COLOtTR, FORM, AND MOVEMENT. 63 



fact that the starfish is radial in form. It behaves as 

 a radial animal. Whether this radial form has in the 

 course of evolution been superimposed upon a bilateral 

 type is a difficult question, the answer to which cannot be 

 discussed here (Fig. 7). 



Unsymmetrical animals are rare. There is the familiar 

 snail, carrying its shell containing many important organs 

 upon its right side. We cannot divide this animal by a 

 single straight line into two symmetric halves. But the 

 snail in this feature exhibits a turning back. It is at 

 bottom bilaterally symmetrical, but this symmetry has 

 been lost 



Symmetry of the Inorganic. 



We have already suggested that the contrast in surface 

 bounding lines between living matter and mineral is that 

 of curved lines and straight lines. We go on now to state 

 the difference more exactly. G-eologists recognise that 

 matter apart from organic matter exists in two main 

 conditions. 



(a] Amorphous, having no definite internal structure, 

 and consequently no characteristic external shape of its 

 own, but taking the form of the cavity or space in which 

 it originated. This is not the place in which to consider 

 the causes leading to matter appearing in this condition, 

 but it may be pointed out that in some cases the assuming 

 of the amorphous state is due to the conditions of forma- 

 tion (e.g. too rapid cooling), and is not an inherent pro- 

 perty of the substance itself. Examples of amorphous 

 matter are natural glasses, e.g. obsidian and pitchstone. 



It has been suggested that amorphous bodies are really 

 made up entirely of matter in the condition of irregularly 

 arranged crystals of microscopic size. 



(&) Crystalline, where the substance has a definite 

 internal structure, which usually finds expression in a 

 definite external form, which is not lost unless the sub- 

 stance be subjected to external forces. One reason why 



