4 MOIU'IIOLOGICAL DEVELOPMENT. 



facts as the hypothesis of Evolution aims at, without content, 

 plating structures and functions in their mutual relations. 

 Everywhere structures in great measure determine functions ; 

 and everywhere functions are incessantly modifying structures. 

 In Nature, the two are inseparable co-operators ; and Science 

 can give no true interpretation of Nature, without keeping 

 their co-operation constantly in view. An account of organic 

 evolution, in its more special aspects, must be essentially an 

 account of the inter-actions of structures and functions, as 

 perpetually altered by changes of conditions. 



Hence, when treating apart Morphological Development 

 and Physiological Development, all we can do is to direct our 

 attention mainly to the one or to the other, as the case may 

 be. In dealing with the facts of structure, we shall consider 

 the facts of function, only in such general way as is needful 

 to explain the facts of structure ; and conversely when deal- 

 ing with the facts of function. 



176. The problems of Morphology fall into two distinct 

 classes, answering respectively to the two leading aspects of 

 Evolution. In things which evolve there go on two processes 

 increase of mass and increase of structure. Increase of 

 mass is primary, and in simple evolution takes place almost 

 alone. Increase of structure is secondary, accompanying or 

 following increase of mass with more or less regularity, 

 wherever evolution rises above that form which small inor- 

 ganic bodies, such as crystals, present to us. The fundamental 

 antagonism between Dissolution and Evolution consisting in 

 this, that while the one is an integration of motion and dis- 

 integration of matter, the other is an integration of matter 

 and disintegration of motion ; and this integration of matter 

 accompanying disintegration of motion, being a necessary 

 antecedent to the differentiation of the matter so inte- 

 grated ; it follows that questions concerning the mode in 

 which the parts are united into a whole, must be dealt with 



