of the cavity in question; and an aquiferous chamber of smaller
dimensions has the same relation to the kidney in Lamellibranchiata--in
Gasteropoda, Heteropoda, Pteropoda, and dibranchiate Cephalopoda. But
although such is likely enough to be the case, we do not know at present
that the aquiferous chambers in any of the last named mollusks attain an
extension similar to that which obtains in Nautilus.
On comparing the observations detailed above with the statements of
previous writers, I find that, in his well-known "Memoir on the Pearly
Nautilus" (1832), Professor Owen describes "on each side, at the roots
of the branchiæ," "a small mamillary eminence with a transverse slit
which conducts from the branchial cavity into the pericardium. There is,
moreover, a foramen at the lower part of the cavity (_o_, pl. 5)
permitting the escape of a small vessel; and by the side of this vessel
a free passage is continued between the gizzard and ovary into the
membranous tube or siphon that traverses the divisions of the shell,
thus establishing a communication between the interior of that tube and
the exterior of the animal."
The foramen here described is easily seen; but, as I have stated, there
are other modes of communication between the so-called pericardium and
the cavity with which the siphuncle communicates, of a far more
extensive nature.
With respect to the pericardium itself, Professor Owen states, "The
peritoneum, after lining the cavity which contains the crop and liver,
and enveloping those viscera, forms two distinct pouches at the bottom
of the pallial sac, in one of which, the left, is contained the gizzard,
and in the other the ovary; anterior to these, and on the ventral aspect
of the liver, is another distinct cavity, of a square shape, which
contains the heart and principal vessels, with the glandular appendages
connected therewith." This is what the author terms the pericardium.
As Van der Hoeven has pointed out, however, the gizzard lies to the
right and the ovary to the left. Moreover, the gizzard is superior to
the ovary, so as only to overlap it a little above; and I can find no
evidence of the existence of such distinct pouches as those described.
Professor Owen states that the branchiæ "arise by a common peduncle from
the inner surface of the mantle." My own observations, however, and Van
der Hoeven's figures, of both male and female, lead me to believe that
the peduncles of the branchiæ are perfectly distinct from one another.
The follicles of the branchial arteries are thus described in the
"Memoir on the Pearly Nautilus:"--"They are short and pyriform and
closely set together. To each of the branchial arteries are appended
three clusters of these glands, of which one is larger than the united
volume of both the others; and the larger cluster is situated on one
side of the vessel and the two smaller on the opposite side. Each of
these clusters is contained in a membranous receptacle proper to itself,
partitioned off, as it were, from the pericardium, but communicating
with it.... The two canals which form the communication between the
pericardium and the branchial cavity commence at the receptacle of the
lesser cluster attached to the superior branchial arteries, and
terminate at the papillæ before mentioned, which are situated at the
roots of the branchiæ. The pericardium and these receptacles of the
glands, when first laid open, were found filled with a coagulated
substance so closely compacted as to require a careful removal, bit by
bit, before the contained follicles and vessels could be brought into
view."
Like Valenciennes and Van der Hoeven, I have been unable to find any
communication between the four sacs in which the small double clusters
of follicles are contained, and the "pericardium;" and I hold it to be
certain that the other four sets of follicles are not contained in sacs
at all, but lie free in the "pericardium" or posterior chamber.
No notice is here taken of the widely different characters of the
anterior and posterior follicles; and the figure gives both a similar
structure.
Valenciennes ("Nouvelles Recherches sur le Nautile Flambé," 'Archives du
Muséum,' ii., 1841) pointed out the existence of three pairs of
apertures opening into the branchial sac, besides the genital and anal
openings; and he affirms that they open into as many closed sacs, which
communicate neither with one another nor with the cavity that contains
the heart. M. Valenciennes indicates the difference in the structure of
the anterior and posterior venous appendages. He seems to me to have
seen something of the part which I have described as the pallio-visceral
ligament; but I cannot clearly comprehend either his figure or his
description.
