46 THE IHFUJENCE OF INANIMATE SUEKOnNDlNGS. 



sable aids to tie assimilation of the true nutriment. They 

 may he compared to the oil needed for the working of every 

 machine; this does not add to the effective power of the 

 machine, whatever that power may be, and yet it cannot work 

 smoothly for any length of time without it. It is in the same 

 way that stimulants enable the body to exert its digestive 

 powers to the utmost. Can other animals dispense with such 

 an ' oiling of the machine ' 1 This gives rise to the question 

 as to what sort of stimulants they need ; and to this other 

 one : Whether substances which certainly are by no means 

 stimulants to man may not prove to be such to the lower 

 animals. And finally a third question occurs : Whether other 

 influences, irrespective of the actual reception of food into the 

 intestinal canal, may not act as powerful stimulants for the 

 absorption of true nourishment. This last question may for 

 the present be regarded as superfluous, but it seems advisable to 

 point out that in Chapter VI., ' On the Influence of Stagnant 

 Water on the Creatures inhabiting it,' evidence will be adduced 

 that in certain of the Mollusca (Lymnoea) the assimilation of 

 nourishment depends not merely on the food itself, on the 

 healthiness of the organs, on the temperature, &c., but also on 

 tlie influence on the skin of a certain constitueiit of the water 

 at present unknown to us. 



Organs for taking in, preparing, and assimilating the 

 food, — Everyone knows that digestive organs of some kind are 

 possessed by every sort of animal, and it may be taken for 

 granted that the general structure of these organs and their 

 mode of action- are geneiuUy weU. known, so that it will be 

 superfluous to describe here the endless variety of such parts ; 

 every text-book of zoology gives ample information on such 

 poLnts. A general outline of the relations of the parts must, 

 however, be briefly given. In the first place, their position, 

 invariably within the body, is worthy of remark, since it is 

 this which necessitates the presence of other organs which 

 have certain auxiliary duties to perform in the service of their 

 masters — the stomach and the intestines. The organs for 

 taking up food, the mouth, teeth, and, more remotely, the fore 

 extremities or other external parts, are specially adapted to 



