30 Traces of Unity in the 



the pectoral fin in fishes are revealed in other ways. In 

 the Gurnards, for example, the three lowest rays, de- 

 tached and free, and more developed than the other 

 rays, have to do work in feeling and holding like true 

 fingers: and in many of the sharks the terminal 

 phalanges of some of the rays have three or four horny 

 filaments which are evidently the homologues of the 

 claws or nails of vertebrate animals of higher grade. 



In the wing of the bat, also, the same plan is trace- 

 able. The bones of the skeleton agree with those of 

 any other hand, and their chief difference is in the wire- 

 like prolongation of some of them. In point of fact, 

 the wing in this case is only a webbed fore-paw, as in 

 the otter, with longer fingers and with much more web, 

 with this in addition, that the web (called patagium in 

 the bat) is extended on the one side between the 

 neck and the arm and forearm, and on the other side, 

 between the fore and hind legs, and between the latter 

 and the tail bone, the edge of the part between the two 

 legs slanting from the top of the little finger in the fore 

 paw to the ankle in the hind paw. The patagium, in- 

 deed, extends along each side from one end of the body 

 to the other, and with the exception of the thumb in 

 the fore limb and the small foot in the hind limb, by 

 the nails of which, as by hooks, the animal suspends 

 itself when at rest, both limbs are wholly enclosed in it. 

 And as in the bat so it would appear to have been in 

 the Pterodactyle, with this difference, that in the extinct 

 animal the four fingers of the fore limb, the ist, 2nd, 

 3rd, and 4th, were free, and that only one, the 5th, was 

 employed in spreading out the patagium. 



In the wing of the bird the part corresponding to 

 the hand is the terminal segment, or pinion, which sup- 

 ports the principal feathers or "primaries," together 



