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

and the obvious and sufficient explanation of this fact is, that they

did _not_ vary enough at the time when variation was required to bring

them into harmony with changed conditions. The objection as to the

"right" or "beneficial" variation occurring when required, seems

therefore to have no weight in view of the actual facts of variation.

_Isolation to prevent Intercrossing._

species a very important factor in the formation of new species, while

others maintain it to be absolutely essential. This latter view has

arisen from an exaggerated opinion as to the power of intercrossing to

keep down any variety or incipient species, and merge it in the parent

stock. But it is evident that this can only occur with varieties which

are not useful, or which, if useful, occur in very small numbers; and

from this kind of variations it is clear that new species do not arise.

Complete isolation, as in an oceanic island, will no doubt enable

natural selection to act more rapidly, for several reasons. In the first

place, the absence of competition will for some time allow the new

immigrants to increase rapidly till they reach the limits of

subsistence. They will then struggle among themselves, and by survival

of the fittest will quickly become adapted to the new conditions of

their environment. Organs which they formerly needed, to defend

themselves against, or to escape from, enemies, being no longer

required, would be encumbrances to be got rid of, while the power of

appropriating and digesting new and varied food would rise in

importance. Thus we may explain the origin of so many flightless and

rather bulky birds in oceanic islands, as the dodo, the cassowary, and

the extinct moas. Again, while this process was going on, the complete

isolation would prevent its being checked by the immigration of new

competitors or enemies, which would be very likely to occur in a

continuous area; while, of course, any intercrossing with the original

unmodified stock would be absolutely prevented. If, now, before this

change has gone very far, the variety spreads into adjacent but rather

distant islands, the somewhat different conditions in each may lead to

the development of distinct forms constituting what are termed

representative species; and these we find in the separate islands of the

Galapagos, the West Indies, and other ancient groups of islands.

peculiar species, descended from the original settlers which happened to

reach the islands; whereas, in wide areas, and in continents, we have

variation and adaptation on a much larger scale; and, whenever important

physical changes demand them, with even greater rapidity. The far

greater complexity of the environment, together with the occurrence of

variations in constitution and habits, will often allow of effective

isolation, even here, producing all the results of actual physical

isolation. As we have already explained, one of the most frequent modes

in which natural selection acts is, by adapting some individuals of a

species to a somewhat different mode of life, whereby they are able to

seize upon unappropriated places in nature, and in so doing they become

practically isolated from their parent form. Let us suppose, for

example, that one portion of a species usually living in forests ranges

into the open plains, and finding abundance of food remains there

permanently. So long as the struggle for existence is not exceptionally

severe, these two portions of the species may remain almost unchanged;

but suppose some fresh enemies are attracted to the plains by the

presence of these new immigrants, then variation and natural selection

would lead to the preservation of those individuals best able to cope

with the difficulty, and thus the open country form would become

modified into a marked variety or into a distinct species; and there

would evidently be little chance of this modification being checked by

intercrossing with the parent form which remained in the forest.

