Sir Richard Francis Burton - "Personal Narrative of a Pilgrimage to Al Madinah & Meccah"

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John Brown, M. D. - "Rab and His Friends"

T.S. Arthur - "The Iron Rule"

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PHILADELPHIA, Pa. -- The Philadelphia literary world will celebrate the launch of two new players today, April 10th: Kay Square Press, a new publishing company focused on Philadelphia-area artists, their stories, and their art; and Kay Square's first release, 'With the Rich and Mighty: Emlen Etting of Philadelphia' (ISBN: 978-0-9815129-0-7), a critical biography by Kenneth C. Kaleta.

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Books: On the Origin of Species

C >> Charles Darwin >> On the Origin of Species




Let us now turn to the nectar-feeding insects in our imaginary case:
we may suppose the plant of which we have been slowly increasing the
nectar by continued selection, to be a common plant; and that certain
insects depended in main part on its nectar for food. I could give
many facts, showing how anxious bees are to save time; for instance,
their habit of cutting holes and sucking the nectar at the bases of
certain flowers, which they can, with a very little more trouble,
enter by the mouth. Bearing such facts in mind, I can see no reason to
doubt that an accidental deviation in the size and form of the body,
or in the curvature and length of the proboscis, etc., far too slight
to be appreciated by us, might profit a bee or other insect, so that
an individual so characterised would be able to obtain its food more
quickly, and so have a better chance of living and leaving
descendants. Its descendants would probably inherit a tendency to a
similar slight deviation of structure. The tubes of the corollas of
the common red and incarnate clovers (Trifolium pratense and
incarnatum) do not on a hasty glance appear to differ in length; yet
the hive-bee can easily suck the nectar out of the incarnate clover,
but not out of the common red clover, which is visited by humble-bees
alone; so that whole fields of the red clover offer in vain an
abundant supply of precious nectar to the hive-bee. Thus it might be a
great advantage to the hive-bee to have a slightly longer or
differently constructed proboscis. On the other hand, I have found by
experiment that the fertility of clover greatly depends on bees
visiting and moving parts of the corolla, so as to push the pollen on
to the stigmatic surface. Hence, again, if humble-bees were to become
rare in any country, it might be a great advantage to the red clover
to have a shorter or more deeply divided tube to its corolla, so that
the hive-bee could visit its flowers. Thus I can understand how a
flower and a bee might slowly become, either simultaneously or one
after the other, modified and adapted in the most perfect manner to
each other, by the continued preservation of individuals presenting
mutual and slightly favourable deviations of structure.

I am well aware that this doctrine of natural selection, exemplified
in the above imaginary instances, is open to the same objections which
were at first urged against Sir Charles Lyell's noble views on "the
modern changes of the earth, as illustrative of geology;" but we now
very seldom hear the action, for instance, of the coast-waves, called
a trifling and insignificant cause, when applied to the excavation of
gigantic valleys or to the formation of the longest lines of inland
cliffs. Natural selection can act only by the preservation and
accumulation of infinitesimally small inherited modifications, each
profitable to the preserved being; and as modern geology has almost
banished such views as the excavation of a great valley by a single
diluvial wave, so will natural selection, if it be a true principle,
banish the belief of the continued creation of new organic beings, or
of any great and sudden modification in their structure.

ON THE INTERCROSSING OF INDIVIDUALS.

I must here introduce a short digression. In the case of animals and
plants with separated sexes, it is of course obvious that two
individuals must always unite for each birth; but in the case of
hermaphrodites this is far from obvious. Nevertheless I am strongly
inclined to believe that with all hermaphrodites two individuals,
either occasionally or habitually, concur for the reproduction of
their kind. This view, I may add, was first suggested by Andrew
Knight. We shall presently see its importance; but I must here treat
the subject with extreme brevity, though I have the materials prepared
for an ample discussion. All vertebrate animals, all insects, and some
other large groups of animals, pair for each birth. Modern research
has much diminished the number of supposed hermaphrodites, and of real
hermaphrodites a large number pair; that is, two individuals regularly
unite for reproduction, which is all that concerns us. But still there
are many hermaphrodite animals which certainly do not habitually pair,
and a vast majority of plants are hermaphrodites. What reason, it may
be asked, is there for supposing in these cases that two individuals
ever concur in reproduction? As it is impossible here to enter on
details, I must trust to some general considerations alone.

