ON INSTiNCl'. 



2m 



j^rther, that Mr. John Hunter had traced this living principle to many of 

 the organized fluids, as well as to the solids ; and that he had especially de 

 veloped it in the blood, which, coincidently with the Mosaic declaration, 

 he believed to be its immediate seat. The difficulty," observes he, " of 

 conceiving that the blood is endowed with life while circulating, arises 

 merely from its being a fluid ; and the mind not being accustomed to the idea 

 of a living fluid. "* And I observed, that by a variety of important and well- 

 defiaed experiments, this enterprising and indefatigable indagator had suc- 

 ceeded in proving, not only that it contributes in a greater degree to the 

 vital action and to the vitai material of the general system than any other 

 constituent part of it, whether sohd or fluid, but has all the essential pro- 

 perties of life ; that it is capable of being acted upon, and contracting, like 

 the muscular fibre, upon the application of an appropriate stimulus, as 

 atmospheric air, for example ; on which occasion it becomes constringed 

 into that cake or coagulum which every one must have beheld in blood 

 drawn from the arm : that in all degrees of atmospherical temperature, of 

 heat or cold, which the body is capable of enduring, it maintains an equality 

 in its own temperature with scarcely any variation : that in the case of para- 

 lytic limbs it is the only power that continues vitality in them and preserves 

 them from corruption ; that though not vascular itself, it is capable by 

 its own energy of producing new vessels out of its own substance, and ves- 

 sels too, of every description, lymphatics, arteries, and even nerves ; and 

 finally, that though like the muscular fibre, it is capable of contracting upon 

 the application of a certain degree of appropriate stimulus, like the mus° 

 cular fibre, also, it is instantly exhausted of its vital power whenever such 

 stimulus is excessive ; and that the stroke of lightning which destroys the 

 muscular fibre, and leaves it flaccid and incontractile, destroys likewise the 

 blood, and leaves it loose and incoagulable. 



In every organized system, then, whether animal or vegetable, and in 

 every part of such system, whether solid or fluid, we trace an evident 

 proof of that controlling and identifying power which physiologists have 

 denominated, and with much propriety, the principle of life. Of its 

 cause and nature we know no more than we do of the cause and nature of 

 gravitation or magnetism. Jt is neither essential mind nor essential mat- 

 ter ; it is neither passion nor sensation ; but though unquestionably distinct 

 from all these, is capable of combining with any of 'them : it is possessed 

 of its own book of laws, to which, under the same circumstances, it ad- 

 . heres without the smallest deviation : and its sole and uniform aim, 

 whether acting generally or locally, is that of health, preservation, or re- 

 production. The agency by which it operates is that which we denomi- 

 nate or should denominate instinct, and the actions by which its sole and 

 uniform aim is accomplished are what we mean or should mean by in- 

 stinctive ACTIONS ; or to speak somewhat more precisely, instinct is the 

 operation of the living principle, whenever manifestly directing its opera- 

 tions to the health, preservation, or reproduction of a living frame, or any 

 part of such frame.! 



The law of instinct, then, is the law of the living principle : instinctive 



* Essay ou the Blood, &c. p. 20. 



t This lecture was delivered January 1S13 ; and Mr. Keith, on Tuesday, December 7, 

 1813, had a valuable paper read before the Lionean Society, in which, like the present 

 system, he opposes Mr. Knight's hypothesis of gravitation as the cause of the peculiar stimii» 

 lus and action of plants, and conceives that the direction of the plumule and radicle of 

 plants must be resolved into vegetable instinct, precisely analogous, and equally inexpli^ 

 cable with animal instinct." — See Thomson's Ann. of Philos. vol. iii. p, 71, or No. IS, 



