74 



RELATIVE SIZE OF THE HEART. 

 Comparative Weights of the Hearts of Reptiles. 



Comparative Weights of the Hearts of Birds. 



By comparing these tables, we see that the heart is proportionally smallest in 

 Fishes and largest in Birds. 



As the organs and apparatus of the animal economy are developed and per- 

 fected, the circulation of the nutritive materials becomes more vigorous. 



As the temperature, intelligence, and activity of animals, with their correspond- 

 ing physical and chemical metamorphoses of the elements of organic structure 

 increase, there is a correspondingly rapid supply of those materials by which the 

 wastes may be repaired, and from which the various secretions and excretions 

 may be elaborated and separated. 



The next consideration is the -rapidity of the circulation in different animals. 