In the first place, I have collected so large a body of facts,
showing, in accordance with the almost universal belief of breeders,
that with animals and plants a cross between different varieties, or
between individuals of the same variety but of another strain, gives
vigour and fertility to the offspring; and on the other hand, that
CLOSE interbreeding diminishes vigour and fertility; that these facts
alone incline me to believe that it is a general law of nature
(utterly ignorant though we be of the meaning of the law) that no
organic being self-fertilises itself for an eternity of generations;
but that a cross with another individual is occasionally--perhaps at
very long intervals--indispensable.

On the belief that this is a law of nature, we can, I think,
understand several large classes of facts, such as the following,
which on any other view are inexplicable. Every hybridizer knows how
unfavourable exposure to wet is to the fertilisation of a flower, yet
what a multitude of flowers have their anthers and stigmas fully
exposed to the weather! but if an occasional cross be indispensable,
the fullest freedom for the entrance of pollen from another individual
will explain this state of exposure, more especially as the plant's
own anthers and pistil generally stand so close together that
self-fertilisation seems almost inevitable. Many flowers, on the other
hand, have their organs of fructification closely enclosed, as in the
great papilionaceous or pea-family; but in several, perhaps in all,
such flowers, there is a very curious adaptation between the structure
of the flower and the manner in which bees suck the nectar; for, in
doing this, they either push the flower's own pollen on the stigma, or
bring pollen from another flower. So necessary are the visits of bees
to papilionaceous flowers, that I have found, by experiments published
elsewhere, that their fertility is greatly diminished if these visits
be prevented. Now, it is scarcely possible that bees should fly from
flower to flower, and not carry pollen from one to the other, to the
great good, as I believe, of the plant. Bees will act like a
camel-hair pencil, and it is quite sufficient just to touch the
anthers of one flower and then the stigma of another with the same
brush to ensure fertilisation; but it must not be supposed that bees
would thus produce a multitude of hybrids between distinct species;
for if you bring on the same brush a plant's own pollen and pollen
from another species, the former will have such a prepotent effect,
that it will invariably and completely destroy, as has been shown by
Gartner, any influence from the foreign pollen.

When the stamens of a flower suddenly spring towards the pistil, or
slowly move one after the other towards it, the contrivance seems
adapted solely to ensure self-fertilisation; and no doubt it is useful
for this end: but, the agency of insects is often required to cause
the stamens to spring forward, as Kolreuter has shown to be the case
with the barberry; and curiously in this very genus, which seems to
have a special contrivance for self-fertilisation, it is well known
that if very closely-allied forms or varieties are planted near each
other, it is hardly possible to raise pure seedlings, so largely do
they naturally cross. In many other cases, far from there being any
aids for self-fertilisation, there are special contrivances, as I
could show from the writings of C. C. Sprengel and from my own
observations, which effectually prevent the stigma receiving pollen
from its own flower: for instance, in Lobelia fulgens, there is a
really beautiful and elaborate contrivance by which every one of the
infinitely numerous pollen-granules are swept out of the conjoined
anthers of each flower, before the stigma of that individual flower is
ready to receive them; and as this flower is never visited, at least
in my garden, by insects, it never sets a seed, though by placing
pollen from one flower on the stigma of another, I raised plenty of
seedlings; and whilst another species of Lobelia growing close by,
which is visited by bees, seeds freely. In very many other cases,
though there be no special mechanical contrivance to prevent the
stigma of a flower receiving its own pollen, yet, as C. C. Sprengel
has shown, and as I can confirm, either the anthers burst before the
stigma is ready for fertilisation, or the stigma is ready before the
pollen of that flower is ready, so that these plants have in fact
separated sexes, and must habitually be crossed. How strange are these
facts! How strange that the pollen and stigmatic surface of the same
flower, though placed so close together, as if for the very purpose of
self-fertilisation, should in so many cases be mutually useless to
each other! How simply are these facts explained on the view of an
occasional cross with a distinct individual being advantageous or
indispensable!

