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Darwinism (1889) by Alfred Russel Wallace

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But such cases as these will only lead to the production of a few
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.

Another mode of isolation is brought about by the variety--either owing
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]

It need hardly be said that the views of Mr. Darwin and myself are
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.

There yet remain a number of difficulties and objections relating to the
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
as of no importance in structure, are continually found to be
functionally important; and I have been especially struck with this fact
in the case of plants, to which my observations have, of late years,
been confined. Therefore it seems to me rather rash to consider slight
differences between representative species, for instance, those
inhabiting the different islands of the same archipelago, as of no
functional importance, and as not in any way due to natural selection"
_(Life of Darwin_, vol. iii. p. 161).]

[Footnote 48: See _Variation of Animals and Plants_, vol. i. p. 86.]

[Footnote 49: _Journal of the Linnean Society, Zoology,_ vol. xx. p.
215.]

[Footnote 50: In Mr. Gulick's last paper (_Journal of Linn. Soc. Zool._,
vol. xx. pp. 189-274) he discusses the various forms of isolation above
referred to, under no less than thirty-eight different divisions and
subdivisions, with an elaborate terminology, and he argues that these
will frequently bring about divergent evolution without any change in
the environment or any action of natural selection. The discussion of
the problem here given will, I believe, sufficiently expose the fallacy
of his contention; but his illustration of the varied and often
recondite modes by which practical isolation may be brought about, may
help to remove one of the popular difficulties in the way of the action
of natural selection in the origination of species.]




CHAPTER VII

ON THE INFERTILITY OF CROSSES BETWEEN DISTINCT SPECIES AND THE USUAL
STERILITY OF THEIR HYBRID OFFSPRING


Statement of the problem--Extreme susceptibility of the
reproductive functions--Reciprocal crosses--Individual
differences in respect to cross-fertilisation--Dimorphism and
trimorphism among plants--Cases of the fertility of hybrids and
of the infertility of mongrels--The effects of close
interbreeding--Mr. Huth's objections--Fertile hybrids among
animals--Fertility of hybrids among plants--Cases of sterility
of mongrels--Parallelism between crossing and change of
conditions--Remarks on the facts of hybridity--Sterility due to
changed conditions and usually correlated with other
characters--Correlation of colour with constitutional
peculiarities--The isolation of varieties by selective
association--The influence of natural selection upon sterility
and fertility--Physiological selection--Summary and concluding
remarks.



One of the greatest, or perhaps we may say the greatest, of all the
difficulties in the way of accepting the theory of natural selection as
a complete explanation of the origin of species, has been the remarkable
difference between varieties and species in respect of fertility when
crossed. Generally speaking, it may be said that the varieties of any
one species, however different they may be in external appearance, are
perfectly fertile when crossed, and their mongrel offspring are equally
fertile when bred among themselves; while distinct species, on the other
hand, however closely they may resemble each other externally, are
usually infertile when crossed, and their hybrid offspring absolutely
sterile. This used to be considered a fixed law of nature, constituting
the absolute test and criterion of a _species_ as distinct from a
_variety_; and so long as it was believed that species were separate
creations, or at all events had an origin quite distinct from that of
varieties, this law could have no exceptions, because, if any two
species had been found to be fertile when crossed and their hybrid
offspring to be also fertile, this fact would have been held to prove
them to be not _species_ but _varieties_. On the other hand, if two
varieties had been found to be infertile, or their mongrel offspring to
be sterile, then it would have been said: These are not varieties but
true species. Thus the old theory led to inevitable reasoning in a
circle; and what might be only a rather common fact was elevated into a
law which had no exceptions.

The elaborate and careful examination of the whole subject by Mr.
Darwin, who has brought together a vast mass of evidence from the
experience of agriculturists and horticulturists, as well as from
scientific experimenters, has demonstrated that there is no such fixed
law in nature as was formerly supposed. He shows us that crosses between
some varieties are infertile or even sterile, while crosses between some
species are quite fertile; and that there are besides a number of
curious phenomena connected with the subject which render it impossible
to believe that sterility is anything more than an incidental property
of species, due to the extreme delicacy and susceptibility of the
reproductive powers, and dependent on physiological causes we have not
yet been able to trace. Nevertheless, the fact remains that most species
which have hitherto been crossed produce sterile hybrids, as in the
well-known case of the mule; while almost all domestic varieties, when
crossed, produce offspring which are perfectly fertile among themselves.
I will now endeavour to give such a sketch of the subject as may enable
the reader to see something of the complexity of the problem, referring
him to Mr. Darwin's works for fuller details.


