OF LIVING FORMS; OR, MORPHOLOGY. 93 



the mechanical resistance moulds the structure. We 

 shall see this more truly by-and-by. 



For the question arises, how far this reference to 

 mechanical conditions may be carried. Evidently 

 that cause is operative, but is it the only one? In 

 answer to this question, \ve may say first, that, since 

 the mechanical conditions present during its forma- 

 tion do', to a certain extent, determine the structure 

 which the growing organism assumes, and may be 

 seen to produce some of the beautiful and useful 

 forms which it displays, we may not assume other 

 causes until it is proved that these are insufficient. 

 Here is a fact : the mechanical conditions under 

 which plants and animals are developed have a 

 power of determining their forms in the right and 

 necessary way. The limit of this power must be 

 learnt by observation. 



Or, if we look at the matter in another way, tne 

 conclusion is equally evident. Let us consider for 

 a moment the circumstances of a developing plant 

 or animal. Here is the living substance ; it is a 

 soft, plastic mass increasing in size; the forces of 

 nature are operating upon it, adding to its bulk. 