If several varieties of the cabbage, radish, onion, and of some other
plants, be allowed to seed near each other, a large majority, as I
have found, of the seedlings thus raised will turn out mongrels: for
instance, I raised 233 seedling cabbages from some plants of different
varieties growing near each other, and of these only 78 were true to
their kind, and some even of these were not perfectly true. Yet the
pistil of each cabbage-flower is surrounded not only by its own six
stamens, but by those of the many other flowers on the same plant.
How, then, comes it that such a vast number of the seedlings are
mongrelized? I suspect that it must arise from the pollen of a
distinct VARIETY having a prepotent effect over a flower's own pollen;
and that this is part of the general law of good being derived from
the intercrossing of distinct individuals of the same species. When
distinct SPECIES are crossed the case is directly the reverse, for a
plant's own pollen is always prepotent over foreign pollen; but to
this subject we shall return in a future chapter.

In the case of a gigantic tree covered with innumerable flowers, it
may be objected that pollen could seldom be carried from tree to tree,
and at most only from flower to flower on the same tree, and that
flowers on the same tree can be considered as distinct individuals
only in a limited sense. I believe this objection to be valid, but
that nature has largely provided against it by giving to trees a
strong tendency to bear flowers with separated sexes. When the sexes
are separated, although the male and female flowers may be produced on
the same tree, we can see that pollen must be regularly carried from
flower to flower; and this will give a better chance of pollen being
occasionally carried from tree to tree. That trees belonging to all
Orders have their sexes more often separated than other plants, I find
to be the case in this country; and at my request Dr. Hooker tabulated
the trees of New Zealand, and Dr. Asa Gray those of the United States,
and the result was as I anticipated. On the other hand, Dr. Hooker has
recently informed me that he finds that the rule does not hold in
Australia; and I have made these few remarks on the sexes of trees
simply to call attention to the subject.

Turning for a very brief space to animals: on the land there are some
hermaphrodites, as land-mollusca and earth-worms; but these all pair.
As yet I have not found a single case of a terrestrial animal which
fertilises itself. We can understand this remarkable fact, which
offers so strong a contrast with terrestrial plants, on the view of an
occasional cross being indispensable, by considering the medium in
which terrestrial animals live, and the nature of the fertilising
element; for we know of no means, analogous to the action of insects
and of the wind in the case of plants, by which an occasional cross
could be effected with terrestrial animals without the concurrence of
two individuals. Of aquatic animals, there are many self-fertilising
hermaphrodites; but here currents in the water offer an obvious means
for an occasional cross. And, as in the case of flowers, I have as yet
failed, after consultation with one of the highest authorities,
namely, Professor Huxley, to discover a single case of an
hermaphrodite animal with the organs of reproduction so perfectly
enclosed within the body, that access from without and the occasional
influence of a distinct individual can be shown to be physically
impossible. Cirripedes long appeared to me to present a case of very
great difficulty under this point of view; but I have been enabled, by
a fortunate chance, elsewhere to prove that two individuals, though
both are self-fertilising hermaphrodites, do sometimes cross.

It must have struck most naturalists as a strange anomaly that, in the
case of both animals and plants, species of the same family and even
of the same genus, though agreeing closely with each other in almost
their whole organisation, yet are not rarely, some of them
hermaphrodites, and some of them unisexual. But if, in fact, all
hermaphrodites do occasionally intercross with other individuals, the
difference between hermaphrodites and unisexual species, as far as
function is concerned, becomes very small.

From these several considerations and from the many special facts
which I have collected, but which I am not here able to give, I am
strongly inclined to suspect that, both in the vegetable and animal
kingdoms, an occasional intercross with a distinct individual is a law
of nature. I am well aware that there are, on this view, many cases of
difficulty, some of which I am trying to investigate. Finally then, we
may conclude that in many organic beings, a cross between two
individuals is an obvious necessity for each birth; in many others it
occurs perhaps only at long intervals; but in none, as I suspect, can
self-fertilisation go on for perpetuity.

CIRCUMSTANCES FAVOURABLE TO NATURAL SELECTION.

This is an extremely intricate subject. A large amount of inheritable
and diversified variability is favourable, but I believe mere
individual differences suffice for the work. A large number of
individuals, by giving a better chance for the appearance within any
given period of profitable variations, will compensate for a lesser
amount of variability in each individual, and is, I believe, an
extremely important element of success. Though nature grants vast
periods of time for the work of natural selection, she does not grant
an indefinite period; for as all organic beings are striving, it may
be said, to seize on each place in the economy of nature, if any one
species does not become modified and improved in a corresponding
degree with its competitors, it will soon be exterminated.

