192 ON THE DISTINCTION OF AN ANIMAL 



themselves to the examination of the most complicated 

 organizations ; so that the modifications which take place 

 in the form of the organs, and the limits by which the 

 faculties of these organs are prescribed, as we trace them 

 down the scale of animal life, have been comparatively 

 neglected. But though there may be no small degree of 

 truth in this remark, the fact is, that a very great difficulty 

 indubitably exists in the subject itself, which, indepen- 

 dently of the manner in which it is treated, opposes almost 

 invincible obstacles to the clear distinction of the two 

 species of organized matter. 



It is not assuredly in the circulation of the fluids in ani- 

 mals that this distinction can be safely said to exist; for 

 while we are as yet ignorant of the true nature of that 

 great motion in the more perfect plants, called the flowing 

 of the sap, there are many animals, and those not of the 

 most simple structure, in which nothing like circulation has- 

 as yet been detected. The distinction does not consist, 

 as some authors will have it, in the nutrition of animals 

 taking place by digestion, and that of plants by suction; for 

 it is difficult to conceive how the simplest form of ani- 

 mals is nourished at all, unless it be by absorption of 

 fluids by their external surfaces. It is not in respiration ; 

 for air is the universal nutriment of organized matter; the 

 penetration of which into an organized body is so neces- 

 sary to its vitality, that whether it takes place by peculiar 

 organs for the purpose, or by the whole of the surface, 

 death appears to be the inevitable consequence of the ex- 

 clusion. It is not in motion, because some animals are as 

 completely destitute of the power of locomotion as plants : 

 unless by motion is meant irritability; in which case, 

 motion and sensation are resolvable into one and the same 



