STRUCTURE AND DEVELOPMENT 47 



difficult and unusual conditions of deep-sea existence 

 have been met by fishes in two ways; some forms 

 possess luminous frilled and weedlike fins, which 

 lure their prey to within easy reach of their jaws, while 

 others have enormous eyes, so as to make use of all 

 possible rays of light in their pursuit of food organisms. 

 But all of these diverse forms are true fishes, possessing 

 a common heritage of structure which demonstrates 

 their unity of origin. 



The brief review of backboned animals has shown 

 how comprehensive are the principles of relationship. 

 The families and tribes of each order, such as the carniv- 

 ora, are like branches arising from a single limb ; the 

 orders in their turn exhibit common qualities of struc- 

 ture which mean that they have grown from the same 

 antecedents, while even the larger divisions or classes 

 of mammals, birds, reptiles, amphibia, and fishes, possess 

 a deep underlying theme whose dominant motif is the 

 backbone, which proves their ultimate unity in an- 

 cestry. The greater and lesser branches have reached 

 different levels, for the fish is clearly simpler in its 

 make-up than the highly specialized bird. But the 

 great fact is that structural evidences demonstrating 

 the reality of genealogical affinities are displayed by 

 the entire series of vertebrates ; although they differ 

 much or little in many or fewer respects they have one 

 and the same ground-plan. 



The lower animals devoid of backbones, and there- 

 fore called invertebrates, are not so well-known except 

 to the student of comparative anatomy, because they 