Van der Hoeven, in his 'Contributions to the Knowledge of the Animal of
_Nautilus pompilius_,' 1850, confirmed the statement of Valenciennes
with regard to the existence of three pairs of apertures; but he showed,
in opposition to him, that one of these pairs of apertures communicated
with the pericardium. The sacs into which the other two pairs open are,
according to this anatomist, blind. In the aperture of the anterior
blind sac he found a concretionary matter which he supposed to contain
uric acid, but chemical analysis did not confirm the supposition. Van
der Hoeven refers to some observations by Vrolik; but as these are in
Dutch, and have not, so far as I can find, been translated into either
French, German, or English, I know not what they may contain.
In his more recent essay, translated in 'Wiegmann's Archiv' for 1857,
under the title of "Beitrag zur Anatomie von _Nautilus pompilius_," Van
der Hoeven states that he has again found hard concretions in the
chamber enclosing the appendage of the anterior branchial artery, and
that these on chemical analysis yielded phosphate of lime and traces of
fat and albumen, but no uric acid.
Mr. Macdonald, in a valuable paper on the anatomy of _Nautilus
umbilicatus_, published in the Philosophical Transactions for 1855, thus
describes the follicular appendages of the branchial arteries:--
"These follicles are subcylindrical in form, somewhat dilated at the
free extremity, to which is appended a folded and funnel-shaped process
of membrane, which expands rather suddenly, presenting a jagged and
irregular border. They open by a smooth and oval or slit-like, orifice
into the afferent pulmonary vessels, on each of which, as Professor Owen
has observed, they are disposed in three clusters. The outer membrane is
smooth and glassy, homogeneous in structure and sprinkled over with
minute rounded and transparent bodies, probably the nuclei of cells.
Beneath this layer, flat bundles of fibres, apparently muscular, are
traceable here and there, principally disposed in a longitudinal
direction, and sometimes branched. The lining membrane consists of a
loose epithelial pavement in many respects similar to that of the
uriniferous tubules of the higher animals, the cells containing, besides
the nuclei, numerous minute oil-globules, or a substance much resembling
concrete fatty matter. This membrane is thrown up into an infinite
number of papillæ and corrugations, so as to augment the extent of
surface considerably. The papillæ are more numerous at the inner part or
towards the attached end; and a circlet of longitudinally disposed folds
radiate from the bottom of the follicles, in which a number of small
pits or fenestrations are sometimes visible. The sides of these folds
are wrinkled transversely so as to present a median zigzag elevation.
The funnel-shaped membranous process above noticed is continuous with
the lining membrane, consisting of an extension of the same epithelial
pavement; but the cells are somewhat larger and more regular in form.
The cavity of each follicle, therefore, communicates with the exterior
through the centre of this process; and the aperture is thus guarded by
a kind of circular valve, permitting the escape of secreted matter, but
effectually preventing the entrance of fluid from without."
In his fig. 9, pl. xv., Mr. Macdonald depicts certain "crystalline
bodies often occurring within the follicles."
From what Mr. Macdonald states, one would be led to conclude that all
the follicles have the same structure; but I suspect this to be an
oversight.
[Illustration: _Nautilus pompilius._ Fig. 1.
Viewed from the left side and a little behind.
Two of the anterior chambers, and the fifth or posterior chamber, laid
open. Natural size.
_a._ Shell muscle. _b._ Ovary. _c._ Intestine. _d._ Heart; _d'._ its
pyriform appendage. _e._ Superior anterior chamber; _e'._ its follicles.
_f._ Inferior anterior chamber; _f'._ its follicles. _g._ Posterior
chamber; _g'._ Follicles. _h._ Cut ends of branchial arteries. _i._
Termination of vena cava. _k._ Pallio-visceral ligament.]