to habits, climate, or constitutional change--breeding at a slightly

different time from the parent species. This is known to produce

complete isolation in the case of many varieties of plants. Yet another

mode of isolation is brought about by changes of colour, and by the fact

that in a wild state animals of similar colours prefer to keep together

and refuse to pair with individuals of another colour. The probable

reason and utility of this habit will be explained in another chapter,

but the fact is well illustrated by the cattle which have run wild in

the Falkland Islands. These are of several different colours, but each

colour keeps in a separate herd, often restricted to one part of the

island; and one of these varieties--the mouse-coloured--is said to breed

a month earlier than the others; so that if this variety inhabited a

larger area it might very soon be established as a distinct race or

species.[48] Of course where the change of habits or of station is still

greater, as when a terrestrial animal becomes sub-aquatic, or when

aquatic animals come to live in tree-tops, as with the frogs and

Crustacea described at p. 118, the danger of intercrossing is reduced to

a minimum.
Several writers, however, not content with the indirect effects of

isolation here indicated, maintain that it is in itself a cause of

modification, and ultimately of the origination of new species. This

was the keynote of Mr. Vernon Wollaston's essay on "Variation of

Species," published in 1856, and it is adopted by the Rev. J.G. Gulick

in his paper on "Diversity of Evolution under one Set of External

Conditions" (_Journ. Linn. Soc. Zool._, vol. xi. p. 496). The idea seems

to be that there is an inherent tendency to variation in certain

divergent lines, and that when one portion of a species is isolated,

even though under identical conditions, that tendency sets up a

divergence which carries that portion farther and farther away from the

original species. This view is held to be supported by the case of the

land shells of the Sandwich Islands, which certainly present some very

remarkable phenomena. In this comparatively small area there are about

300 species of land shells, almost all of which belong to one family (or

sub-family), the Achatinellidae, found nowhere else in the world. The

interesting point is the extreme restriction of the species and

varieties. The average range of each species is only five or six miles,

while some are restricted to but one or two square miles, and only a

very few range over a whole island. The forest region that extends over

one of the mountain-ranges of the island of Oahu, is about forty miles

in length and five or six miles in breadth; and this small territory

furnishes about 175 species, represented by 700 or 800 varieties. Mr.

Gulick states, that the vegetation of the different valleys on the same

side of this range is much the same, yet each has a molluscan fauna

differing in some degree from that of any other. "We frequently find a

genus represented in several successive valleys by allied species,

sometimes feeding on the same, sometimes on different plants. In every

such case the valleys that are nearest to each other furnish the most

nearly allied forms; and a full set of the varieties of each species

presents a minute gradation of forms between the more divergent types

found in the more widely separated localities." He urges, that these

constant differences cannot be attributed to natural selection, because

they occur in different valleys on the same side of the mountain, where

food, climate, and enemies are the same; and also, because there is no

greater difference in passing from the rainy to the dry side of the

mountains than in passing from one valley to another on the same side

an equal distance apart. In a very lengthy paper, presented to the

Linnean Society last year, on "Divergent Evolution through Cumulative

Segregation," Mr. Gulick endeavours to work out his views into a

complete theory, the main point of which may perhaps be indicated by the

following passage: "No two portions of a species possess exactly the

same average character, and the initial differences are for ever

reacting on the environment and on each other in such a way as to ensure

increasing divergence in each successive generation as long as the

individuals of the two groups are kept from intercrossing."[49]

inconsistent with the notion that, if the environment were absolutely

similar for the two isolated portions of the species, any such necessary

and constant divergence would take place. It is an error to assume that

what seem to us identical conditions are really identical to such small

and delicate organisms as these land molluscs, of whose needs and

difficulties at each successive stage of their existence, from the

freshly-laid egg up to the adult animal, we are so profoundly ignorant.

The exact proportions of the various species of plants, the numbers of

each kind of insect or of bird, the peculiarities of more or less

exposure to sunshine or to wind at certain critical epochs, and other

slight differences which to us are absolutely immaterial and

unrecognisable, may be of the highest significance to these humble

creatures, and be quite sufficient to require some slight adjustments of

size, form, or colour, which natural selection will bring about. All we

know of the facts of variation leads us to believe that, without this

action of natural selection, there would be produced over the whole area

a series of inconstant varieties mingled together, not a distinct

segregation of forms each confined to its own limited area.
Mr. Darwin has shown that, in the distribution and modification of

species, the biological is of more importance than the physical

environment, the struggle with other organisms being often more severe

than that with the forces of nature. This is particularly evident in the

case of plants, many of which, when protected from competition, thrive

in a soil, climate, and atmosphere widely different from those of their

native habitat. Thus, many alpine plants only found near perpetual snow

thrive well in our gardens at the level of the sea; as do the tritomas

from the sultry plains of South Africa, the yuccas from the arid hills

of Texas and Mexico, and the fuchsias from the damp and dreary shores of

the Straits of Magellan. It has been well said that plants do not live

where they like, but where they can; and the same remark will apply to

the animal world. Horses and cattle run wild and thrive both in North

and South America; rabbits, once confined to the south of Europe, have

established themselves in our own country and in Australia; while the

domestic fowl, a native of tropical India, thrives well in every part of

the temperate zone.
If, then, we admit that when one portion of a species is separated from

