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seems to me to be no great difficulty in believing that natural selection 1859 1860
seems to me to be no extreme difficulty in believing that natural selection 1861 1866
is no reason to doubt that the swimbladder 1869 1872

converted a swimbladder into a 1859 1860 1861 1866
been converted into 1869 1872

I can, indeed, hardly doubt 1859 1860
On this view it may be inferred 1861 1866
According to this view it may be inferred 1869 1872

prototype, of which we know nothing, 1859 1860 1861 1866
and unknown prototype, which was 1869 1872

the 1859 1860 1861 1866
in the embryo the 1869
but in the embryo the 1872

slits 1859 1860 1861 1869 1872
embryo the slits 1866

marking in the embryo 1859 1860 1861
marking 1866
mark 1869 1872

in the same manner as, on the view entertained by some naturalists that 1859 1860 1861 1866
for instance, 1869
for instance, Landois has shown that 1872

branchiæ and dorsal scales of Annelids are homologous with the wings and wing-covers 1859 1860 1861 1866
branchiæ and dorsal scales of Annelids are believed to be homologous with the wings and wing-covers 1869
wings 1872

it is 1859 1860 1861 1866
and it is not 1869
are developed from the tracheæ; it is therefore highly 1872

organs which at a very ancient period 1859 1860 1861 1866
with our existing insects, organs, which at an ancient period 1869
in this great class organs which once 1872

All physiologists admit that the
swim-bladder
swimbladder
is homologous, or "ideally
similar"
similar,"
in position and structure with the lungs of the higher vertebrate animals: hence there seems to me to be no great difficulty in believing that natural selection has actually converted a swimbladder into a
lungs,
lung,
or
an organ
organ
used exclusively for respiration.
I can, indeed, hardly doubt that all vertebrate animals
with
having
true lungs
are
have
descended by ordinary generation from an ancient prototype, of which we know nothing, furnished with a floating apparatus or
swim-bladder.
swimbladder.
We can thus, as I infer from
Professor
Professor
Owens
Owen's
interesting description of these parts, understand the strange fact that every particle of food and drink which we swallow has to pass over the orifice of the trachea, with some risk of falling into the lungs, notwithstanding the beautiful contrivance by which the glottis is closed. In the higher
Vertebrate
Vertebrata
the branchiæ have wholly
disappeared—in
disappeared—but
disappeared—
the slits on the sides of the neck and the loop-like course of the arteries still marking in the embryo their former position. But it is conceivable that the now utterly lost branchiæ might have been gradually worked in by natural selection for some
quite
quite
distinct purpose: in the same manner as, on the view entertained by some naturalists that the branchiæ and dorsal scales of Annelids are homologous with the wings and wing-covers of
insects
insects,
it is
improbable
probable
that organs which at a very ancient period served for
respiration,
respiration
have
actually been
been actually
converted into organs
for
of
flight.
In considering transitions of organs, it is so important to bear in mind the probability of conversion from one function to another, that I will give
another
one more
instance. Pedunculated cirripedes have two minute folds of skin,