EVOLUTION, VARIATION AND HEREDITY 451 



thoroughly dug over and examined. Of the millions of animals 

 that die yearly, but few are so situated at the time of their death 

 that they can be preserved. Preservation usually takes place in a 

 river or sea deposit or in formations like a bog. It is only the hard 

 parts that will be preserved, save in exceptional cases, and so as a 

 rule only those animals with hard parts are capable of leaving a 

 record behind them. The geological record is for these and many 

 other reasons very incomplete, but in spite of this the evidence it 

 affords is overwhelming. We shall now examine a case in some little 

 detail in order to see the nature of the evidence. The classical 

 example is that of the horse, but before considering the actual details 

 of this, we must note briefly how the records were made. 



Geology, the study of the earth's crust, tells us that in remote 

 times the land surface of the globe was composed of granites and 

 basalts and similar hard rocks. The action of the rain, the wind, 

 the gases of the atmosphere, etc., led to the slow breaking up of these 

 rocks, and the loose matter was carried away by the rivers to be 

 accumulated in layers or deposits. As the conditions under which 

 these deposits were made varied, so the character of the layers 

 themselves altered. We find these sedimentary rocks, as they are 

 termed, superimposed one upon another in a succession of layers or 

 strata. It naturally follows from their mode of formation that the 

 lower the stratum is in a series the older it is, and also that the thick- 

 ness and number of layers will give us some idea at any rate of the 

 time taken in their formation. A number of such strata have been 

 recognised by Geologists, and they have been divided into three 

 main divisions or eras : the Palceozoic, the Mesozoic, and the Canozoic, 

 each representing an enormously long period of time, and each com- 

 posed of a number of smaller periods. It is only the last of these, 

 the Caenozoic or Tertiary period, that immediately concerns us, for 

 no distinct ancestral horse form has been found prior to it. This 

 division itself is divided into Eocene, Oligocene, Miocene, Pliocene, 

 Pleistocene, and Recent periods, each of which is in turn composed 

 of several distinct layers. It will be seen then that under certain 

 conditions it is possible for an animal to be swept away and deposited 

 by the sediment and so its bones or hard parts preserved. 



The surface of the earth is constantly undergoing slow movements, 

 whereby certain portions are subsiding and others being elevated. 

 Sometimes this transformation, always a gradual one, takes place 

 relatively faster than at others. In this way it comes about that the 

 strata laid down under water get raised up to form part of the land, 

 and they bring with them animal remains or fossils. Sometimes they 

 are raised so high that they form mountain ranges, to become subject 

 to the same weathering forces and so help to form further sediments. 



