown away. If inorganic particles
comparable in weight, size, or form with such seeds are carried for
great distances, we may be sure that seeds will also be occasionally
carried in the same way. It will, therefore, be necessary to give a few
examples of wind-carriage of small objects.

On 27th July 1875 a remarkable shower of small pieces of hay occurred at
Monkstown, near Dublin. They appeared floating slowly down from a great
height, as if falling from a dark cloud which hung overhead. The pieces
picked up were wet, and varied from single blades of grass to tufts
weighing one or two ounces. A similar shower occurred a few days earlier
in Denbighshire, and was observed to travel in a direction contrary to
that of the wind in the lower atmosphere.[174] There is no evidence of
the distance from which the hay was brought, but as it had been carried
to a great height, it was in a position to be conveyed to almost any
distance by a violent wind, had such occurred at the time.


_Mineral Matter carried by the Wind._

The numerous cases of sand and volcanic dust being carried enormous
distances through the atmosphere sufficiently prove the importance of
wind as a carrier of solid matter, but unfortunately the matter
collected has not been hitherto examined with a view to determine the
maximum size and weight of the particles. A few facts, however, have
been kindly furnished me by Professor Judd, F.R.S. Some dust which fell
at Genoa on 15th October 1885, and was believed to have been brought
from the African desert, consisted of quartz, hornblende, and other
minerals, and contained particles having a diameter of 1/500 inch, each
weighing 1/200,000 grain. This dust had probably travelled over 600
miles. In the dust from Krakatoa, which fell at Batavia, about 100 miles
distant, during the great eruption, there are many solid particles even
larger than those mentioned above. Some of this dust was given me by
Professor Judd, and I found in it several ovoid particles of a much
larger size, being 1/50 inch long, and 1/70 wide and deep. The dust from
the same eruption, which fell on board the ship _Arabella_, 970 miles
from the volcano, also contained solid particles 1/500 inch diameter.
Mr. John Murray of the _Challenger_ Expedition writes to me that he
finds in the deep sea deposits 500 and even 700 miles west of the coast
of Africa, rounded particles of quartz, having a diameter of 1/250 inch,
and similar particles are found at equally great distances from the
south-west coasts of Australia; and he considers these to be atmospheric
dust carried to that distance by the wind. Taking the sp. gr. of quartz
at 2.6, these particles would weigh about 1/25,000 grain each. These
interesting facts can, however, by no means be taken as indicating the
extreme limits of the power of wind in carrying solid particles. During
the Krakatoa eruption no gale of special violence occurred, and the
region is one of comparative calms. The grains of quartz found by Mr.
Murray more nearly indicate the limit, but the very small portions of
matter brought up by the dredge, as compared with the enormous areas of
sea-bottom, over which the atmospheric dust must have been scattered,
render it in the highest degree improbable that the maximum limit either
of size of particles, or of distance from land has been reached.

Let us, however, assume that the quartz grains, found by Mr. Murray in
the deep-sea ooze 700 miles from land, give us the extreme limit of the
power of the atmosphere as a carrier of solid particles, and let us
compare with these the weights of some seeds. From a small collection of
the seeds of thirty species of herbaceous plants sent me from Kew, those
in the above table were selected, and small portions of eight of them
carefully weighed in a chemical balance.[175] By counting these portions
I was able to estimate the number of seeds weighing one grain. The three
very minute species, whose numbers are marked with an asterisk (*), were
estimated by the comparison of their sizes with those of the smaller
weighed seeds.


No| Species. |Approximate | Approximate | Remarks.
| |No. of Seeds| Dimensions. |
| |In one Grain| |
| | | in. in. in. |
1|Draba verna | 1,800 |1/60 x 1/90 x 1/160|Oval, flat.
2|Hypericum perforatum | 520 | 1/30 x 1/80 |Cylindrical.
3|Astilbe rivularis | 4,500 | 1/50 x 1/100 |Elongate, flat, tailed,
| | | | wavy.
4|Saxifraga coriophylla| 750 | 1/40 x 1/75 |Surface rough, adhere
| | | | to the dry capsules.
5|Oenothera rosea | 640 | 1/40 x 1/80 |Ovate.
6|Hypericum hirsutum | 700 | 1/30 x 1/100 |Cylindrical, rough.
7|Mimulus luteus | 2,900 | 1/60 x 1/100 |Oval, minute.
8|Penthorum sedoides | 8,000* | 1/70 x 1/150 |Flattened, very minute.
9|Sagina procumbens | 12,000* | 1/120 |Sub-triangular, flat.
10|Orchis maculata | 15,000* | - - |Margined, flat,
| | | | very minute.
11|Gentiana purpurea | 35 | 1/25 |Wavy, rough, with this
| | | | coriaceous margins.
12|Silene alpina | - - | 1/30 |Flat, with fringed
| | | | margins.
13|Adenophora communis | - - | 1/20 x 1/40 |Very thin, wavy, light.
|Quartz grains | 25,000 | 1/250 |Deep sea ... 700 miles.
|Do. |200,000 | 1/500 |Genoa ... 600 miles.


