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. ‘lhis 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 ina 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, im 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 perceived, is an animalcule which resides 
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