132 EVOLUTION OF TO-DAY. 



moment. We can then readily see why the chick 

 should have lost its gills but have retained the gill- 

 slits. The ancestral stage in the history of the 

 chick which possessed gills occurred very long ago ; 

 and since that time the embryo has slowly lost the 

 habit of developing such useless structures. If it 

 had been only a short time since that stage, we 

 might still expect to find them. This we do find, 

 for instance, in a frog of Guadaloupe, which, from 

 the absence of stagnant water, is obliged to skip the 

 tadpole stage, or rather is obliged to pass through 

 this stage before hatching from the egg. In this 

 stage, though the animal is still in the egg, there 

 are developed gills and a tail. We know that frogs 

 are a recent introduction into the Guadaloupe, and 

 thus see that it takes time even for the disappear- 

 ance of such useless organs as gills. But in the case 

 of the chick the time has been much longer, and 

 every trace of gills has disappeared. We might ex- 

 pect that the g\\\-slits would disappear too ; but 

 they have not; they still remain to mark the former 

 aquatic life of the ancient vertebrates. 



If a shipbuilder wishes to build a steamboat, he 

 moulds his material directly into the form of boat 

 he wishes ; we would think little of his ability if he 

 began by building a canoe, modified this into a row- 

 ing boat, this into a Roman trireme, this into a 

 simple sailing vessel, this into a large ship, and 

 finally into a steamboat. Such a method would 

 be repeating the history of the steamship, but 

 would be a poor way to build a boat. Now the 

 development of the chick is a compromise between 



