1S2 THE ORIGIN OF SPECIES 



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 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 ovige- 

 rous 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 pedunculated 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 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 Professor Cope and others in the 

 United States. It is now known that some animals are capable of 

 reproduction at a very early age, before they have acquired their 

 perfect characters; and if this power became thoroughly well de- 

 veloped in a species, it seems probable that the adult stage of 

 development would sooner or later be lost; and in this case, 

 especially if the larva differed much from the mature form, the 

 character of the species would be greatly changed and degraded. 

 Again, not a few animals, after arriving at maturity, go on chang- 

 ing in character during nearly their whole lives. With mammals, 

 for instance, the form of the skull is often much altered with age, 

 of which Dr. Murie has given some striking instances with seals. 

 Every one knows how the horns of stags become more and more 

 branched, and the plumes of some birds become more finely devel- 

 oped, as they grow older. Professor Cope states that the teeth of 

 certain lizards change much in shape with advancing years. With 

 crustaceans not only many trivial, but some important, parts as- 

 sume a new character, as recorded by Fritz Muller, after maturity. 

 In all such cases — and many could be given — if the age for repro- 