In the second edition of Professor Owen's Lectures on the Invertebrata
(1855), I find no mention of Valenciennes' discovery of the additional
four apertures; but the author states that "on each side, at the roots
of the anterior branchiæ, there is a small mamillary eminence with a
transverse slit, which conducts from the branchial cavity to one of the
compartments of the pericardium containing two clusters of venous
glands. There are also two similar, but smaller, slits, contiguous to
one another, near the root of the posterior branchia on each side, which
lead to and may admit sea-water into the compartments containing the
posterior cluster of the venous follicles." In this work the ovary is
not only described, but _figured_, on the right side of the gizzard. The
figure, however, rightly places the greater part of the ovary below that
organ.
[Illustration: _Nautilus pompilius._ Fig. 2.
Natural Size.
The pallio-visceral ligament seen from below: torn on the right side to
show the rectum and oviduct; cut through on the left side along the
dotted line close to _d'_ in the preceding figure.
_a._ Anus. _b._ Oviducal aperture. _c._ Heart. _d._ Left branchial
veins. _e._ Right branchial veins. _f._ Oviduct cut through. _g._ Ovary.
_h._ Rectum. _i._ Mantle. _k k k._ Pallio-visceral ligament; _k'._ its
torn portion. The oval "aperture for the siphonal artery" is seen to the
left of _c'_, and the right-hand style in _Fig._ 1 passes through it.]
On the Tendency of Species to form Varieties; and on the Perpetuation of
Varieties and Species by Natural Means of Selection. By CHARLES DARWIN,
Esq., F.R.S., F.L.S., & F.G.S., and ALFRED WALLACE, Esq. Communicated by
Sir CHARLES LYELL, F.R.S., F.L.S., and J. D. HOOKER, Esq., M.D.,
V.P.R.S., F.L.S., &c.
[Read July 1st, 1858.]
London, June 30th, 1858.
MY DEAR SIR,--The accompanying papers, which we have the honour of
communicating to the Linnean Society, and which all relate to the same
subject, viz. the Laws which affect the Production of Varieties, Races,
and Species, contain the results of the investigations of two
indefatigable naturalists, Mr. Charles Darwin and Mr. Alfred Wallace.
These gentlemen having, independently and unknown to one another,
conceived the same very ingenious theory to account for the appearance
and perpetuation of varieties and of specific forms on our planet, may
both fairly claim the merit of being original thinkers in this important
line of inquiry; but neither of them having published his views, though
Mr. Darwin has for many years past been repeatedly urged by us to do so,
and both authors having now unreservedly placed their papers in our
hands, we think it would best promote the interests of science that a
selection from them should be laid before the Linnean Society.
Taken in the order of their dates, they consist of:--
1. Extracts from a MS. work on Species[A], by Mr. Darwin, which was
sketched in 1839, and copied in 1844, when the copy was read by Dr.
Hooker, and its contents afterwards communicated to Sir Charles Lyell.
The first Part is devoted to "The Variation of Organic Beings under
Domestication and in their Natural State;" and the second chapter of
that Part, from which we propose to read to the Society the extracts
referred to, is headed, "On the Variation of Organic Beings in a state
of Nature; on the Natural Means of Selection; on the Comparison of
Domestic Races and true Species."
2. An abstract of a private letter addressed to Professor Asa Gray, of
Boston, U.S., in October 1857, by Mr. Darwin, in which he repeats his
views, and which shows that these remained unaltered from 1839 to 1857.
3. An Essay by Mr. Wallace, entitled "On the Tendency of Varieties to
depart indefinitely from the Original Type." This was written at Ternate
in February 1858, for the perusal of his friend and correspondent Mr.
