44, PHILOSGPHY OF ZOOLOGY. 
species. In the animals of the higher classes, the nervous 
filaments can be distinctly traced ; but in proportion as the 
absorbing, secreting, and circulating vessels of the system 
diminish in size, the nerves experience a corresponding de- 
crease ; and when these vessels can no longer be detected, 
on account of the smallness of their diameter, the nervous 
filaments likewise elude our observation. But in every ani- 
mal, the most minute, we infer, from analogy, the exis- 
tence of these vessels ; and the belief of the corresponding 
existence of the nerves, is sanctioned by the same authori- 
ty. Since all animals may thus be considered as possessing 
nerves, the faculty of sensation may be regarded as com- 
mon to every individual of the Animal Kingdom. 
Let us now turn our attention to the most perfect plants ; 
to those in which the organs are most numerous, and most 
complicated in their structure. In these, however, there are 
no nerves, nor any substance which resembles them in form, 
structure and function; for the claims of the pith of plants 
to be considered as analogous, have been abandoned by 
common consent. If nerves do not exist in the most per- 
fect plants, have we reason to anticipate their discovery in 
imperfect plants, which occupy a lower scale, whose organi- 
zation is more simple, and whose living principles possess 
fewer attributes? Here analogy forbids the expectation ; 
the microscope and the scalpel announce that it is hopeless. . 
We conclude, therefore, that as vegetables are destitute of 
nerves, they are likewise destitute of the faculty of sensa- 
tion. 
When any object touches my finger, and excites the 
idea of its presence, it is the nerve which communicates the 
sensation. But I can remove my finger from the object, 
and thus cause the sensation to cease, or bring it again in 
contact, and renew the impression. These actions are ex- 
pressed by the phrase Voluntary Motton. This faculty is 
