8 ESSENTIALS OF BIOLOGY 



land surrounded by water and a lake is a mass of water completely 

 surrounded by land. The forms of the island and lake are not 

 essential to their being islands or lakes and neither is the nature 

 of the rocks of the island, nor the density of the water that is 

 the lake. 



The characteristics of other natural things may depend upon 

 the processes by which they were formed and by the materials of 

 which they are composed. Thus a river delta is made up of 

 sands and muds laid down by the current when it enters the ocean. 

 Its form is determined, to some extent, by the condition that 

 the materials are borne in suspension in rapidly moving water 

 and fall to the bottom when the current loses its velocity, but it 

 is also determined by the coarseness, or fineness, of the particles 

 and by their specific gravity. Again, a volcanic cone acquires 

 its characteristic shape because of the way in which its materials 

 are ejected into the air and then fall down, forming slopes with 

 characteristic " angles of repose." 



Thus islands, continents, capes, lakes, volcanoes, mountains, 

 rivers, etc., are amorphous-heterogeneous things. They may have 

 many forms and they may be composed of many kinds of materials 

 without ceasing to be what they are. It is in their nature to 

 have certain definite relations to other natural things : thus if 

 the sea round about an island falls in level the island may cease 

 to exist although its materials and form persist. 



Natural things may be formed and heterogeneous. Thus the 

 earth itself has definite spheroidal form though its materials 

 are of many kinds, while the moon and sun have also forms that 

 are characteristic though their materials are also heterogeneous 

 and are not quite the same as the earth-materials. Water- worn 

 pebbles in the bed of a river may have definite spheroidal form 

 though they may be composed of many different kinds of rocks 

 and minerals. 



Very many kinds of inanimate things are characterized by their 

 chemical composition. Calcium carbonate, for instance, may 

 be an amorphous precipitate, or the limestone composing a fossil 

 coral, or it may be globigerina ooze, or a crystal of Iceland spar. 

 Regarded as the chemical substance, CaCOg, the form of the thing 

 that we see and handle and analyse does not matter : it is still 

 CaCOg in all its forms. We say that it is amorphous and homo- 

 geneous, for it is essential to its being calcium carbonate that. 