_Extreme Susceptibility of the Reproductive Functions._

One of the most interesting facts, as showing how susceptible to changed
conditions or to slight constitutional changes are the reproductive
powers of animals, is the very general difficulty of getting those which
are kept in confinement to breed; and this is frequently the only bar to
domesticating wild species. Thus, elephants, bears, foxes, and numbers
of species of rodents, very rarely breed in confinement; while other
species do so more or less freely. Hawks, vultures, and owls hardly ever
breed in confinement; neither did the falcons kept for hawking ever
breed. Of the numerous small seed-eating birds kept in aviaries, hardly
any breed, neither do parrots. Gallinaceous birds usually breed freely
in confinement, but some do not; and even the guans and curassows, kept
tame by the South American Indians, never breed. This shows that change
of climate has nothing to do with the phenomenon; and, in fact, the same
species that refuse to breed in Europe do so, in almost every case, when
tamed or confined in their native countries. This inability to reproduce
is not due to ill-health, since many of these creatures are perfectly
vigorous and live very long.

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A way with tears makes Magorian a worthy Costa winner
Articles published by guardian.co.uk Books

Perfumes: the Guide – a portal to a whole new art

Michelle Magorian scooped the 2008 Costa Children's Book Award with Just Henry, a huge 700-page book that made me cry. Not many authors can do that but Magorian handles dangerously emotional stuff and pulls it off without slipping into mawkish sentimentality. Hence tears.

The same quality marked out Goodnight Mister Tom, her first novel, which won the 1980 Guardian children's book prize and has been read by every child in year 6 and many others both younger and older – rightly so – ever since. Goodnight Mister Tom is avowedly weepy. Only the hardest heart could remain unmoved. I once met a child who'd sticky-taped three pages together because they made her cry too much – I'm sure everyone who's read the book will know which three.

In Goodnight Mister Tom, Magorian had the external drama of the second world war as an emotional backdrop: put simply, there was a lot to weep over. In Just Henry, however, the setting is 1949 and there should be – and is – a feeling of optimism and hope. It is a period that's rarely used in fiction but Just Henry reveals it to be one that's worth exploring. The effect of the war is still being felt in the social changes it brought about. Life didn't just "slip back": few families were lucky enough to remain unaffected. Fathers were lost or altered; mothers found themselves raising families alone, or having to return abruptly to a subordinate role; children were forced to make adjustments either way.

In her big, bold novel, knitted together with more mysteries and coincidences than are credible, Magorian wonderfully captures that uncertainty and shows children's ability to move forward and embrace change far faster than their parents or grandparents. Lest this realism and the solving of the mysteries is too mundane, Michelle adds an extra layer of emotion by weaving in the stories of film stars from the movies of the day. For once, the current fashion of long, long, long books is justified. Just Henry is a wallowing great read. Just don't forget your hanky.

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Leona Lewis to write autobiography

I touched on Perfumes: the Guide by Luca Turin and Tania Sanchez in today's G2 arts diary. What a wonderful book (I've just opened it and, in a synaesthesic overture, it's offering me Amouage Gold, a spray-sample of which I left between its leaves). It offers a critical analysis – smelling notes, if you will – of 1,500 perfumes. I suppose the authors' language and technique relates somewhat to wine criticism. But the art of writing about something so emotionally rich and elusive (and at the same time entirely unlockable by the proper technical expertise) reminded me a bit of how people write (or try to write) about music (a subject I'm speaking about at this year's Association of British Orchestras annual conference). As it happens, Turin and Sanchez often use musical metaphor to help explain the nature of a perfume (they talk in terms of "brassy" or "melodic line" or "string section"; Shalimar has a "uniquely sweet, penetrating tune"; Yatagan a "high-pitched, hissing tone"). What about a job swap between these two and Andrew Clements or Alexis Petridis, I wonder.

Over Christmas I did a lot of smelling in the various perfumery halls and perfumery shops of London, and had enormous fun trying to get to grips with the artform. The joy of it is that most perfumes are widely available and can be squirted by the curious with impunity. The scent I've been wearing for the past few years – Irisia by Creed – Turin and Sanchez write off, with a cutting one-star review, as a "green floral chypre of exceptional banality and unpleasantness", so I have have had the amusement of trying to find a replacement.

By the way, Sanchez notes the importance of the web in writing and sharing knowledge about perfume. She writes: "Until recently, talking intelligently about the art of perfume seemed impossible. Then suddenly it seemed inevitable. What changed? The obvious: the Internet. Online now you can read historical and technical information, find discontinued or otherwise elusive perfumes, order samples of raw materials to smell out of curiosity, and, most important, find communities of people clustered around this single obsession. Half of what I know I owe to the 24-hour-a-day pajama party that is the fragrance board of Makeup Alley... Online communities can criticise perfume in a way that magazines have never dared: there's no advertising to lose... Perfume blogs now seem to outnumber the sample vials around my desk: there are men and women of intelligence sitting down every day and thinking and writing about perfume."

It's a bit like the way Alex Ross talks about the way the web has affected his relationship with and access to contemporary music. It's that old long tail.

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