70 



HABIT AND INTELLIGENCE. 



[chap. 



Crystals 

 are bound- 

 ed by plane 

 surfaces : 



may be 

 described 

 mathe- 

 matically. 



Spiral 

 shells. 



Crystals 

 have a 

 limit of 

 size : 



are hard : 



imperme- 

 able hy 

 water : 



two fundaraentally distinct forms ; that is to say, forms of 

 which, the " crystallographic elements " are different, and 

 between which, consequently, no gradation is possible. 

 Such substances are said to be dimorphous. Carbonate of 

 lime is one of these : it forms crystals of the rhombohedral 

 system which are known as calcspar, and crystals of the 

 right prismatic system which are known as aragonite. 

 Such crystals are to be regarded as distinct species. 



As I shall have to show farther on, there are great 

 differences of form within the limits of the sanie species ; 

 but these are not fundamental, and gradation between 

 them is possible. 



A crystal, when normally formed, is always bounded 

 by plane surfaces which meet each other at angles. In 

 organisms, on the contrary, there is no such thing as two 

 plane surfaces meeting at an angle. 



In every crystal, all the faces stand in simple mathe- 

 matical relations to each other. In organisms, on the con- 

 trary, the forms are too complex to admit of mathematical 

 description. Spiral shells are an exception to this : they 

 present regular equiangular spirals. But thoitgh this is an 

 exception, it is one that rather confirms the rule ; for 

 though shells are vital products, no vital 2}'i'occss goes on in 

 their substance. 



Every crystaRized species, as well as every organic 

 species, has an approximate limit of growth, and conse- 

 quently a specific size, which is seldom exceeded. In 

 crystals this is not nearly so definite as in the higher orders 

 of organisms; but it is quite as definite as in such low 

 organisms as lichens. 



A crystal is generally harder and more brittle than the 

 same substance when in the non- crystalline state. In this 

 crystals are contrasted with organisms, the substance of 

 which is generally soft, and is always so in those parts of 

 the organism in which growth, or any other vital process, 

 is going on with greatest rapidity. 



The substance of crystals is impermeable by water. This 

 is true even of those crystals the substance of which is 

 soluble in water, such as common salt : their substance 



