CHAPTER IV. 
THE MORPHOLOGY OF THE COMMON CRAYFISH: THE STRUC- 
TURE AND THE DEVELOPMENT OF THE INDIVIDUAL. 
In the two preceding chapters the crayfish has been 
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 pheno- 
mena can be expressed in the language of teleology. 
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- 
