HEADS AND TAILS. 55 



throw a flood of light upon questions of evolution 

 which confront us at every turn. They will bear 

 us back to the misty past and compel us to com- 

 pare afresh the revelations of the rocks with the 

 revelations of the microscope. 



The fish of to-day, then, is "the heir of all the 

 ages," and the last of a long line, every member 

 of which was bound, albeit unconsciously, to 

 contribute something towards the greater per- 

 fection of his race. But there is no royal road 

 to perfection, and none may do more than bear 

 a share in its attainment. Furthermore, that 

 each may be perfect after his own kind, it is 

 necessary that eich should proceed towards the 

 desired goal along definite lines. Thus it comes 

 to be that every animal, in the course of its 

 development, is obliged, as it has been said, 

 to climb its own ancestral tree. Thus it comes 

 to pass that what was the adult condition at one 

 period, is represented only as a passing phase 

 in later periods, and out of this phase a new 

 form is evolved. This rule is not, however, 

 absolute, for occasionally omissions are made, 

 and newer developments come into being without 

 recording the track along which they have come, 

 or without revealing the frame on which they 

 were modelled, so to speak. Generally speak- 

 ing, however, the forms of animals are reached 

 by a route along definite lines — by addition to 

 previously existing structures. 



And so then with the fish's tail. If we turn 

 to the earliest known fossil fishes, we shall find 

 the tail diphycercal in type. The heterocercal 

 type, however, soon made its appearance, as a 



