Seite - 196 - in The Origin of Species

196 ORIGIN OF SPECIES ogous, or "ideally similar" in position and structure with the lungs of the higher vertebrate animals : hence there is no reason to doubt that the swimbladder has actually been con- verted into lungs, or an organ used exclusively for respi- ration. According to this view it may be inferred that all verte- brate animals with true lungs are descended by ordinary gen- eration from an ancient and unknown prototype, which was furnished with a floating apparatus or swimbladder. We can thus, as I infer from 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, notwith- standing the beautiful contrivance by which the glottis is closed. In the higher Vertebrata the branchise have wholly disappeared—but in the embryo the slits on the sides of the neck and the loop-like course of the arteries still mark their former position. But it is conceivable that the now utterly lost branchise might have been gradually worked in by nat- ural selection for some distinct purpose : for instance, Lan- dois has shown that the wings of insects are developed from the tracheae; it is therefore highly probable that in this great class organs which once served for respiration have 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 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 branchiae, the whole surface of the body and of the sack, together with the small frena, serving for respiration. The Balanidae 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, in the same relative position with the frena, large, much-folded membranes, which freely communicate with the circulatory lacunae of the sack and body, and which have been considered by all naturalists to act as branchiae. Now I think no one will dispute that the ovigerous frena in