166 MODES OF TRANSITION. 



lungs, notwithstanding the beautiful contrivance by which 

 the glottis is closed. In the higher vertebrata the branchiae 

 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 branchiae might have been gradually 

 worked in by natural selection for some distinct purpose : 

 for instance, Landois has shown that the wings of insects 

 are developed from the trachea ; it is therefore highly prob- 

 able 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 func- 

 tion to another, that I will give another instance. Pedun- 

 culated 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 mem- 

 branes, which freely communicate with the circulatory 

 lacunae of the sack and body, and which have been consid- 

 ered by all naturalists to act as branchiae. Now I think no 

 one will dispute that the ovigerous frena in the one family 

 are strictly homologous with the branchiae of the other 

 family ; indeed, they graduate into each other. Therefore 

 it need not be doubted that the two little folds of skin, 

 which originally served as ovigerous frena, but which, like- 

 wise, very slightly aided in the act of respiration, have been 

 gradually converted by natural selection into branchiae, 

 simply through an increase in their size and the oblitera- 

 tion of their adhesive glands. If all pedunculated cirripedes 

 had become extinct, and they have suffered far more extinc- 

 tion than have sessile cirripedes, who would ever have im- 

 agined that the branchiae in this latter family had originally 

 existed as organs for preventing the ova from being washed 

 out of the sack ? 



There is another possible mode of transition, namely, 

 through the acceleration or retardation of the period of 



