THE DISTINCTIONS BETWEEN, ANIMALS AND PLANTS. 475 
Thus, no one would feel any uncertainty as to the characteristics 
proper to a forest tree or a quadruped; but, as we gradually descend 
to the lower tribes of the two kingdoms, we find the distinctive 
characters becoming gradually fainter and fainter, until we arrive 
at last at a simple cell, which appears to extend upon the confines 
of the two kingdoms, and to partake of the characters of both, and 
might with equal justice be referred to either. Indeed, there are 
not wanting physiologists of the highest ability, as Kiitsung, 
Unger, and many others, who believe that there are natural bodies 
which are vegetables at one period of their lives, and animals at 
the other. They positively assert this to be the case in some of 
the lower algce , as Ulothrix zonata , Vaucheria clavata, &c. 
Linnaeus has said, that vegetables grow and live, but animals 
grow, live, and feel; and this definition is nearly in accordance 
with the generally received opinion upon the subject, for animals 
are commonly regarded as beings which live, grow, and reproduce 
themselves, agreeing in these characters with plants; but they also 
possess the powers of voluntary motion and consciousness of 
external impressions, characters which we usually consider plants 
not to possess. But we cannot deny the power of locomotion to 
many tribes of plants, as the Diatomacece , and there are numerous 
plants which move with as much appearance of consciousness as 
many of the lower animals: for example, compare the movements 
of the Oscillator ece, and the reproductive particles of some of the 
lower aquatic tribes of plants, with the movements of many 
polypes, and the lower mollusca. Again; there are large tribes 
usually considered as belonging to the animal kingdom, and of 
which we may take the sponge as an example, which, as far as 
we have ascertained at present by observation and experiment, 
possess neither sensation nor power of voluntary motion. 
Another character commonly regarded as distinctive of animal 
life is the presence of a stomach; but, strictly speaking, we might 
consider a plant as composed of many stomachs, for every vege¬ 
table cell of which it is composed is physiologically a stomach. 
In animals, however, the stomach is not a closed one as a vegeta¬ 
ble cell, but it is open to the external medium, and food is thus 
introduced into its exterior, consequently absorption of nutriment 
takes place from within; whereas, in plants, absorption of nutri¬ 
ment takes place from without, into a cavity which is a closed 
one. 
The respiratory process also affords a character, by which 
animals are considered by some physiologists as distinguished from 
plants; for, in the respiration of animals, oxygen is absorbed and 
carbonic acid evolved; whereas, in what has usually been consi¬ 
dered the respiration of plants, we have a fixation of carbon and 
