14 ZOOLOGICAL EXERCISES. 
But the study of distribution extends back to those 
animals which formerly lived on the earth, and which we 
know now only by their fossil remains, and the study of 
Palseontology has shown us that each species has had a certain 
distribution in time; that is, each came into existence, attained 
its maximum, and gradually became extinct. Each larger 
group has also come into existence at a definite time: some 
have become extinct, while others still exist. There is 
therefore a geological distribution as well as a geographical 
distribution of animals. Unfortunately the geological record 
is necessarily very imperfect. Only the hard parts of 
animals can be preserved. Very large numbers of animals 
live on the land, or in deep water, where they have no 
chance of being buried. Of those that live in favourable 
situations a very small percentage are covered up before they 
are destroyed. Many of the hard parts that are covered up 
are dissolved by the percolation of water through the rock, 
or are obliterated by the action of heat. An enormous 
portion of the sedimentary rocks has been removed by 
denudation, for the denudation must have been at least 
equal to the amount of deposition. A very small portion of 
the fossiliferous rocks is available for our examination ; the 
larger portion is covered with water or other rocks. Of the 
portion that is available, only fragments here and there have 
been as yet examined. Consequently very limited trust can be 
put on negative evidence ; nevertheless an immense amount 
of positive evidence has been accumulated. . 
Geologists have divided geological time into four eras, 
called Eozoic, Palzeozoic, Mesozoic, and Cainozoic, the last 
being the youngest, or the one nearest to the present day ; 
and these eras are subdivided into different periods. These 
periods are not supposed to represent equal lengths of time. 
On the contrary, it is known that they must be very 
