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

further check upon their migrations. In these respects almost all other

organisms have great advantages over mammals. Birds can often fly long

distances, and can thus cross arms of the sea, deserts, or mountain

ranges; insects not only fly, but are frequently carried great distances

by gales of wind, as shown by the numerous cases of their visits to

ships hundreds of miles from land. Reptiles, though slow of movement,

have advantages in their greater capacity for enduring hunger or thirst,

their power of resisting cold or drought in a state of torpidity, and

they have also some facilities for migration across the sea by means of

their eggs, which may be conveyed in crevices of timber or among masses

of floating vegetable matter. And when we come to the vegetable kingdom,

the means of transport are at their maximum, numbers of seeds having

special adaptations for being carried by mammalia or birds, and for

floating in the water, or through the air, while many are so small and

so light that there is practically no limit to the distances they may be

carried by gales and hurricanes.
We may, therefore, feel quite certain that the means of distribution

that have enabled the larger mammalia to reach the most remote regions

from a common starting-point, will be at least as efficacious, and

usually far more efficacious, with all other land animals and plants;

and if in every case the existing distribution of this class can be

explained on the theory of oceanic and continental permanence, with the

limited changes of sea and land already referred to, no valid objections

can be taken against this theory founded on anomalies of distribution in

other orders. Yet nothing is more common than for students of this or

that group to assort that the theory of oceanic permanence is quite

inconsistent with the distribution of its various species and genera.

Because a few Indian genera and closely allied species of birds are

found in Madagascar, a land termed "Lemuria" has been supposed to have

united the two countries during a comparatively recent geological epoch;

while the similarity of fossil plants and reptiles, from the Permian and

Miocene formations of India and South Africa, has been adduced as

further evidence of this connection. But there are also genera of

snakes, of insects, and of plants, common to Madagascar and South

America only, which have been held to necessitate a direct land

connection between these countries. These views evidently refute

themselves, because any such land connections must have led to a far

greater similarity in the productions of the several countries than

actually exists, and would besides render altogether inexplicable the

absence of all the chief types of African and Indian mammalia from

Madagascar, and its marvellous individuality in every department of the

organic world.[169]

_Powers of Dispersal as illustrated by Insular Organisms._
Having arrived at the conclusion that our existing oceans have remained

practically unaltered throughout the Tertiary and Secondary periods of

geology, and that the distribution of the mammalia is such as might

have been brought about by their known powers of dispersal, and by such

changes of land and sea as have probably or certainly occurred, we are,

of course, restricted to similar causes to explain the much wider and

sometimes more eccentric distribution of other classes of animals and of

plants. In doing so, we have to rely partly on direct evidence of

dispersal, afforded by the land organisms that have been observed far

out at sea, or which have taken refuge on ships, as well as by the

periodical visitants to remote islands; but very largely on indirect

evidence, afforded by the frequent presence of certain groups on remote

oceanic islands, which some ancestral forms must, therefore, have

reached by transmission across the ocean from distant lands.

_Birds._
These vary much in their powers of flight, and their capability of

traversing wide seas and oceans. Many swimming and wading birds can

continue long on the wing, fly swiftly, and have, besides, the power of

resting safely on the surface of the water. These would hardly be

limited by any width of ocean, except for the need of food; and many of

them, as the gulls, petrels, and divers, find abundance of food on the

surface of the sea itself. These groups have a wide distribution

_across_ the oceans; while waders--especially plovers, sandpipers,

snipes, and herons--are equally cosmopolitan, travelling _along_ the

coasts of all the continents, and across the narrow seas which separate

them. Many of these birds seem unaffected by climate, and as the

organisms on which they feed are equally abundant on arctic, temperate,

and tropical shores, there is hardly any limit to the range even of some

of the species.

usually limited powers of flight, their inability to rest on the surface

of the sea or to obtain food from it, and their greater specialisation,

which renders them less able to maintain themselves in the new countries

they may occasionally reach. Many of them are adapted to live only in

woods, or in marshes, or in deserts; they need particular kinds of food

or a limited range of temperature; and they are adapted to cope only

with the special enemies or the particular group of competitors among

which they have been developed. Such birds as these may pass again and

again to a new country, but are never able to establish themselves in

it; and it is this organic barrier, as it is termed, rather than any

physical barrier, which, in many cases, determines the presence of a

species in one area and its absence from another. We must always

remember, therefore, that, although the presence of a species in a

remote oceanic island clearly proves that its ancestors must at one time

have found their way there, the absence of a species does not prove the

contrary, since it also may have reached the island, but have been

unable to maintain itself, owing to the inorganic or organic conditions

not being suitable to it. This general principle applies to all classes

of organisms, and there are many striking illustrations of it. In the

Azores there are eighteen species of land-birds which are permanent

residents, but there are also several others which reach the islands

almost every year after great storms, but have never been able to

establish themselves. In Bermuda the facts are still more striking,

since there are only ten species of resident birds, while no less than

twenty other species of land-birds and more than a hundred species of

waders and aquatics are frequent visitors, often in great numbers, but

are never able to establish themselves. On the same principle we account

for the fact that, of the many continental insects and birds that have

been let loose, or have escaped from confinement, in this country,

hardly one has been able to maintain itself, and the same phenomenon is

still more striking in the case of plants. Of the thousands of hardy

plants which grow easily in our gardens, very few have ever run wild,

and when the experiment is purposely tried it invariably fails. Thus A.

