454 NATURAL SCIENCE. June. 



developed. The latter animals, possessing true flight — in fact, they 

 are the most aerial of all vertebrates — still show, in their tree- and 

 rock-frequenting habits, and activity in climbing, traces of their 

 scansorial ancestor. In mammals, in this way, we can trace an almost 

 complete transition from a terrestrial to an aerial life, the intermediate 

 stage being spent in the trees. 



It will, however, doubtless be objected, that the flying-fish are 

 the animals which present the nearest approach to true flight among 

 the parachutist vertebrates. Indeed, after watching these creatures, 

 one is inclined to agree with Pettigrew, that they really do fly for 

 short distances ; and we know that the power of flight is in some 

 birds quite limited. 



Darwin, also, points out that there is no difficulty in imagining 

 that it might be of advantage to a penguin-Uke bird, first to flap along 

 the surface of the water like the logger-head duck {Microptems), and 

 ultimately to rise in the air. 



But against this it may be urged that aquatic animals do not 

 tend towards complexity of epidermic clothing, as witness the 

 hairless Cetacea and Sirenia ; so that it is difficult to see how feathers, 

 those most complex of epidermic structures, could have arisen on an 

 aquatic animal. Besides which, I hope to be able to show that the 

 evidence is all in favour of aquatic birds being descended from 

 terrestrial, as indicated above, and not vice versa. This brings me to 

 the point contained in the title of this paper. The avian foot is, in 

 its way, almost as characteristic as the fore-limb ; and its typical 

 form is one adapted, by its backwardly-directed hallux, for grasping 

 boughs. This form is found in Archcsopteryx, and in many important 

 existing groups, to wit, Passeres, Pigeons, and Herons, and other 

 modifications may easily be derived from it. Thus, zygodadylotis 

 birds become so by a gradual shifting backwards of the fourth digit, 

 a change which is apparently still in process in the owls, which 

 always perch in the zygodadyle position. An analogous turning back 

 of the second toe gives the heterodactylotis foot of the trogons. As this 

 backwardly-directed hallux is unnecessary for terrestrial and aquatic 

 birds, we find in these a tendency towards its disappearance ; and in 

 some groups the process can be traced .Thus, in the Gallinae we find 

 the Curassows and Megapodes with a'well-developed thumb, while in 

 the terrestrial game-birds it is small, elevated, and almost function- 

 less ; indeed, I have noticed that domestic fowls, when roosting, turn 

 the hallux forwards. It is noteworthy that the Megapodes, with 

 their grasping feet, also display the reptilian character of 

 hatching their eggs otherwise than by incubation, and the 

 young emerge from the shell in an almost adult condition. 

 Similarly, among the Anseres, we find that in Anseranas, where the 

 webs of the feet are least developed, the hallux is well-developed and 

 incumbent, whereas the rest of the family show it in a nearly or quite 

 functionless condition. Likewise those archaic Anseriformes, the 