Darwin, and sent to him with the expressed wish that it should be
forwarded to Sir Charles Lyell, if Mr. Darwin thought it sufficiently
novel and interesting. So highly did Mr. Darwin appreciate the value of
the views therein set forth, that he proposed, in a letter to Sir
Charles Lyell, to obtain Mr. Wallace's consent to allow the Essay to be
published as soon as possible. Of this step we highly approved, provided
Mr. Darwin did not withhold from the public, as he was strongly inclined
to do (in favour of Mr. Wallace), the memoir which he had himself
written on the same subject, and which, as before stated, one of us had
perused in 1844, and the contents of which we had both of us been privy
to for many years. On representing this to Mr. Darwin, he gave us
permission to make what use we thought proper of his memoir, &c.; and in
adopting our present course, of presenting it to the Linnean Society, we
have explained to him that we are not solely considering the relative
claims to priority of himself and his friend, but the interests of
science generally; for we feel it to be desirable that views founded on
a wide deduction from facts, and matured by years of reflection, should
constitute at once a goal from which others may start, and that, while
the scientific world is waiting for the appearance of Mr. Darwin's
complete work, some of the leading results of his labours, as well as
those of his able correspondent, should together be laid before the
public.
We have the honour to be yours very obediently,
CHARLES LYELL.
JOS. D. HOOKER.
_J. J. Bennett, Esq.,_
_Secretary of the Linnean Society._
I. _Extract from an unpublished Work on Species, by_ C. DARWIN, Esq.,
_consisting of a portion of a Chapter entitled, "On the Variation of
Organic Beings in a state of Nature; on the Natural Means of Selection;
on the Comparison of Domestic Races and true Species._"
De Candolle, in an eloquent passage, has declared that all nature is at
war, one organism with another, or with external nature. Seeing the
contented face of nature, this may at first well be doubted; but
reflection will inevitably prove it to be true. The war, however, is not
constant, but recurrent in a slight degree at short periods, and more
severely at occasional more distant periods; and hence its effects are
easily overlooked. It is the doctrine of Malthus applied in most cases
with tenfold force. As in every climate there are seasons, for each of
its inhabitants, of greater and less abundance, so all annually breed;
and the moral restraint which in some small degree checks the increase
of mankind is entirely lost. Even slow-breeding mankind has doubled in
twenty-five years; and if he could increase his food with greater ease,
he would double in less time. But for animals without artificial means,
the amount of food for each species must, _on an average_, be constant,
whereas the increase of all organisms tends to be geometrical, and in a
vast majority of cases at an enormous ratio. Suppose in a certain spot
there are eight pairs of birds, and that _only_ four pairs of them
annually (including double hatches) rear only four young, and that these
go on rearing their young at the same rate, then at the end of seven
years (a short life, excluding violent deaths, for any bird) there will
be 2048 birds, instead of the original sixteen. As this increase is
quite impossible, we must conclude either that birds do not rear nearly
half their young, or that the average life of a bird is, from accident,
not nearly seven years. Both checks probably concur. The same kind of
calculation applied to all plants and animals affords results more or
less striking, but in very few instances more striking than in man.
Many practical illustrations of this rapid tendency to increase are on
record, among which, during peculiar seasons, are the extraordinary
numbers of certain animals; for instance, during the years 1826 to 1828,
in La Plata, when from drought some millions of cattle perished, the
whole country actually _swarmed_ with mice. Now I think it cannot be
doubted that during the breeding-season all the mice (with the exception
of a few males or females in excess) ordinarily pair, and therefore that
this astounding increase during three years must be attributed to a
greater number than usual surviving the first year, and then breeding,
and so on till the third year, when their numbers were brought down to
their usual limits on the return of wet weather. Where man has
introduced plants and animals into a new and favourable country, there
are many accounts in how surprisingly few years the whole country has
become stocked with them. This increase would necessarily stop as soon
as the country was fully stocked; and yet we have every reason to
believe, from what is known of wild animals, that _all_ would pair in
the spring. In the majority of cases it is most difficult to imagine
where the checks fall--though generally, no doubt, on the seeds, eggs,
and young; but when we remember how impossible, even in mankind (so much
better known than any other animal), it is to infer from repeated casual
observations what the average duration of life is, or to discover the
