Enibrijoloij II of certain of the Lower Fishes. 205 



rod-like " stylopterygia," used for clambering about amongst 

 the vegetation in the primitive shallow- water habitat. 



4. As the struggle for existence became more intense in 

 the shallow marginal zone, certain forms spread into deep 

 water, and their stylopterygia became paddle-like archi- 

 pterygia, while others took to a more terrestrial existence ; 

 and in them the stylopterygium becoming flattened at the 

 tip, and taking on an S-shaped curve, became developed 

 into a cheiropterygium. 



It will be seen that the view implies the acceptance of the 

 belief that the biserial archipterygium is really the primitive 

 type of existing fin. That this is so is rendered probable 

 by the fact of the predominance of this type of fin in the 

 earlier periods of the history of the main groups of fishes, 

 and, further, by the anatomical structure of the fin, which is 

 clearly less highly differentiated as an organ of swimming 

 than the other forms of paired fin. It will be noted further, 

 that the view involves the acceptance of Gegenbaur's view 

 that the limb girdle has been evolved from a branchial 

 arch. 



Development of the Nervous System. 



In regard to the central nervous system, there appear to 

 emerge from the facts of development, as shown by the 

 forms under consideration, two points of cardinal importance. 

 The first of these relates to the general topography of the 

 brain region. In Polypterus, and in Lepidosiren and 

 Protopterns, the brain shows at an early stage a division into 

 two regions, the posterior one of which will give rise to the 

 hind brain region of the adult, the other to the whole of the 

 brain lying anterior to this. 



In none of these cases do we find the division into three 

 primitive brain vesicles — the familiar fore, mid, and hind 

 brain. And when we look into the matter we find that the 

 view that this division is a primitive one rests upon the 

 developmental phenomena as seen in the bird, and selachian, 

 and mammal. Apart from the relatively high systematic 

 position of two out of the three groups mentioned, which 

 would in itself suggest caution in basing general ideas upon 



