vii OEIGIN OF LIMBS AND TAIL 453 



Embryology offers no explanation of the number of digits being 

 so generally five. The physiological advantage of the expanded foot 

 being divided up into separate radiating digits is obvious, as is that 

 of the double nature of the adjoining portion of the limb skeleton to 

 facilitate rotation round the axis of the limb. There are also 

 mechanical advantages in there being a central digit with one on 

 each side of it. Possibly the presence of an additional digit outside 

 of these is to be looked on as of the nature of simple reinforcement. 



The modification of the pectoral limb in the case of Birds for 

 purposes of flight is of great interest, but nothing is known as to the 

 phylogenetic transition from Beptile to Bird in this connexion. To 

 the present writer it seems most probable that the Birds were evolved 

 out of aquatic, Beptiles in which the fore-limb was specialized for use 

 in swimming under water, after the manner of existing Penguins, and 

 that the function of aerial flight was evolved directly from such 

 movement under water. On this hypothesis the more or less terres- 

 trial habits of modern Birds would be regarded as a secondary 

 acquirement. 



(4) Evolutionary Origin of the Tail Eegion. — It is charac- 

 teristic of Vertebrates that the anus loses its practically terminal 

 position and comes to be situated some distance forwards on the 

 ventral side, the overhanging hinder end of the body forming the 

 tail. This opens up a question of much morphological interest — 

 though one to which we are not yet in a position to give any certain 

 answer — as to the phylogenetic origin of the tail. 



It seems clear that the tail arose in ancient aquatic Vertebrates as 

 an adaptation to swimming and on the whole it seems most probable 

 that it came into existence through the gradual migration forwards 

 of the anus upon the ventral side. Such a shifting forwards of the 

 anal opening from the hinder end of the body is a familiar feature in 

 many groups of invertebrates where it is associated as a rule with a 

 tubicolous habit and has doubtless for its object the getting rid of 

 excretory products which would otherwise be discharged into the 

 depths of the tube, or burrow, or shell. In the Vertebrate the 

 forward shifting of the anal opening has probably its physiological 

 significance in the increasing efficiency of the tail as the main motive 

 organ — the disappearance from it of the alimentary canal, and its 

 surrounding splanchnocoele, being correlated with the conversion of 

 the tissues on each side of the skeletal axis into a solid mass of 

 muscle. Probability is added to this conjecture by the fact that we 

 see what appears to be a continuation of the same process in the 

 most efficient group of modern swimming Vertebrates (Teleostei) 

 where in the most highly developed forms the alimentary canal and 

 splanchnocoele come to be restricted to a relatively small region 

 immediately behind the head, the remaining and main part of the 

 body being entirely " tail." 



In actual ontogeny the tail region is developed not by the with- 

 drawal from it of gut and splanchnocoele but as an actual outgrowth, 



