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 branchiae, 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 Pr