different percentage of deaths to births in different countries, we
ought to feel no surprise at our being unable to discover where the
check falls in any animal or plant. It should always be remembered, that
in most cases the checks are recurrent yearly in a small, regular
degree, and in an extreme degree during unusually cold, hot, dry, or wet
years, according to the constitution of the being in question. Lighten
any check in the least degree, and the geometrical powers of increase in
every organism will almost instantly increase the average number of the
favoured species. Nature may be compared to a surface on which rest ten
thousand sharp wedges touching each other and driven inwards by
incessant blows. Fully to realize these views much reflection is
requisite. Malthus on man should be studied; and all such cases as those
of the mice in La Plata, of the cattle and horses when first turned out
in South America, of the birds by our calculation, &c., should be well
considered. Reflect on the enormous multiplying power _inherent and
annually in action_ in all animals; reflect on the countless seeds
scattered by a hundred ingenious contrivances, year after year, over the
whole face of the land; and yet we have every reason to suppose that the
average percentage of each of the inhabitants of a country usually
remains constant. Finally, let it be borne in mind that this average
number of individuals (the external conditions remaining the same) in
each country is kept up by recurrent struggles against other species or
against external nature (as on the borders of the Arctic regions, where
the cold checks life), and that ordinarily each individual of every
species holds its place, either by its own struggle and capacity of
acquiring nourishment in some period of its life, from the egg upwards;
or by the struggle of its parents (in short-lived organisms, when the
main check occurs at longer intervals) with other individuals of the
_same_ or _different_ species.
But let the external conditions of a country alter. If in a small
degree, the relative proportions of the inhabitants will in most cases
simply be slightly changed; but let the number of inhabitants be small,
as on an island, and free access to it from other countries be
circumscribed, and let the change of conditions continue progressing
(forming new stations), in such a case the original inhabitants must
cease to be as perfectly adapted to the changed conditions as they were
originally. It has been shown in a former part of this work, that such
changes of external conditions would, from their acting on the
reproductive system, probably cause the organization of those beings
which were most affected to become, as under domestication, plastic.
Now, can it be doubted, from the struggle each individual has to obtain
subsistence, that any minute variation in structure, habits, or
instincts, adapting that individual better to the new conditions, would
tell upon its vigour and health? In the struggle it would have a better
_chance_ of surviving; and those of its offspring which inherited the
variation, be it ever so slight, would also have a better _chance_.
Yearly more are bred than can survive; the smallest grain in the
balance, in the long run, must tell on which death shall fall, and which
shall survive. Let this work of selection on the one hand, and death on
the other, go on for a thousand generations, who will pretend to affirm
that it would produce no effect, when we remember what, in a few years,
Bakewell effected in cattle, and Western in sheep, by this identical
principle of selection?
To give an imaginary example from changes in progress on an island:--let
the organization of a canine animal which preyed chiefly on rabbits, but
sometimes on hares, become slightly plastic; let these same changes
cause the number of rabbits very slowly to decrease, and the number of
hares to increase; the effect of this would be that the fox or dog would
be driven to try to catch more hares: his organization, however, being
slightly plastic, those individuals with the lightest forms, longest
limbs, and best eyesight, let the difference be ever so small, would be
slightly favoured, and would tend to live longer, and to survive during
that time of the year when food was scarcest; they would also rear more
young, which would tend to inherit these slight peculiarities. The less
fleet ones would be rigidly destroyed. I can see no more reason to doubt
that these causes in a thousand generations would produce a marked
effect, and adapt the form of the fox or dog to the catching of hares
instead of rabbits, than that greyhounds can be improved by selection
and careful breeding. So would it be with plants under similar
circumstances. If the number of individuals of a species with plumed
seeds could be increased by greater powers of dissemination within its
own area (that is, if the check to increase fell chiefly on the seeds),
those seeds which were provided with ever so little more down, would in
the long run be most disseminated; hence a greater number of seeds thus
formed would germinate, and would tend to produce plants inheriting the
slightly better-adapted down[B].