1 8 ESSENTIALS OF BIOLOGY 



animals. Generally these tissues undergo complete disintegration 

 so that their substances are decomposed, either purely chemically, 

 or by the activities of bacteria, into the stable substances, carbonic 

 acid, water, simple nitrogenous compounds, sulphates, and 

 phosphates, etc. But occasionally the organic tissues may be 

 partially preserved, after death, with the results that artifacts 

 such as we have mentioned may be formed. 



Fossils are artifacts of a different kind. In them the chemical 

 elements of the original organic body do not (normally) exist. 

 Thus coal contains some of the carbon that was present in the 

 cellulose of the plants that gave rise to it, but tree-trunks that 

 may be found in coal-beds, or elsewhere, as fossils may not 

 contain carbon. In typical fossils the tissues become mineralized, 

 that is, the carbonaceous materials, or the calcareous ones, become 

 replaced by other materials (usually lime or silica). Yet the 

 replacement occurs in such a way that the formation of the tissues 

 may be preserved. The structure of the latter may be the 

 same in the fossil as it was in the living organism, though the 

 actual materials that manifest this structure may be entirely 

 different. 



What we generally call an artifact is an inanimate thing, or 

 a number of such, that have been modified in some ways so as to 

 subserve a purpose. Thus many marine worms pick up sand 

 grains and cement these together to form tubes in which they 

 live ; a hermit crab chooses an empty gastropod shell and backs 

 into this so that it inhabits it ; a fox may find a natural hole in 

 the earth and there enlarge it to form a home, or a bird may 

 collect and arrange twigs of wood to make a nest. All these 

 things are artifacts in the ways in which they are used. The 

 stone that is used by a thrush in order to crack snail-shells upon 

 is also (though in a rather different way) an artifact. 



Typical artifacts are houses, clothing, tools, etc., made and 

 utilized by man. In these cases the choice of the natural things 

 is conscious and deliberate (though in lower animals the choice 

 may be instinctive and perhaps not even conscious). The natural 

 things are modified and shaped and this too may be instinctive 

 in the lower animals but deliberated, or even designed^ in the case 

 of man. Thus in artifacts, the notion of purpose may be implicit : 

 the thing, whatever it may be, has had, or has usefulness of some 

 kind. It is employed to do something and it may even be 



