ANATOMY AND PHYSIOLOGY 



water which about half covered them they converted 

 the vital air into fixed air, or carbonic - acid gas, in 

 the same manner as in animal respiration. 



"7. The circulation in the lungs or leaves of plants 

 is very similar to that of fish. In fish the blood, after 

 having passed through their gills, does not return to 

 the heart as from the lungs of air-breathing animals, but 

 the pulmonary vein taking the structure of an artery 

 after having received the blood from the gills, which 

 there gains a more florid color, distributes it to the 

 other parts of their bodies. The same structure oc- 

 curs in the livers of fish, whence we see in those ani- 

 mals two circulations independent of the power of the 

 heart viz., that beginning at the termination of the 

 veins of the gills and branching through the muscles, 

 and that which passes through the liver; both which 

 are carried on by the action of those respective ar- 

 teries and veins." ' 



Darwin is here a trifle fanciful in forcing the analogy 

 between plants and animals. The circulatory system 

 of plants is really not quite so elaborately comparable 

 to that of fishes as he supposed. But the all-important 

 idea of the uniformity underlying the seeming diver- 

 sity of Nature is here exemplified, as elsewhere in the 

 writings of Erasmus Darwin ; and, more specifically, a 

 clear grasp of the essentials of the function of respira- 

 tion is fully demonstrated. 



ZOOLOGY AT THE CLOSE OF THE EIGHTEENTH CENTURY 



Several causes conspired to make exploration all tin- 

 fashion during the closing epoch of the eighteenth cen- 



99 



