EXPLAINING THE CONTRADICTIONS. 131 



able to move to any great extent, and certainly 

 unable to breathe by means of gills, and, moreover, 

 passing through its entire development in twenty- 

 one days. Can we expect, therefore, that even if 

 the tendency to repeat ancestral forms is very 

 strong, there will be time and opportunity to com- 

 plete such useless structures as gills? Now the 

 facts are, that while the gills themselves have dis- 

 appeared, the embryo chick does possess undoubted 

 remains of them in two features. There are found 

 in the embryo openings upon the sides of the neck, 

 which mark the position of the former gill-slits ; 

 and the blood-vessels of the embryo at first run to 

 these slits in a manner which shows that they once 

 supplied gills here situated. No one can doubt 

 that here are the homologues of a former aquatic 

 respiratory apparatus. 



This case is then most readily explained. For, if 

 the gills are of no use, their development would be 

 of positive disadvantage. Other things being equal, 

 it is desirable that the embryo should go through its 

 development as rapidly as possible, since the species 

 will thus multiply more rapidly, and also because it 

 will give less opportunity for the destruction of the 

 helpless embryos by their enemies. The chick de- 

 velops in twenty-one days, and it is therefore prac- 

 tically impossible that all of the stages of its 

 ancestral history can be exactly repeated. Much 

 will be, of necessity, skipped, much faintly sug- 

 gested, and the whole will be condensed into small 

 compass. Stages of fundamental importance will 

 be more likely to be retained than those of less 



