HOMOLOGY AND ADAPTATION 



185 



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 

 to different modes of life, interpreted on the principle of homol- 

 ogy, furnish evidence of the origin of species from generalized 

 types. The appendages of the lobster are originally all alike, 

 and of a primitive biramous type. From this primitive type by 



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



various modifications, several different forms of the adult ap- 

 pendages are derived. These appendages, furthermore, are 

 utilized for different purposes, thus illustrating the principle of 

 adaptation a generalized type of organ may become adapted to 

 several 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 

 adaptations which we can interpret best on the theory of com- 

 mon origin. Thus, in this one group of Crustacea to which the 



