238 ELEMENTS OF SCIENCE 



the snail in structure, but with the following exceptions : 

 there is no head and no air sac, but there are plate-like 

 gills, and the shell, instead of being a single, spirally 

 coiled cone, consists of two lateral halves, united dorsally 

 by a hinge, and bearing to the creature between them, 

 somewhat the relation of the two sides of a frock coat to 

 the man who wears it. 



Having, it is hoped, now said enough to stimulate our 

 readers to have recourse to works on zoology in order to 

 pass beyond a mere introduction to the elements of that 

 science, we may revert to general considerations which 

 refer to the whole of the organic or living world. 



We have already seen* what are the properties of that 

 substance, protoplasm, which is common to all animals 

 and plants, but the simplest facts of physiology suffice to 

 establish the great distinction between the living and the 

 non-living world. A seed, under suitable conditions, will 

 give rise to a plant which will again prod Lice a seed, and 

 from the kitten there will similarly be produced a cat, 

 and thence a kitten once more. So the changes of 

 organic life tend to recur in cycles the necessary condi- 

 tions of heat, moisture, and gaseous material, &c., being 

 supplied. Thus the existence of an innate tendency to 

 go through a definite cycle of changes when exposed to 

 certain fixed conditions, forms a distinction, not only 

 between mineral substances and living organic bodies, 

 but also between living organisms and those which have 

 died. The latter will go through changes indeed, but not 

 a cycle of changes they never return to the point whence 

 they set out. 



Inorganic substances tend simply to persist as they 

 are, and have no definite relations either to the past or 



* See ante, pp. 192-197. 