the rest, there will necessarily be a slight difference in the average

characters of the two portions, it does not follow that this difference

has much if any effect upon the characteristics that are developed by a

long period of isolation. In the first place, the difference itself will

necessarily be very slight unless there is an exceptional amount of

variability in the species; and in the next place, if the average

characters of the species are the expression of its exact adaptation to

its whole environment, then, given a precisely similar environment, and

the isolated portion will inevitably be brought back to the same average

of characters. But, as a matter of fact, it is impossible that the

environment of the isolated portion can be exactly like that of the bulk

of the species. It cannot be so physically, since no two separated areas

can be absolutely alike in climate and soil; and even if these are the

same, the geographical features, size, contour, and relation to winds,

seas, and rivers, would certainly differ. Biologically, the differences

are sure to be considerable. The isolated portion of a species will

almost always be in a much smaller area than that occupied by the

species as a whole, hence it is at once in a different position as

regards its own kind. The proportions of all the other species of

animals and plants are also sure to differ in the two areas, and some

species will almost always be absent in the smaller which are present in

the larger country. These differences will act and react on the

isolated portion of the species. The struggle for existence will differ

in its severity and in its incidence from that which affects the bulk of

the species. The absence of some one insect or other creature inimical

to the young animal or plant may cause a vast difference in its

conditions of existence, and may necessitate a modification of its

external or internal characters in quite a different direction from that

which happened to be present in the average of the individuals which

were first isolated.
On the whole, then, we conclude that, while isolation is an important

factor in effecting some modification of species, it is so, not on

account of any effect produced, or influence exerted by isolation _per

se_, but because it is always and necessarily accompanied by a change of

environment, both physical and biological. Natural selection will then

begin to act in adapting the isolated portion to its new conditions, and

will do this the more quickly and the more effectually because of the

isolation. We have, however, seen reason to believe that geographical or

local isolation is by no means essential to the differentiation of

species, because the same result is brought about by the incipient

species acquiring different habits or frequenting a different station;

and also by the fact that different varieties of the same species are

known to prefer to pair with their like, and thus to bring about a

physiological isolation of the most effective kind. This part of the

subject will be again referred to when the very difficult problems

presented by hybridity are discussed.[50]

_Cases in which Isolation is Ineffective._
One objection to the views of those who, like Mr. Gulick, believe

isolation itself to be a cause of modification of species deserves

attention, namely, the entire absence of change where, if this were a

_vera causa_, we should expect to find it. In Ireland we have an

excellent test case, for we know that it has been separated from Britain

since the end of the glacial epoch, certainly many thousand years. Yet

hardly one of its mammals, reptiles, or land molluscs has undergone the

slightest change, even although there is certainly a distinct difference

in the environment both inorganic and organic. That changes have not

occurred through natural selection, is perhaps due to the less severe

struggle for existence owing to the smaller number of competing species;

but, if isolation itself were an efficient cause, acting continuously

and cumulatively, it is incredible that a decided change should not have

been produced in thousands of years. That no such change has occurred in

this, and many other cases of isolation, seems to prove that it is not

in itself a cause of modification.

question of hybridity, which are so important as to require a separate

chapter for their adequate discussion.
FOOTNOTES:
[Footnote 41: See _Origin of Species_, pp. 176-198.]
[Footnote 42: See Kerner's _Flowers and their Unbidden Guests_ for

numerous other structures and peculiarities of plants which are shown to

be adaptive and useful.]
[Footnote 43: _Nature_, vol. xx. p. 603.]
[Footnote 44: _Nature_, vol. xxxviii. p. 328.]
[Footnote 45: A very remarkable illustration of function in an

apparently useless ornament is given by Semper. He says, "It is known

that the skin of reptiles encloses the body with scales. These scales

are distinguished by very various sculpturings, highly characteristic of

the different species. Irrespective of their systematic significance

they appear to be of no value in the life of the animal; indeed, they

are viewed as ornamental without regard to the fact that they are

microscopic and much too delicate to be visible to other animals of

their own species. It might, therefore, seem hopeless to show the

necessity for their existence on Darwinian principles, and to prove that

they are physiologically active organs. Nevertheless, recent

investigations on this point have furnished evidence that this is

possible.
"It is known that many reptiles, and above all the snakes, cast off the

