THE SILURIAN BEACH. 
lived ; for there is nothing in nature without a 
purpose, and when so complicated an organ was 
made to receive the light, there must have been 
light to enter it. The immense vegetable deposits 
in the Carboniferous period announce the intro- 
duction of an extensive terrestrial vegetation ; 
and the impressions loft by the wood and leaves 
of the trees show that these first forests must have 
grown in a damp soil and a moist atmosphere. 
In short, all the remains of animals and plants 
hidden in the rocks have something to tell of the 
climatic conditions and the general circumstances 
under which they lived, and the study of fossils is 
to the naturalist a thermometer by which he reads 
the variations of temperature in past times, a 
plummet by which he sounds the depths of the 
ancient oceans, — a register, in fact, of all the im- 
portant physical changes the earth has undergone. 
But although the animals of the early geologi- 
cal deposits indicate shallow seas by their simi- 
larity to our shoal-water animals, it must not be 
supposed that they are by any means the same. 
On the contrary, the old shells, Crustacea, corals, 
etc., represent types which have existed in all 
times with the same essential structural elements, 
but under different specific forms in the several 
geological periods. And here it may not be amiss 
to say something of what are called by naturalists 
representative types. 
