456 SCIENCE PROGRESS 



bird works. In our own case if the hand be held horizontally 

 with the palm downwards the wrist works so that the hand 

 rises and falls; in the bird and its ancestors it worked so 

 that the hand moved sideways and outwards until the little 

 finger lay flat against the fore-arm. The outer digits thus 

 became crowded together in the angle between the arm and 

 the hand. Under these circumstances the fourth and fifth digits 

 disappeared. 



To return to fishes. The rotation of the glenoid line indi- 

 cated in the sturgeon does actually take place during the develop- 

 ment of some bony fishes,^ e.g. salmon. In others the rotation is 

 upwards instead of inwards, so that this line runs down the 

 side of the body instead of across its ventral surface. But in 

 both cases the same end is attained, both allowing the fin 

 to be extended so that its plane is at right angles to the 

 body. 



Thus it is possible to recognise a complete sequence from 

 the broad-based fin acting as a keel, through the narrow-based 

 fin with limited freedom acting as a lateral rudder, to the 

 narrow-based fin with great freedom of movement which enables 

 it to be used either as a keel, a lateral rudder or a brake. 



' H. H. Svvinnerton, "A Contribution to the Morphology and Development of 

 the Pectoral Skeleton of Teleosteans," Quart. Joiirn. Micro. Set. 1905. 