In man's methodical selection, a breeder selects for some definite
object, and free intercrossing will wholly stop his work. But when
many men, without intending to alter the breed, have a nearly common
standard of perfection, and all try to get and breed from the best
animals, much improvement and modification surely but slowly follow
from this unconscious process of selection, notwithstanding a large
amount of crossing with inferior animals. Thus it will be in nature;
for within a confined area, with some place in its polity not so
perfectly occupied as might be, natural selection will always tend to
preserve all the individuals varying in the right direction, though in
different degrees, so as better to fill up the unoccupied place. But
if the area be large, its several districts will almost certainly
present different conditions of life; and then if natural selection be
modifying and improving a species in the several districts, there will
be intercrossing with the other individuals of the same species on the
confines of each. And in this case the effects of intercrossing can
hardly be counterbalanced by natural selection always tending to
modify all the individuals in each district in exactly the same manner
to the conditions of each; for in a continuous area, the conditions
will generally graduate away insensibly from one district to another.
The intercrossing will most affect those animals which unite for each
birth, which wander much, and which do not breed at a very quick rate.
Hence in animals of this nature, for instance in birds, varieties will
generally be confined to separated countries; and this I believe to be
the case. In hermaphrodite organisms which cross only occasionally,
and likewise in animals which unite for each birth, but which wander
little and which can increase at a very rapid rate, a new and improved
variety might be quickly formed on any one spot, and might there
maintain itself in a body, so that whatever intercrossing took place
would be chiefly between the individuals of the same new variety. A
local variety when once thus formed might subsequently slowly spread
to other districts. On the above principle, nurserymen always prefer
getting seed from a large body of plants of the same variety, as the
chance of intercrossing with other varieties is thus lessened.

Even in the case of slow-breeding animals, which unite for each birth,
we must not overrate the effects of intercrosses in retarding natural
selection; for I can bring a considerable catalogue of facts, showing
that within the same area, varieties of the same animal can long
remain distinct, from haunting different stations, from breeding at
slightly different seasons, or from varieties of the same kind
preferring to pair together.

Intercrossing plays a very important part in nature in keeping the
individuals of the same species, or of the same variety, true and
uniform in character. It will obviously thus act far more efficiently
with those animals which unite for each birth; but I have already
attempted to show that we have reason to believe that occasional
intercrosses take place with all animals and with all plants. Even if
these take place only at long intervals, I am convinced that the young
thus produced will gain so much in vigour and fertility over the
offspring from long-continued self-fertilisation, that they will have
a better chance of surviving and propagating their kind; and thus, in
the long run, the influence of intercrosses, even at rare intervals,
will be great. If there exist organic beings which never intercross,
uniformity of character can be retained amongst them, as long as their
conditions of life remain the same, only through the principle of
inheritance, and through natural selection destroying any which depart
from the proper type; but if their conditions of life change and they
undergo modification, uniformity of character can be given to their
modified offspring, solely by natural selection preserving the same
favourable variations.

Isolation, also, is an important element in the process of natural
selection. In a confined or isolated area, if not very large, the
organic and inorganic conditions of life will generally be in a great
degree uniform; so that natural selection will tend to modify all the
individuals of a varying species throughout the area in the same
manner in relation to the same conditions. Intercrosses, also, with
the individuals of the same species, which otherwise would have
inhabited the surrounding and differently circumstanced districts,
will be prevented. But isolation probably acts more efficiently in
checking the immigration of better adapted organisms, after any
physical change, such as of climate or elevation of the land, etc.;
and thus new places in the natural economy of the country are left
open for the old inhabitants to struggle for, and become adapted to,
through modifications in their structure and constitution. Lastly,
isolation, by checking immigration and consequently competition, will
give time for any new variety to be slowly improved; and this may
sometimes be of importance in the production of new species. If,
however, an isolated area be very small, either from being surrounded
by barriers, or from having very peculiar physical conditions, the
total number of the individuals supported on it will necessarily be
very small; and fewness of individuals will greatly retard the
production of new species through natural selection, by decreasing the
chance of the appearance of favourable variations.

