46 PHILOSOPHY OF ZOOLOGY. 



Though plants possess many organs apparently well a- 

 dapted to the display of voluntary motion, yet the total ab- 

 sence of any thing like a nervous system, the essential con- 

 dition of the faculty, and the want of animal matter on which 

 it may exert its energies, forbid us to entertain a belief of its 

 existence. 



Besides possessing the faculty of sensation and voluntary 

 motion, I likewise am able to move my limbs in such a 

 manner, as to change the position not of one organ merely, 

 but of my whole body, or to shift from one place to ano- 

 ther. This new action is termed Locomotion. It requires 

 for its performance not merely the conditions requisite for 

 sensation and voluntary motion ; but likewise an arrange- 

 ment of organs so constructed, as by their action on the sur- 

 rounding elements, whether of air, earth or water, the body 

 may be displaced. Quadrupeds, birds, reptiles and fishes, 

 possess such an arrangement of organs, and exhibit the loco- 

 motive power in a great degree of perfection. But as we 

 descend in the scale, we find many animals in which such 

 an organization does not exist, and that live on the same 

 spot from the commencement to the termination of their 

 existence. These animals, however, are all natives of the 

 water ; and although they be thus stationary themselves, 

 the fluctuations of the element in which they live, produce 

 a variety in the scene, and daily bring new objects in con- 

 tact with their organs of sensation. 



Among the invertebral animals, in which this faculty is 

 not present in every species, there does not appear to be 

 any link of the chain, or any system of organs connected 

 with other functions, which regulate the presence or ab- 

 sence of locomotion. The Monas, usually considered as 

 the lowest term of animal life, and in which, neither mouth 

 nor vessels can be perc eived, is an animalcule which resides 



