196 ON THE DISTINCTION OF AN ANIMAL 



source of that exquisite sensation which gives them their 

 superiority over the vegetable kingdom. And this hypothe- 

 sis assumes the characterof probability, when it is consi- 

 dered that in four out of five distinct sorts of animals four 

 different sorts of nervous systems have been discovered, 

 but none for the fifth ; that in the most perfect class of ani- 

 mals there prevails a diametrically opposite system to the 

 one here supposed to exist in the least perfect class ; and 

 lastly, that the system here proposed would be of exactly 

 such a nature as to accord with the actual phenomena, — 

 for instance, the nervous system itself would be indistinct, 

 while the sensitive molecules being dispersed throughout 

 the mass, would render the animals themselves peculiarly 

 irritable. 



In the next place, the difference between the pheno- 

 menon which occurs when the feelers of a polype are 

 touched and that which the leaves of the sensitive plant 

 exhibit on a similar occasion has been accurately stated 

 by M. Lamarck. The first is a true contraction of the 

 part as it were into itself, which contraction appears to 

 result from the injury experienced by that process of the 

 nervous system which ramifies through the feeler touched. 

 In the case of the sensitive plant there is nothing like this 

 nervous contraction of the part touched, but only, as La- 

 marck styles it, an articular plication of the neighbouring 

 parts, without any of their dimensions being altered. To 

 this last phenomenon however this author will not allow 

 the appellation of irritability, though I cannot but think 

 that the distinction he draws between animal sensation 

 and animal irritability is merely verbal, and by no means 

 founded on observation or analogy ; while, on the other 

 hand, it seems no easy matter to understand how the change 



