chap. vi. Modes of Transition. 149 



they graduate into each other. Therefore it need not be doubted 

 that the two little folds of skin, which originally served as 

 ovio-erous 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 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 

 they grow older. Prof. 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 assume a new 

 character, as recorded by Fritz Muller, after maturity. In all such 

 cases, — and many could be given, — if the age for reproduction were 

 retarded, the character of the species, at least in its adult state, 

 Mould be modified; nor is it improbable that the previous and 

 earlier stages of development would in some cases be hurried 

 through and finally lost. Whether species have often or ever been 

 modified through this comparatively sudden mode of transition, I 

 •can form no opinion ; but if this has occurred, it is probable that 

 the differences between the young and the mature, and between 

 the mature and the old, were primordially acquired by graduated 

 steps. 



