HIS ZOOLOGICAL EELATIONS. 43 



members are but modifications of the original type- 

 forms, just as those type-forms themselves are modi- 

 fications of a wider and more comprehensive plan. 



It is true that modification of any important organ 

 implies a corresponding modification in all the other 

 organs which colistitute the entirety of any living 

 being. This is the great physiological doctrine of the 

 "co-relation of parts" by which, for instance, the 

 simpler stomach and shorter intestines of the carnivore 

 is co-adapted to the trenchant tooth and seizing fore- 

 limb, and the more complicated stomach and longer 

 intestines of the ruminant co-adapted to the grinding 

 tooth and the harmless fore-foot. No important 

 modification, then, can take place in one member 

 without affecting the others, and hence the numerous 

 forms in nature according to the function to be 

 performed and the element to be occupied.* But 

 difference in form and function does not necessarily 

 imply a separate origin, and seeing the gradual shading 

 of form into form in nature, it is easier, and indeed 

 more rational, to believe in modification of original 

 type-forms than in the creation of new forms for every 



* " Every organised teing," says CuTier, " forms a whole, a 

 single circumacrilied system, the parts of which mutually correspond 

 and concur to the same definite action by a reciprocal reaction. 

 None of these parts can change without the others also changing, 

 and consequently each part, taken separately, indicates and gives 

 all the others." 



