ANIMAL NATURE OF DIATOMEiE. 361 



may decide the question. The comparison of animals 

 with vegetables has been treated and discussed in all its 

 forms so many times, and by so many celebrated authors, 

 that the mere mention of it may excite a fear that we 

 are about to recite things antiquated or stale, or at least 

 well known by every one. Therefore I shall abstain 

 from the useless display of cheap erudition, and shall 

 apply myself solely to the most recent results of scientific 

 investigation. 



The most certain manifestations of animal life are those 

 of sensibility and motion, either of the whole body or of 

 its various parts. It is useless to enter into minute par- 

 ticulars in order to prove that the word sensibility, like 

 aU other abstract terms, is merely a name by which we 

 understand a class of phenomena whose cause is unknown. 

 Hence the abstract expression has come to denote the 

 cause itself, and by conventional agreement we say that 

 sensibility causes the phenomena of sensation. It is 

 sensation only that really exists, for that is a change that 

 we have experienced. Analogy leads us to conjecture, 

 according to certain indications, that similar changes 

 occur in all other animals, and, therefore, that there is a 

 property common in all these beings, which we call 

 sensibility. We see that in many cases these indications 

 become so transient that we are entitled to attribute 

 their apparent absence to the imperfection of our means 

 of observation. 



With respect to motion, since it manifests itself 

 objectively, and this its manifestation is always induced 

 directly or indirectly by external causes, the distinction 

 between vegetable and animal motion has been sug- 

 gested to our minds, not by the action itself, but rather 

 by the quaUty of those beings hitherto regarded as plants 

 or animals on other grounds. And if we wish to find 

 elements of distinction in the phenomenon itself, we look 

 for them in the connection between external stimulants 

 and the motion produced, as also in the instruments by 

 which it is effected. We endeavour to prove that in 



