HISTORY OF LIVING ORGANISMS 79 



forms arose, and so we find that the buried record 

 shows a gradual progress from simple to more complex 

 forms. 



If this record of bygone times were complete, we 

 should be able to draw up a genealogical tree showing 

 all the stages from the earliest Protist to Man in their 

 proper places, but such a perfect record must always 

 remain a dream, and necessarily so when we take into 

 account the nature of the rocks in which the fossils are 

 formed, and the great changes they are liable to. More- 

 over the structure of many animals themselves and the 

 medium in which they lived would often be a bar to 

 their preservation. 



Imperfect however as the record is, there are numerous 

 marvellously complete series. For example, we can 

 trace the gradual evolution of the horse through a series 

 of fossil-forms that leave but few stages unpresented, 

 and so also is it with the ancestry of the elephants. 

 In such cases the mass of evidence proves without a 

 doubt that there was a continuously progressive 

 evolution. 



It is true that some forms, such as the Lamp-shells, 

 have persisted almost unchanged to the present day, 

 but in the majority, the characters of the race have 

 gradually changed, and though the old forms are no 

 longer represented, their lineal descendants are still with 

 us. One of the peculiar features of the geological record 

 is the large number of forms that have become extinct, 

 and it is difficult to explain why they have become so. 

 In some cases it may have been the struggle between 

 the various competitors, in others it may have been that 





