CHAPTEE IV. 



THE MORPHOLOGY OF THE COMMON CRAYFISH: THE STRUC- 

 TURE AND THE DEVELOPMENT OF THE INDIVTDUAL. 



In the two iDrececling chapters the crayfish has heen 

 studied from the point of view of the physiologist, who, 

 regarding an animal as a mechanism, endeavours to dis- 

 cover how it does that which it does. And, practically, this 

 way of looking at the matter is the same as that of the 

 teleologist. For, if all that we know concerning the pur- 

 pose of a mechanism is derived from observation of the 

 manner in which it acts, it is all one, whether we say 

 that the properties and the connexions of its parts 

 account for its actions, or that its structure is adapted 

 to the performance of those actions. 



Hence it necessarily follows that physiological plieno- 

 mena can be expressed in the language of teleolog}'. 

 On the assumption that the preservation of the indi- 

 vidual, and the continuance of the species, are the 

 final causes of the organization of an animal, the exist- 

 ence of that organization is, in a certain sense, explained, 

 when it is shown that it is fitted for the attainment of 

 those ends ; although, perhaps, the importance of de- 



