230 THE INFANCY OF ANIMALS 



supporting rays springing from a rounded base, and hence 

 the tail is called " homocercal." As a matter of fact 

 this symmetry is deceptive. Sooner or later, the up- 

 wardly directed portion degenerates and finally vanishes, 

 leaving the fin-rays originally belonging to the under- 

 surface of the upturned portion to arrange themselves 

 in the fan-wise fashion just referred to. 



Thus we see that the peculiar arrangement of the 

 tail-fin of the most highly developed species passes through 

 the ancestral stage before arriving at the form with which 

 most of us are familiar. And it is quite possible that 

 the whole appearance of the angler at this third stage 

 is that which characterised the adult stage of some remote 

 ancestor. A more striking illustration of the general 

 truth of the contention that every animal chmbs its own 

 ancestral tree it would not be easy to find. 



In some cases, on the other hand, we are confronted 

 with the evolution of new characters, and these often of 

 a very remarkable kind. The young of the " flat-fish " — 

 plaice, sole, turbot, and their kin — illustrate this in a 

 very striking manner. In the adults of these fishes the 

 dark-coloured upper surface answers, apparently, to the 

 back, and is generally so regarded, more especially since 

 the eyes are also uppermost. But, as a matter of fact, 

 this surface answers to the right or left side, as the 

 case may be : in some species the right, and in others 

 the left being uppermost. The back is only to be dis- 

 tinguished after an examination of the mouth, and a 

 search for the pair of fins answering to the hind-legs of 

 terrestrial animals. How has this singular condition 

 come to be ? The answer again can only be found after 

 an examination of the larval stages. These show us that 

 the young of all the flat fish enter the world as " round " 



