DEVELOPMENT. 3 



loDg intervals of time. A lifeless structure, as a crystal, 

 is subject to no such, laws ; neither decay nor repair is a 

 necessary condition of its existence. That which is true of 

 structures which never had to do with life is true also with 

 respect to those which, though they are formed by living 

 parts, are not themselves alive. Thus, an oyster shell is 

 formed by the living animal which it encloses, but it is as 

 lifeless as any other mass of saline matter ; and in accord- 

 ance with this circumstance its growth takes place not in- 

 terstitially , but layer by layer, and it is not subject to the 

 constant decay and reconstruction which belong to the 

 living. The hair and nails are examples of the same 

 fact. 



Thirdly. In connection with the growth of lifeless 

 masses there is no alteration in composition or properties 

 of the material which is taken up and added to the pre- 

 viously existing mass. For example, when a crystal of 

 common salt grows on being placed in a fluid which con- 

 tains the same material, the properties of the salt are not 

 changed by being taken out of the liquid by the crystal 

 and added to its surface in a solid form. But the case is 

 essentially different from this in living beings, both animal 

 and vegetable. A plant, like a crystal, can only grow when 

 fresh material is presented to it ; and this is absorbed by 

 its leaves and roots ; and animals for the same purpose of 

 getting new matter for growth and nutrition, take food into 

 their stomachs. But in both these cases the materials are 

 much altered before they are finally assimilated by the 

 structures they are destined to nourish. 



Fourthly. The growth of all living things has a definite 

 limit, and the law which governs this limitation of increase 

 in size is so invariable that we should be as much astonished 

 to find an individual plant or animal without limit as to 

 growth as without limit to life. 



Development is as constant an accompaniment of life as 

 growth. The term is used to indicate that change to 



