44 THE INFLUENCE OF INANIMATE SUEROUNDINGS. 



knowledge, in itself but small, has been .acquired either by 

 casual observations or by experiments which relate almost 

 exclusively to such animals as are useful or injurious to man, 

 and the general biological bearing of these proportions has as 

 yet been in no way verified by investigation. Hence I shall 

 avoid giving any specific data, and it will suffice to repeat once 

 more that every deviation from the optimum of nutrition (as 

 to quantity) must be more or less injurious to the creature. 



The quality of the nourishment has, if possible, an even 

 greater influence on the life of the individual and consequently 

 on the species, and it constitutes one of the most powerful 

 influences for adjusting the relations between animals and their 

 surrounding circumstances. There is scarcely a constituent of 

 the earth's crust, whether on land or in water not an animal 

 nor a plant, whether living, dead, or even in decomposition 

 which does not afford nourishment to some living animal. Some 

 injects live in dried wood, others on living leaves or roots. 

 Almost all the species of Holothuria (sea-cucumbers), many 

 sea hedgehogs, and one genus of Mollusca (Onchidiuni) swallow 

 sand or mud, while neglecting the animals and plants which 

 lie close at hand. Parasites suck the blood of their host or 

 absorb the juices of a particular organ ; certain larvte of Ascaris 

 (Ascaris nigrovenosa, in the frog) consume the organs of their 

 own parent ; and human flesh is a tit-bit to some of the human 

 race. But in these, as in all other cases, animals require two 

 quite different kinds of food ; it must be of organic and of in- 

 organic origin. If one kind of nutrition is omitted, the other 

 kind, exclusively supplied, will no longer have the same favour- 

 able effect on growth and the other vital processes that it had 

 when duly mixed with the other kind. This fact is universally 

 recognised with regard to man and the domestic animals ; but 

 it obtains throughout the animal kingdom, though it is not in 

 all cases so plainly apparent. Thus, for instance, Parasites 

 siich as tapeworms, threadworms, &c. seem to require one kind 

 only of organic food, since they live in certain organs only, 

 one species in the liver, another in the intestines, others again 

 in the brain (as the worm which gives sheep the staggers) or in 

 the eye, the skin, and even in the bones. All these species of 



