138 THE MORPHOLOGY OF THE COMMON CRAYFISH. 



monstrating the proposition that a thing is fitted to do 

 that which it does, is not very great. 



But whatever may he the value of Ideological ex- 

 planations, there is a large series of facts, which have as 

 yet heen passed over, or touched only incidentally, of 

 which they take no account. These constitute the sub- 

 ject matter of Morphology, which is related to physiology 

 much as, in the not-living world, crystallography is 

 related to the study of the chemical and physical pro- 

 perties of minerals. 



Carbonate of lime, for example, is a definite compound 

 of calcium, carbon, and oxygen, and it has a great variety 

 of physical and chemical properties. But it may be 

 studied under another aspect, as a substance capable of 

 assuming crystalline forms, which, though extraordinarily 

 various, may all be reduced to certain geometrical types. 

 It is the business of the crystallographer to work out 

 the relations of these forms ; and, in so doing, he takes no 

 note of the other properties of carbonate of lime. 



In like manner, the morphologist directs his attention 

 to the relations of form between different parts of the 

 same animal, and between different animals; and these 

 relations would be unchanged if animals were mere 

 dead matter, devoid of all physiological properties a 

 kind of mineral capable of a peculiar mode of growth. 



A familiar exemplification of the difference between 

 teleology and morphology may be found in such works 

 of human art as houses. 



