232 MODES OP TRANSITION. [Chap. VL 



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 branchia3, the whole surface of 

 the body and of the sack, together with the small frena, 

 serving for respiration. The Balanidse or sessile cirri- 

 pedes, 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. ISTow 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. There- 

 fore 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 respira- 

 tion, 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 

 pedunculated cirripedes had become extinct, and they 

 have suffered far more extinction than have sessile cirri- 

 pedes, who would ever have imagined 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 

 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 