If we turn to nature to test the truth of these remarks, and look at
any small isolated area, such as an oceanic island, although the total
number of the species inhabiting it, will be found to be small, as we
shall see in our chapter on geographical distribution; yet of these
species a very large proportion are endemic,--that is, have been
produced there, and nowhere else. Hence an oceanic island at first
sight seems to have been highly favourable for the production of new
species. But we may thus greatly deceive ourselves, for to ascertain
whether a small isolated area, or a large open area like a continent,
has been most favourable for the production of new organic forms, we
ought to make the comparison within equal times; and this we are
incapable of doing.

Although I do not doubt that isolation is of considerable importance
in the production of new species, on the whole I am inclined to
believe that largeness of area is of more importance, more especially
in the production of species, which will prove capable of enduring for
a long period, and of spreading widely. Throughout a great and open
area, not only will there be a better chance of favourable variations
arising from the large number of individuals of the same species there
supported, but the conditions of life are infinitely complex from the
large number of already existing species; and if some of these many
species become modified and improved, others will have to be improved
in a corresponding degree or they will be exterminated. Each new form,
also, as soon as it has been much improved, will be able to spread
over the open and continuous area, and will thus come into competition
with many others. Hence more new places will be formed, and the
competition to fill them will be more severe, on a large than on a
small and isolated area. Moreover, great areas, though now continuous,
owing to oscillations of level, will often have recently existed in a
broken condition, so that the good effects of isolation will
generally, to a certain extent, have concurred. Finally, I conclude
that, although small isolated areas probably have been in some
respects highly favourable for the production of new species, yet that
the course of modification will generally have been more rapid on
large areas; and what is more important, that the new forms produced
on large areas, which already have been victorious over many
competitors, will be those that will spread most widely, will give
rise to most new varieties and species, and will thus play an
important part in the changing history of the organic world.

We can, perhaps, on these views, understand some facts which will be
again alluded to in our chapter on geographical distribution; for
instance, that the productions of the smaller continent of Australia
have formerly yielded, and apparently are now yielding, before those
of the larger Europaeo-Asiatic area. Thus, also, it is that
continental productions have everywhere become so largely naturalised
on islands. On a small island, the race for life will have been less
severe, and there will have been less modification and less
extermination. Hence, perhaps, it comes that the flora of Madeira,
according to Oswald Heer, resembles the extinct tertiary flora of
Europe. All fresh-water basins, taken together, make a small area
compared with that of the sea or of the land; and, consequently, the
competition between fresh-water productions will have been less severe
than elsewhere; new forms will have been more slowly formed, and old
forms more slowly exterminated. And it is in fresh water that we find
seven genera of Ganoid fishes, remnants of a once preponderant order:
and in fresh water we find some of the most anomalous forms now known
in the world, as the Ornithorhynchus and Lepidosiren, which, like
fossils, connect to a certain extent orders now widely separated in
the natural scale. These anomalous forms may almost be called living
fossils; they have endured to the present day, from having inhabited a
confined area, and from having thus been exposed to less severe
competition.

To sum up the circumstances favourable and unfavourable to natural
selection, as far as the extreme intricacy of the subject permits. I
conclude, looking to the future, that for terrestrial productions a
large continental area, which will probably undergo many oscillations
of level, and which consequently will exist for long periods in a
broken condition, will be the most favourable for the production of
many new forms of life, likely to endure long and to spread widely.
For the area will first have existed as a continent, and the
inhabitants, at this period numerous in individuals and kinds, will
have been subjected to very severe competition. When converted by
subsidence into large separate islands, there will still exist many
individuals of the same species on each island: intercrossing on the
confines of the range of each species will thus be checked: after
physical changes of any kind, immigration will be prevented, so that
new places in the polity of each island will have to be filled up by
modifications of the old inhabitants; and time will be allowed for the
varieties in each to become well modified and perfected. When, by
renewed elevation, the islands shall be re-converted into a
continental area, there will again be severe competition: the most
favoured or improved varieties will be enabled to spread: there will
be much extinction of the less improved forms, and the relative
proportional numbers of the various inhabitants of the renewed
continent will again be changed; and again there will be a fair field
for natural selection to improve still further the inhabitants, and
thus produce new species.