206 THE WORLD OF LIFE 



that enormous lapse of time. Yet directly we pass from the 

 Cretaceous to the Tertiary rocks, not only are Mammalia 

 abundant and of fairly large size, but ancestral types of all 

 the chief orders occur, and such highly specialised forms as 

 bats, lemurs, and sea-cows (Sirenia) are found in its earliest 

 division, the Eocene. 



Either there is no record of the missing links in the Sec- 

 ondary formations, or, what is perhaps more probable, the 

 break between the Secondary and Tertiary beds was of such 

 enormous duration as to afford time for the simultaneous dvinsr 

 out of numerous groups of gigantic reptiles and the develop- 

 ment in all the large continents of much higher and more 

 varied mammals. This seems to imply that a large portion 

 of all our existing continents was dry land during this vast 

 period of time ; the result being that the skeletons of very few 

 of these unknown forms were fossilised; or if there were anv 

 they have been subsequently destroyed by denudation during 

 the depression and elevation of the land which we know to 

 have occurred. 



We will now consider these great geological periods sep- 

 arately, in order to form some conception of the changes in 

 the world of life which characterised each of them. 



The Primary or Pdloeozoic Era 



The Palaeozoic differs from the two later eras of geology 

 in having no known beginning. The earliest fossils are found 

 in the Cambrian rocks ; they consist of a few obscure aquatic 

 plants allied to our Charas and Algae, and some lowly marine 

 animals allied to sponges, crinoids, and annelids. But there 

 are also many forms of shell-bearing Mollusca, which had al- 

 ready developed into the four great classes, lamellibranchs, 

 pteropods, gasteropods, and cephalopods ; while some groups 

 of the highly organised crustaceans were abundant, being rep- 

 resented by water-fleas (ostracods) and numerous large and 

 varied trilobites. Besides these, the curious Molluscoidea 

 were fairly abundant, Terebratulae now first appear, and, as 



