232 MODES OF TBAXSITION. [Chap. YI. 



Pedunculated cirripedes have two minute folds of skin, 

 called by nie the ovio-erous 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 Balanidas or sessile cirripedes, on the 

 other hand, have no ovigerous frena, the eggs lying loose 



O ' CO J o 



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 lacuna? of the sack 

 and body, and which have been considered by all 

 naturalists to act as branchiae. Xow 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, 

 likewise, 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 obliteration of their adhesive glands. If all pedun- 

 culated cirripedes had become extinct, and they have 

 suffered far more extinction than have sessile cirripedes, 

 who would ever have imagined that the branchiae in this 

 latter family had originally existed as organs for pre- 

 venting 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 

 reproduction. This has lately been insisted on by Prof. 

 Cope and others in the United States. It is now known 

 that some animals are capable of reproduction at a very 



