X.] ORGANIZATION. 119 



a group depends, are determined by the " law of develop- 

 ment " in a way that is altogether peculiar to biological 

 science, as I shall have to explain farther on. 



But the description, comparison, and classification of 

 organic species — that is to say, organic morphology — con- Morpho- 

 stitute only one-half of the science of biology, and that °^^' 

 its least characteristic half Crystals, like organisms, have 

 their morphology and their classification ; but in crystals 

 there is nothing but structure to consider : there is no 

 adaptation of structure to function, because there is no 

 function. Function (in the organic, not the mathematical 

 sense) is impossible and inconceivable in such bodies as 

 crystals, which can continue to exist only under the con- 

 dition of perfect molecular immobility, and cease to be 

 crystals if this is disturbed. In organisms, on the contrary, 

 every structure is adapted to its function, and structure 

 depends on function. Consequently, the second, and most 

 characteristic part of biology, consists in physiology, which Physi- 

 is defined as the science of organic functions, and of the ° °^' 

 relations of structures to their functions. 



I call physiology the most characteristic part of biology, 

 because, by the definition, physiology alone takes cognizance 

 of that relation of structure to function, or, what comes to . 

 the same thing, of means to purpose, which is peculiar to the 

 organic creation. We have seen that the relation of cause 

 and effect is the characteristic relation in the dynamical 

 sciences, and that of resemblance and difference is the 

 characteristic one in the classificatory sciences. Biology 

 is a classificatory science ; but the relation of means and 

 purpose is also a characteristic one in biology, and is found 

 there alone. 



From this point of view, we may say that morpho- 

 logy is the classificatory side of biological science, and 

 physiology its functional side. 



I may be told, that when I say that the relation of 

 structure to function is the same thing with the relation 

 of means to purpose, I am assuming as true an hypothesis 

 which has not been, and cannot be, verified. 



I reply, that the relation of special structure to special 



