FISHES 157 



selection on account of their dangers, which were 

 increased by life on land. Here we have for the 

 first time the negative action of natural selection. We 

 see that it can not only create new organs, but also 

 destroy actual ones when this becomes necessary for 

 their possessor. 



It often happens that when an organ has to be 

 adapted to a new function parts of it will degenerate. 



When the fore - extremity of the reptile was con- 

 verted into a wing in its descendants, the birds, it 

 was useful to the latter to have the new organ in one 

 piece, in order to work it like an oar. Projecting 

 fingers would have offered resistance to the air, and 

 would have brought continual wounds to the bird by 

 getting broken off. Hence, in the interest of the 

 flying-organ, the fingers had to disappear. As a 

 matter of fact, in the bird's wing to - day we find 

 only the rudiments of five fingers, though the archse- 

 opteryx had them well developed and active. There 

 are plenty of instances of a similar decay in the 

 interest of a unified support ; amongst others, we 

 might mention the leg of the birds and the ungulates. 

 What seems to be the knee in the fore -leg of the 

 horse corresponds to our wrist. The large bone that 

 passes from this to the joint of the hoof is the upper 

 part of the middle finger; it has been developed to 

 this extent because it can bear the weight of the body 

 better as a single support. But at the side of it we 

 find two other small bones, which are attached to the 

 joint above, and are called the styloid bones. They 

 have no function, and cannot be understood except 



