Origin of Species Variorum

→ 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

→ sack, including 1859 1860 1861

of the sack, including 1866

of the sack, together with 1869 1872

→ in the same relative position, large, much folded membranes, which freely communicate with the circulatory lacunæ of the sack and body, and which have been considered to be branchiæ by Prof. Owen and all other naturalists who have treated on the subject. 1861

large folded branchiæ. 1859 1860

in the same relative position with the frena, large, much-folded membranes, which freely communicate with the circulatory lacunæ of the sack and body, and which have been considered to be branchiæ by Prof. Owen and all other naturalists who have treated on the subject. 1866

in the same relative position with the frena, large, much-folded membranes, which freely communicate with the circulatory lacunæ of the sack and body, and which have been considered to be branchiæ by Prof. Owen and by all other naturalists who have treated on the subject. 1869

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

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, called by me the ovigerous frena, which serve, through the means of a sticky secretion, to retain the eggs until they are hatched within the sack. These cirripedes have no branchiæ, the whole surface of the body and →sack, including the small frena, serving for respiration. The Balanidæ or sessile cirripedes, on the other hand, have no ovigerous frena, the eggs lying loose at the bottom of the sack,

within

the well-enclosed shell; but they

have

have,

→in the same relative position, large, much folded membranes, which freely communicate with the circulatory lacunæ of the sack and body, and which have been considered to be branchiæ by Prof. Owen and all other naturalists who have treated on the subject. Now I think no one will dispute that the ovigerous frena in the one family are strictly homologous with the branchiæ