270 THE REIGN OF LAW. 



Correlation of Growth ? It is, first, the Internal 

 Correlation of those parts to each other, but 

 secondly and principally, it is the External Corre- 

 lation of both to their function of propelling 

 under water. The form of the foot is correlated 

 to the function of opposing the largest possible 

 area of resistance to that medium, exactly where, 

 for the purpose of swimming, the maximum of 

 resistance is required ; the knife-shaped leg-bone 

 is correlated to the function of opposing the 

 least possible resistance, precisely where, for the 

 same purpose, the minimum of resistance is re- 

 quired. In Australia we have, in the Ornitho- 

 rynchus paradoxus, the webbed feet correlated 

 with the Duck-shaped Bill in an animal which 

 does not belong to the Class of Birds at all. 



There is another case of what may be called 

 Correlated Correlations, which brings out very 

 clearly the distinction which is so important in the 

 philosophy of this great subject. Feathers are a 

 kind of covering peculiar to the Class of Birds. 

 Under every variety of modification they have one 

 fundamental plan — a central shaft or quill to which 

 lateral filaments are attached. Now there is a 



