The Project Gutenberg ebook of Darwinism (1889), by Alfred Russel Wallace

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._

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.

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.

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._

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._

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

Galapagos, and the Sandwich Islands, each possess a fairly rich--the

last a very rich--indigenous flora; and the means which sufficed to

stock them with a great variety of plants would probably suffice to

transmit others from mountain-top to mountain-top in various parts of

the globe. In the case of the Azores, we have large numbers of species

identical with those of Europe, and others closely allied, forming an

exactly parallel case to the species found on the various mountain

summits which have been referred to. The distances from Madagascar to

the South African mountains and to Kilimandjaro, and from the latter to

Abyssinia, are no greater than from Spain to the Azores, while there are

other equatorial mountains forming stepping-stones at about an equal

distance to the Cameroons. Between Java and the Himalayas we have the

lofty mountains of Sumatra and of North-western Burma, forming steps at

about the same distance apart; while between Kini Balu and the

Australian Alps we have the unexplored snow mountains of New Guinea,

the Bellenden Ker mountains in Queensland, and the New England and Blue

Mountains of New South Wales. Between Brazil and Bolivia the distances

are no greater; while the unbroken range of mountains from Arctic

America to Tierra-del-Fuego offers the greatest facilities for

transmission, the partial gap between the lofty peak of Chiriqui and the

high Andes of New Grenada being far less than from Spain to the Azores.

Thus, whatever means have sufficed for stocking oceanic islands must

have been to some extent effective in transmitting northern forms from

mountain to mountain, across the equator, to the southern hemisphere;

while for this latter form of dispersal there are special facilities, in

the abundance of fresh and unoccupied surfaces always occurring in

mountain regions, owing to avalanches, torrents, mountain-slides, and

rock-falls, thus affording stations on which air-borne seeds may

germinate and find a temporary home till driven out by the inroads of

the indigenous vegetation. These temporary stations may be at much lower

altitudes than the original habitat of the species, if other conditions

are favourable. Alpine plants often descend into the valleys on glacial

moraines, while some arctic species grow equally well on mountain

summits and on the seashore. The distances above referred to between the

loftier mountains may thus be greatly reduced by the occurrence of

suitable conditions at lower altitudes, and the facilities for

transmission by means of aerial currents proportionally increased.[181]

_Facts Explained by the Wind-Carriage of Seeds._
But if we altogether reject aerial transmission of seeds for great

distances, except by the agency of birds, it will be difficult, if not

impossible, to account for the presence of so many identical species of

plants on remote mountain summits, or for that "continuous current of

vegetation" described by Sir Joseph Hooker as having apparently long

existed from the northern to the southern hemisphere. It may be admitted

that we can, possibly, account for the greater portion of the floras of

remote oceanic islands by the agency of birds alone; because, when blown

out to sea land-birds must reach some island 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.

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.[182] 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 bird-transport, 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