44 COMPARATIVE ZOOLOGY. 



These four are possessed by all animals, but in a variety 

 of ways. No two species have exactly the same mech- 

 anism and method of life. We must learn to distinguish 

 between what is vital and what is only accessory. That 

 only is essential to life which is common to all forms of 

 life. Our brains, stomachs, livers, hands, and feet are 

 luxuries. They are necessary to make us human, but not 

 living, beings. Half of our body is taken up with a com- 

 plicated system of digestion ; but the Amoeba has neither 

 mouth nor stomach. We have an elaborate apparatus of 

 motion ; the adult Oyster cannot stir an inch. 



Nutrition, Motion, and Sensation indicate three steps 

 up the grade of life. Thus, the first is the prominent 

 function in the Coral, which simply "vegetates," the pow- 

 ers of moving and feeling being very feeble. In the 

 higher Insect, as the Bee, there is great activity with sim- 

 ple organs of nutrition. In the still higher Mammal, as 

 Man, there is less power of locomotion, though the most 

 perfect nutritive system ; but both functions are subordi- 

 nate to sensation, which is the crowning development. 



In studying the comparative anatomy and physiology 

 of the animal kingdom, our plan will be to trace the vari- 

 ous organs and functions, from their simplest expression 

 upward to the highest complexity. Thus Nutrition will 

 begin with absorption, which is the simplest method of 

 taking food; going higher, we find digestion, but in no 

 particular spot in the body; next, we see it confined to a 

 tube; then to a tube with a sac, or stomach; and, finally, 

 we reach the complex arrangement of the higher animals. 



