CHAP. VI.] MODES OF TKANSITION. 139 



Landois 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 Balauidae 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 

 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 pedunculated cirri- 

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

 tion 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 1 



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 repro- 

 duction at a very early age, before they have acquired their perfect 

 characters ; and if this power became thoroughly well developed 

 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 changing 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 developed, as 



6 



