LIVING FORMS. THE LAW OF FORM. 109 



It was found that this was true only to a certain 

 extent, and to varying degrees ; just so does the 

 special formative power supposed in living bodies 

 produce peculiar forms only to a limited and varying 

 degree of accuracy. 



A word may be said here, also, respecting the 

 doctrine of " types," or standards, to which all living 

 forms are referred. As a guide to the investigation 

 of the organic world, this idea has proved itself 

 invaluable ; and the doctrine of corresponding parts 

 in different organisms, to which it has been made 

 subservient, constitutes, and must continue to consti- 

 tute, a beautiful branch of physiological science.* 

 But it is hardly necessary to say that no formative 

 power is to be ascribed to those types or standards. 

 The body needs some efficient cause to determine 

 its form just as much, being conformed to such 

 a type, as it would if it were not so conformed. 

 Constancy of form proves constancy of conditions, 

 and must do so equally upon every hypothesis. 



* These corresponding pails arc called " homologous ; " as, for 

 example, in plants, leaves and stamens are homologous ; they corre- 

 spond in their nature, although performing different offices. 