whole skin at once, whereas human beings do so by degrees. If by any

accident they are prevented doing so, they infallibly die, because the

old skin has grown so tough and hard that it hinders the increase in

volume which is inseparable from the growth of the animal. The casting

of the skin is induced by the formation on the surface of the inner

epidermis, of a layer of very fine and equally distributed hairs, which

evidently serve the purpose of mechanically raising the old skin by

their rigidity and position. These hairs then may be designated as

_casting hairs_. That they are destined and calculated for this end is

evident to me from the fact established by Dr. Braun, that the casting

of the shells of the river crayfish is induced in exactly the same

manner by the formation of a coating of hairs which mechanically loosens

the old skin or shell from the new. Now the researches of Braun and

Cartier have shown that these casting hairs--which serve the same

purpose in two groups of animals so far apart in the systematic

scale--after the casting, are partly transformed into the concentric

stripes, sharp spikes, ridges, or warts which ornament the outer edges

of the skin-scales of reptiles or the carapace of crabs."[1] Professor

Semper adds that this example, with many others that might be quoted,

shows that we need not abandon the hope of explaining morphological

characters on Darwinian principles, although their nature is often

difficult to understand.
During a recent discussion of this question in the pages of _Nature_,

Mr. St. George Mivart adduces several examples of what he deems useless

specific characters. Among them are the aborted index finger of the

lemurine Potto, and the thumbless hands of Colobus and Ateles, the

"life-saving action" of either of which he thinks incredible. These

cases suggest two remarks. In the first place, they involve _generic_,

not _specific_, characters; and the three genera adduced are somewhat

isolated, implying considerable antiquity and the extinction of many

allied forms. This is important, because it affords ample time for great

changes of conditions since the structures in question originated; and

without a knowledge of these changes we can never safely assert that any

detail of structure could not have been useful. In the second place, all

three are cases of aborted or rudimentary organs; and these are admitted

to be explained by non-use, leading to diminution of size, a further

reduction being brought about by the action of the principle of economy

of growth. But, when so reduced, the rudiment might be inconvenient or

even hurtful, and then natural selection would aid in its complete

abortion; in other words, the abortion of the part would be _useful_,

and would therefore be subject to the law of survival of the fittest.

The genera Ateles and Colobus are two of the most purely arboreal types

of monkeys, and it is not difficult to conceive that the constant use of

the elongated fingers for climbing from tree to tree, and catching on to

branches while making great leaps, might require all the nervous energy

and muscular growth to be directed to the fingers, the small thumb

remaining useless. The case of the Potto is more difficult, both because

it is, presumably, a more ancient type, and its actual life-history and

habits are completely unknown. These cases are, therefore, not at all to

the point as proving that positive specific characters--not mere

rudiments characterising whole genera--are in any case useless.
Mr. Mivart further objects to the alleged rigidity of the action of

natural selection, because wounded or malformed animals have been found

which had evidently lived a considerable time in their imperfect

condition. But this simply proves that they were living under a

temporarily favourable environment, and that the real struggle for

existence, in their case, had not yet taken place. We must surely admit

that, when the pinch came, and when perfectly formed stoats were dying

for want of food, the one-footed animal, referred to by Mr. Mivart,

would be among the first to succumb; and the same remark will apply to

his abnormally toothed hares and rheumatic monkeys, which might,

nevertheless, get on very well under favourable conditions. The struggle

for existence, under which all animals and plants have been developed,

is intermittent, and exceedingly irregular in its incidence and

severity. It is most severe and fatal to the young; but when an animal

has once reached maturity, and especially when it has gained experience

by several years of an eventful existence, it may be able to maintain

itself under conditions which would be fatal to a young and

inexperienced creature of the same species. The examples adduced by Mr.

Mivart do not, therefore, in any way impugn the hardness of nature as a

taskmaster, or the extreme severity of the recurring struggle for

existence. (See _Nature_, vol. xxxix. p. 127.)]
[Footnote 46: _Origin of Species,_ p. 72.]
[Footnote 47: Darwin's latest expression of opinion on this question is

interesting, since it shows that he was inclined to return to his

earlier view of the general, or universal, utility of specific

characters. In a letter to Semper (30th Nov. 1878) he writes: "As our

knowledge advances, very slight differences, considered by systematists