REPRODUCTION AND GROWTH 167 



Other cases of natural, inanimate growth may be analysed in 

 the same way. Prominent features are the accretion of materials 

 and the condition that in growth of this kind the materials 

 added to the growing thing are similar to those that make up the 

 thing. 



59^. Crystal Growth. This process we must examine in 

 detail because of the analogy, which has often been suggested, 

 between it and organic growth. A crystal grows in size when 

 it is placed in a strong solution of the same substance as that of 

 which it is composed (or exceptionally of some other chemical 

 substance which crystallizes in the same geometrical form). If 

 certain precautions are taken the crystal that grows in the mother- 

 liquor preserves its geometrical form very closely. As a rule, 

 irregular masses of crystals form and there may be apparent 

 deviations from the typical form, but growth may be so regulated 

 that a small crystal grows to be a giant one which has almost 

 exactly the same arrangement of faces and angles. 



The materials of the crystal are the same as are the materials 

 in the mother-liquor. In the latter there are molecules of the 

 chemical substance and these have certain very definite forms and 

 orientations. When they leave the liquid state and become added 

 (by accretion) to the growing crystal they take up definite relations 

 to the parts of the latter. Molecules do not exist, as such, in the 

 solid crystal, for the latter is a lattice in three dimensions and is 

 one thing, but the molecules that become added to the existing 

 lattice are added on in a certain invariable way, so that the new 

 parts of the crystal are similar to the old ones and, in fact, form 

 a continuous structure that has no internal divisions or boundaries. 



Organic growth is quite diiferent from this process. 



60. ON ORGANIC GROWTH 

 Inanimate things usually grow by accretion of material to their 

 external parts — thus the sand-bank grows by the deposition of 

 materials to its margins and surface. This is sometimes regarded 

 as a distinction between inanimate and organic growth : organisms 

 are said to grow by " intussusception," that is by the addition of 

 materials to their internal parts. But a small block of dry gelatine 

 that is placed in w^ater will increase in size by the addition of 

 water to the internal parts, into which the liquid soaks. 



