138 THE MORPHOLOGY OF THE COMMON CRAYFISIL 
monstrating the proposition that a thing is fitted to do 
that which it does, is not very great. 
But whatever may be the value of teleological ex- 
planations, there is a large series of facts, which have as 
yet been 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. 
