m Organized Beings. 113 



ment from the parent. The analogy, then, between the spore 

 and the seed, is something like that of the tadpole and the frog, 

 both of tlie former being less developed states of the latter, bul 

 modified to maintain an independent existence. Taking this 

 view of the case, the fronds of such plants as the marchantia, 

 which are permanent, and never develope a distinct axis of 

 growth, must be regarded as cotyledons, and are obviously ana- 

 logous to the perennial branchiae of the lower classes of animals. 

 It is interesting to remark that the simple cell, which is the type 

 of the lowest plant as well as of the lowest animal, is also the 

 type of the earliest embryonic condition in both kingdoms ; and 

 there is no more perceptible difference between the germ of a 

 plant and an animal, than there is between those of the differ- 

 ent classes of either kingdom. 



In tracing the gradual development of the functions peculiar 

 to animals, namely, sensation and voluntary motion, we may, I 

 think, even here find that the special type is evolved from one more 

 general. If the views which I have elsewhere stated be correct, 

 it follows that the irritability of certain tissues in plants is ana- 

 logous to that of the muscular fibre of animals; and that the 

 actions immediately connected with the maintenance of the or- 

 ganic functions of the latter, are the direct result of external 

 stimuli on their organism. The possession of a nervous system 

 must, I think, certainly be regarded as the distinguishing charac 

 teristic of animals, although our means of investigation will not 

 always enable us to detect it ; but the functions of this system 

 in the lowest classes of animals, would appear to be almost en- 

 tirely confined to the conduction of impressions from one part 

 of the organism to another. As we rise in the scale, we observe 

 that the instinctive actions which are the necessary respondence 

 of the organism to external stimuli, are gradually overpowered 

 by the influence of the will ; and although, as has been recently 

 observed,* we may regard the nervous system as living and 

 growing and carrying on its actions within the body of an ani- 

 mal as a parasitic plant does in a vegetable, and as "not commu- 

 nicating any influence which is immediately essential to its or- 

 ganic functions, yet we must perceive (to use the words of the 



• British and Foreign Medical Re view,* vol. iii. p. 10. 

 VOL. XXIII. NO. XLV.— JULY 1837. H 