de Candolle informs us that several botanists of Paris, Geneva, and

especially of Montpellier, have sown the seeds of many hundreds of

species of exotic hardy plants, in what appeared to be the most

favourable situations, but that in hardly a single case has any one of

them become naturalised.[170] Still more, then, in plants than in

animals the absence of a species does not prove that it has never

reached the locality, but merely that it has not been able to maintain

itself in competition with the native productions. In other cases, as

we have seen, facts of an exactly opposite nature occur. The rat, the

pig, and the rabbit, the water-cress, the clover, and many other plants,

when introduced into New Zealand, nourish exceedingly, and even

exterminate their native competitors; so that in these cases we may feel

sure that the species in question did not exist in New Zealand simply

because they had been unable to reach that country by their natural

means of dispersal. I will now give a few cases, in addition to those

recorded in my previous works, of birds and insects which have been

observed far from any land.

_Birds and Insects at Sea._

occurrence of a great number of small land-birds about the ship on 15th

March 1886, when in Lat. 48° 31' N., Long. 8° 16' W. He says: "A great

many small land-birds about us; put about sixty into a coop, evidently

tired out." And two days later, 17th March, "Over fifty of the birds

cooped on 15th died, though fed. Sparrows, finches, water-wagtails, two

small birds, name unknown, one kind like a linnet, and a large bird like

a starling. In all there have been on board over seventy birds, besides

some that hovered about us for some time and then fell into the sea

exhausted." Easterly winds and severe weather were experienced at the

time.[171] The spot where this remarkable flight of birds was met with

is about 160 miles due west of Brest, and this is the least distance the

birds must have been carried. It is interesting to note that the

position of the ship is nearly in the line from the English and French

coasts to the Azores, where, after great storms, so many bird stragglers

arrive annually. These birds were probably blown out to sea during their

spring migration along the south coast of England to Wales and Ireland.

During the autumnal migration, however, great flocks of

birds--especially starlings, thrushes, and fieldfares--have been

observed every year flying out to sea from the west coast of Ireland,

almost the whole of which must perish. At the Nash Lighthouse, in the

Bristol Channel on the coast of Glamorganshire, an enormous number of

small birds were observed on 3d September, including nightjars,

buntings, white-throats, willow-wrens, cuckoos, house-sparrows, robins,

wheatears, and blackbirds. These had probably crossed from

Somersetshire, and had they been caught by a storm the larger portion of

them must have been blown out to sea.[172]
These facts enable us to account sufficiently well for the birds of

oceanic islands, the number and variety of which are seen to be

proportionate to their facilities for reaching the island and

maintaining themselves in it. Thus, though more birds yearly reach

Bermuda than the Azores, the number of residents in the latter islands

is much larger, due to the greater extent of the islands, their number,

and their more varied surface. In the Galapagos the land-birds are still

more numerous, due in part to their larger area and greater proximity to

the continent, but chiefly to the absence of storms, so that the birds

which originally reached the islands have remained long isolated and

have developed into many closely allied species adapted to the special

conditions. All the species of the Galapagos but one are peculiar to the

islands, while the Azores possess only one peculiar species, and Bermuda

none--a fact which is clearly due to the continual immigration of fresh

individuals keeping up the purity of the breed by intercrossing. In the

Sandwich Islands, which are extremely isolated, being more than 2000

miles from any continent or large island, we have a condition of things

similar to what prevails in the Galapagos, the land-birds, eighteen in

number, being all peculiar, and belonging, except one, to peculiar

genera. These birds have probably all descended from three or four

original types which reached the islands at some remote period, probably

by means of intervening islets that have since disappeared. In St.

Helena we have a degree of permanent isolation which has prevented any

land-birds from reaching the island; for although its distance from the

continent, 1100 miles, is not so great as in the case of the Sandwich

Islands, it is situated in an ocean almost entirely destitute of small

islands, while its position within the tropics renders it free from

violent storms. Neither is there, on the nearest part of the coast of

Africa, a perpetual stream of migrating birds like that which supplies

the innumerable stragglers which every year reach Bermuda and the

Azores.

_Insects._

their power of flight, aided by violent or long-continued winds. Being

so small, and of such low specific gravity, they are occasionally

carried to still greater distances; and thus no islands, however remote,

are altogether without them. The eggs of insects, being often deposited

in borings or in crevices of timber, may have been conveyed long

distances by floating trees, as may the larvae of those species which

feed on wood. Several cases have been published of insects coming on

board ships at great distances from land; and Darwin records having

caught a large grasshopper when the ship was 370 miles from the coast of

Africa, whence the insect had probably come.
In the _Entomologists' Monthly Magazine_ for June 1885, Mr. MacLachlan

has recorded the occurrence of a swarm of moths in the Atlantic ocean,

from the log of the ship _Pleione_. The vessel was homeward bound from

New Zealand, and in Lat. 6° 47' N., Long. 32° 50' W., hundreds of moths

appeared about the ship, settling in numbers on the spars and rigging.

The wind for four days previously had been very light from north,

north-west, or north-east, and sometimes calm. The north-east trade wind

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.

_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._

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._

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.

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._

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.

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