132 HOPKINS! EVOLUTION 



of the same or similar elements of structure and activity, in widely 

 separated classes and orders, must have been independent. It 

 is also evident that the progressive modification of these elements 

 from the simple to the complex must have been along parallel 

 lines in response to the momentum of a dominant tendency to- 

 wards a common plan under the strain of similar requirements in 

 the struggle for individual and racial existence and supremacy. 



