94 LIFE IN NATURE. 



Around it is a more or less resisting envelope. 

 Will it not necessarily grow in those directions in 

 which its extension is the least resisted ? The case 

 is, to a certain extent, like that of taking the copy 

 of a medal in wax it is a very rough comparison, 

 but still it may help us to grasp the general idea 

 the plastic substance, under the pressure of the 

 artist's hand, moulds itself into the desired form by 

 extending where the resistance is the least. There 

 is no possibility of its doing otherwise. The case 

 is as demonstrable as a proposition in Euclid. And 

 it is equally so in respect to the growing plant or 

 animal ; under the pressure arising from the increase 

 of its mass, it will mould itself by extending where 

 the resistance is the least. 



But the process, of course, is much more complex 

 than in this simple illustration. Perpetual changes 

 and modifications are taking place, and especially in 

 this respect, that every step in the development has 

 its share in determining all that follow. Every 

 newly-formed part or organ, each minutest fold, 

 becomes at once a factor in the process. Thus it 

 is, of course, that from seeds, all of them so much 