If now we compare the seeds with the quartz grains, we find that
several are from twice to three times the weight of the grains found by
Mr. Murray, and others five times, eight times, and fifteen times as
heavy; but they are proportionately very much larger, and, being usually
irregular in shape or compressed, they expose a very much larger surface
to the air. The surface is often rough, and several have dilated margins
or tailed appendages, increasing friction and rendering the uniform rate
of falling through still air immensely less than in the case of the
smooth, rounded, solid quartz grains. With these advantages it is a
moderate estimate that seeds ten times the weight of the quartz grains
could be carried quite as far through the air by a violent gale and
under the most favourable conditions. These limits will include five of
the seeds here given, as well as hundreds of others which do not exceed
them in weight; and to these we may add some larger seeds which have
other favourable characteristics, as is the case with numbers 11-13,
which, though very much larger than the rest, are so formed as in all
probability to be still more easily carried great distances by a gale of
wind. It appears, therefore, to be absolutely certain that every
autumnal gale capable of conveying solid mineral particles to great
distances, must also carry numbers of small seeds at least as far; and
if this is so, the wind alone will form one of the most effective agents
in the dispersal of plants.

Hitherto this mode of conveyance, as applying to the transmission of
seeds for great distances across the ocean, has been rejected by
botanists, for two reasons. In the first place, there is said to be no
direct evidence of such conveyance; and, secondly, the peculiar plants
of remote oceanic islands do not appear to have seeds specially adapted
for aerial transmission. I will consider briefly each of these
objections.


_Objection to the Theory of Wind-Dispersal._

To obtain direct evidence of the transmission of such minute and
perishable objects, which do not exist in great quantities, and are
probably carried to the greatest distances but rarely and as single
specimens, is extremely difficult. A bird or insect can be seen if it
comes on board ship, but who would ever detect the seeds of Mimulus or
Orchis even if a score of them fell on a ship's deck? Yet if but one
such seed per century were carried to an oceanic island, that island
might become rapidly overrun by the plant, if the conditions were
favourable to its growth and reproduction. It is further objected that
search has been made for such seeds, and they have not been found.
Professor Kerner of Innsbruck examined the snow on the surface of
glaciers, and assiduously collected all the seeds he could find, and
these were all of plants which grew in the adjacent mountains or in the
same district. In like manner, the plants growing on moraines were found
to be those of the adjacent mountains, plateaux, or lowlands. Hence he
concluded that the prevalent opinion that seeds may be carried through
the air for very great distances "is not supported by fact."[176] The
opinion is certainly not supported by Kerner's facts, but neither is it
opposed by them. It is obvious that the seeds that would be carried by
the wind to moraines or to the surface of glaciers would be, first and
in the greatest abundance, those of the immediately surrounding
district; then, very much more rarely, those from more remote mountains;
and lastly, in extreme rarity, those from distant countries or
altogether distinct mountain ranges. Let us suppose the first to be so
abundant that a single seed could be found by industrious search on each
square yard of the surface of the glacier; the second so scarce that
only one could possibly be found in a hundred yards square; while to
find one of the third class it would be necessary exhaustively to
examine a square mile of surface. Should we expect that _one_ ever to be
found, and should the fact that it could not be found be taken as a
proof that it was not there? Besides, a glacier is altogether in a bad
position to receive such remote wanderers, since it is generally
surrounded by lofty mountains, often range behind range, which would
intercept the few air-borne seeds that might have been carried from a
distant land. The conditions in an oceanic island, on the other hand,
are the most favourable, since the land, especially if high, will
intercept objects carried by the wind, and will thus cause more of the
solid matter to fall on it than on an equal area of ocean. We know that
winds at sea often blow violently for days together, and the rate of
motion is indicated by the fact that 72 miles an hour was the average
velocity of the wind observed during twelve hours at the Ben Nevis
observatory, while the velocity sometimes rises to 120 miles an hour. A
twelve hours' gale might, therefore, carry light seeds a thousand miles
as easily and certainly as it could carry quartz-grains of much greater
specific gravity, rotundity, and smoothness, 500 or even 100 miles; and
it is difficult even to imagine a sufficient reason why they should not
be so carried - perhaps very rarely and under exceptionally favourable
conditions, - but this is all that is required.

