HOMOLOGY AND ADAPTATION 



181 



II. GENERAL BIOLOGICAL INTEREST OF THE LOBSTER 



The structures and life history of the lobster teach, by anal- 

 ogy, the story of evolution. Structural adaptations of animals 

 :o different modes of life, interpreted on the principle of homol- 

 ogy, furnish evidence of the origin of species from generalized 

 :ypes. The appendages of the lobster are originally all alike 

 ;tnd of a primitive biramous type. From this primitive type by 



FIG. 80. A young lobster leaving the egg case (on left). (From Herrick.) 



direct metamorphosis, highly modified thoracic appendages of 

 the adult are derived. These appendages furthermore are 

 utilized for different purposes. Here is the principle of adapta- 

 tion a generalized type of organ may become adapted to sev- 

 eral different kinds of uses. 



What happens among homologous parts in the individual 

 lobster can, theoretically, take place in allied organisms of a 

 given group although the process cannot be watched as it can be 

 in the lobster. We find in existing animals structural adapta- 

 tions which we can interpret best on the theory of common 



