202 HABIT AND INTELLIGENCE. [chap. 



Mental and motor characters are more variable, as well 

 as more modifiable, than formative ones. 



Among formative characters, the minutest structures are 

 the least variable. 



The lowest organisms vary most ; and when there are 

 many similar parts, their number is variable. 



I believe that very little more than what I have stated 

 is known on the subject of ordinary spontaneous variation. 

 But, as I have mentioned at the beginning of this chapter, 

 besides fvmctionally-produced modifications and sponta- 

 Third class neous variations, there is a third kind which it is difficult 

 o c anges. ^^ dass. These take place only in organisms of low type. 

 Crystals It has been mentioned in the chapter on Crystallization, 



the me- that crystals of the same species — that is to say, of the 

 (Imm from game chemical composition and the same " crystallographic 

 they are elements " — often differ very much in form, according to 

 deposited. |.|-^g character of the medium from which they have crystal- 

 lized. This variability is regular ; tl^at is to say, the same 

 alteration in the medium determines the same peculiarity 

 in the form of the crystal. Thus, " common salt crystal- 

 lizing from pure water forms cubes ; but if the water con- 

 tains a little boracic acid, the angles of the cubes are 

 truncated." " Carbonate of copper, crystallizing from a 

 solution containing sulphuric acid, forms hexagonal tabular 

 prisms ; but if a little ammonia is added, the form changes 

 to that of a long rectangular prism with secondary planes 

 on the angles ; if a little more ammonia is added, several 

 varieties of rhombic octahedra appear." ^ 

 Similar Similar variations from similar causes appear to take 



variations pjace among many low organisms, especially among those 

 Fungi which constitute mould. We have not what can be 

 called direct evidence of this ; but there is evidence that 

 the forms of the inferior Fungi are extremely variable ; ^ 

 and it is scarcely possible to doubt that the various forms 

 of Fungi which are characteristic of particular situations 



1 P. 75. 



2 " It is asserted by Fries, that out of a single species of ThelejiJiora 

 more than eight genera have been consti-ucted by various authors." 

 (Carpenter's Comparative Physiology, p. 214.) Compare what has been 

 said above on the variabUitj' of the Foraminifera. 



