GENERAL LAWS OF ANIMAL STRUCTURE. 28 1 



2. In all plans the ideal similarity of the repeated 

 parts in the animal and of the repeated animals in the 

 department is clearest in the lower part of the scale, and 

 adaptive modification for function becomes more con- 

 spicuous as we rise, until finally the essential identity 

 may be wholly obscured by adaptive modification. This 

 is the law of progressive differentiation so universal in 

 Nature. 



3. All these phenomena may be completely explained 

 by the origin of organic forms by derivation — i. e., by 

 "descent with modifications"; in a word, by the theory 

 of evolution, and can not be explained in any other way. 



