GROWTH AND DEVELOPMENT. 13 



How, by means of external force, the embryo can 

 build up its structure with, the materials around, we 

 know not in the least. But we are nearly as ignorant 

 of many other transformations of force. The means by 

 which an electric current decomposes a chemical com- 

 pound are as mysterious as those by which vital decom- 

 position and re-construction are effected. When we know 

 the cause in the one case we shall probably in the other 

 also. 



Growth and development after birth are but continua- 

 tions of the same processes that were active in the embryo. 

 The material wherewith construction is effected is taken 

 from a different source, and elaborated by different organs ; 

 but all the vital acts concerned in the further progress of a 

 new being to maturity are dependent for their continuance 

 on external force. This may be applied to them directly, 

 and from without, as when trees absorb nutrient matter, 

 and grow under the light and heat of the sun ; or it may be 

 derived from material taken into the body as food, part of 

 which food is used for constructive purposes, and part as 

 material for the production of force, especially heat, by the 

 transformation of which, construction and other acts cha- 

 racteristic of life are alone effected. 



Capacity of growth, however, is not a property of living 

 beings only. Crystals grow as well as plants or animals, 

 if placed under appropriate conditions. But the manner 

 of growth in the two cases is widely different. A crystal 

 increases in size by fresh additions to its exterior only; 

 while a living form is built up by increase throughout its 

 whole substance. The nutrition is not superficial only but 

 interstitial. There is, however, a greater difference than 

 this. In the case of an inorganic body, there is addition 

 simply, and together with this, there is no waste going on 

 at the same time. Throughout the substance of a living 

 organism, on the contrary, there is constant waste at the 

 same time with repair. In the case of a being that has 