As regards the second objection, it has been observed that orchideae,
which have often exceedingly small and light seeds, are remarkably
absent from oceanic islands. This, however, may be very largely due to
their extreme specialisation and dependence on insect agency for their
fertilisation; while the fact that they do occur in such very remote
islands as the Azores, Tahiti, and the Sandwich Islands, proves that
they must have once reached these localities either by the agency of
birds or by transmission through the air; and the facts I have given
above render the latter mode at least as probable as the former. Sir
Joseph Hooker remarks on the composite plant of Kerguelen Island (Cotula
plumosa) being found also on Lord Auckland and MacQuarrie Islands, and
yet having no pappus, while other species of the genus possess it. This
is certainly remarkable, and proves that the plant must have, or once
have had, some other means of dispersal across wide oceans.[177] One of
the most widely dispersed species in the whole world (Sonchus oleraceus)
possesses pappus, as do four out of five of the species which are common
to Europe and New Zealand, all of which have a very wide distribution.
The same author remarks on the limited area occupied by most species of
Compositae, notwithstanding their facilities for dispersal by means of
their feathered seeds; but it has been already shown that limitations
of area are almost always due to the competition of allied forms,
facilities for dispersal being only one of many factors in determining
the wide range of species. It is, however, a specially important factor
in the case of the inhabitants of remote oceanic islands, since, whether
they are peculiar species or not, they or their remote ancestors must at
some time or other have reached their present position by natural means.

I have already shown elsewhere, that the flora of the Azores strikingly
supports the view of the species having been introduced by aerial
transmission only, that is, by the agency of birds and the wind, because
all plants that could not possibly have been carried by these means are
absent.[178] In the same way we may account for the extreme rarity of
Leguminosae in all oceanic islands. Mr. Hemsley, in his Report on
Insular Floras, says that they "are wanting in a large number of oceanic
islands where there is no true littoral flora," as St. Helena, Juan
Fernandez, and all the islands of the South Atlantic and South Indian
Oceans. Even in the tropical islands, such as Mauritius and Bourbon,
there are no endemic species, and very few in the Galapagos and the
remoter Pacific Islands. All these facts are quite in accordance with
the absence of facilities for transmission through the air, either by
birds or the wind, owing to the comparatively large size and weight of
the seeds; and an additional proof is thus afforded of the extreme
rarity of the successful floating of seeds for great distances across
the ocean.[179]


_Explanation of North Temperate Plants in the Southern Hemisphere._

If we now admit that many seeds which are either minute in size, of thin
texture or wavy form, or so fringed or margined as to afford a good hold
to the air, are capable of being carried for many hundreds of miles by
exceptionally violent and long-continued gales of wind, we shall not
only be better able to account for the floras of some of the remotest
oceanic islands, but shall also find in the fact a sufficient
explanation of the wide diffusion of many genera, and even species, of
arctic and north temperate plants in the southern hemisphere or on the
summits of tropical mountains. Nearly fifty of the flowering plants of
Tierra-del-Fuego are found also in North America or Europe, but in no
intermediate country; while fifty-eight species are common to New
Zealand and Northern Europe; thirty-eight to Australia, Northern Europe,
and Asia; and no less than seventy-seven common to New Zealand,
Australia, and South America.[180] On lofty mountains far removed from
each other, identical or closely allied plants often occur. Thus the
fine Primula imperialis of a single mountain peak in Java has been found
(or a closely allied species) in the Himalayas; and many other plants of
the high mountains of Java, Ceylon, and North India are either identical
or closely allied forms. So, in Africa, some species, found on the
summits of the Cameroons and Fernando Po in West Africa, are closely
allied to species in the Abyssinian highlands and in Temperate Europe;
while other Abyssinian and Cameroons species have recently been found on
the mountains of Madagascar. Some peculiar Australian forms have been
found represented on the summit of Kini Balu in Borneo. Again, on the
summit of the Organ mountains in Brazil there are species allied to
those of the Andes, but not found in the intervening lowlands.


_No Proof of Recent Lower Temperature in the Tropics._

Now all these facts, and numerous others of like character, were
supposed by Mr. Darwin to be due to a lowering of temperature during
glacial epochs, which allowed these temperate forms to migrate across
the intervening tropical lowlands. But any such change within the epoch
of existing species is almost inconceivable. In the first place, it
would necessitate the extinction of much of the tropical flora (and with
it of the insect life), because without such extinction alpine
herbaceous plants could certainly never spread over tropical forest
lowlands; and, in the next place, there is not a particle of direct
evidence that any such lowering of temperature in inter-tropical
lowlands ever took place. The only alleged evidence of the kind is that
adduced by the late Professor Agassiz and Mr. Hartt; but I am informed
by my friend, Mr. J.C. Branner (now State Geologist of Arkansas, U.S.),
who succeeded Mr. Hartt, and spent several years completing the
geological survey of Brazil, that the supposed moraines and glaciated
granite rocks near Rio Janeiro and elsewhere, as well as the so-called
boulder-clay of the same region, are entirely explicable as the results
of sub-aerial denudation and weathering, and that there is no proof
whatever of glaciation in any part of Brazil.


_Lower Temperature not needed to Explain the Facts._

But any such vast physical change as that suggested by Darwin, involving
as it does such tremendous issues as regards its effects on the tropical
fauna and flora of the whole world, is really quite uncalled for,
because the facts to be explained are of the same essential nature as
those presented by remote oceanic islands, between which and the nearest
continents no temperate land connection is postulated. In proportion to
their limited area and extreme isolation, the Azores, St. Helena, the