ITS FAUNA. 53 



her and in like manner all the other appurtenances of the 

 vertebrate skeleton. This is the great doctrine of HOM- 

 OLOGY, or science of similar parts, as it is termed, through 

 which we arrive at the conclusion that the arm and hand of 

 man, the fore-limb and foot of the quadruped, the wing of 

 the bird, and the fore-fin of the fish, are one and the same 

 primal organ, composed of the same or homologous parts, 

 and merely modified or altered for the performance of cer- 

 tain special functions. As the stationary engine that turns 

 the spindles of the factory, the locomotive that drags the 

 railway cars, and the marine engine that propels the steam- 

 ship, are but modifications of the same primal machine ; so 

 the mammal that runs, the mammal that flies, and the 

 mammal that swims, are but specialised expressions of the 

 same primal plan, the creation of a new type being unne- 

 cessary where a modification of an existing one would suffice. 

 Knowing these modifications in the limbs, jaws, teeth, and 

 other organs, and the ends they were meant to subserve in 

 living races, we can predicate of forms long since extinct, 

 and can associate with co-relation of structure the functions 

 that creatures were meant to perform in the economy of 

 former ages. It is by this " law of the co-relation of parts," 

 and faith in the uniformity of nature's method, that Cuvier 

 and Owen, and other great anatomists, have been enabled 

 to accomplish their wonderful restorations of extinct life, 

 and from a few sorely mutilated and scattered fragments to 

 present us with forms of harmonious entirety. "Every 

 organised being," says the great French anatomist, " forms a 

 whole, a single circumscribed 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." 

 As with the vertebrate type, so with the niolluscan, the 



