44 EVOLUTION 



Primary (or Paleozoic) stretch of the earth's history. 

 As far as the story of living things is concerned, it was 

 mainly a period of preparation of the land for the higher 

 phases of animal life, and therefore the broad changes 

 we have noticed must be clearly borne in mind. The ex- 

 tension and solidification of the continents, the purifica- 

 tion of the atmosphere, and the lowering of the earth's 

 temperature and initiation of seasons and zones of 

 climate, were the chief changes that took place, from the 

 general evolutionary point of view. But these changes 

 occupied an enormous period of time. If we grant 

 50 million years for the entire formation of the stratified 

 crust of the earth, we must assign more than 30 millions 

 of this to the Primary epoch. How long it may actually 

 have lasted it is difficult to say. Professor Sollas, in 

 one of the most recent and careful estimates (in his Age 

 of the Earth) concludes that the formation of the crust 

 occupied probably between 50 and 60 million years. 

 Lord Avebury in a recent work (The Scenery of England) 

 suggests 100 million years. Walcott thinks 27 millions 

 enough. We may say, in a word, that the more weighty 

 estimates of geological time (for the stratified rocks) 

 range from 20 to 100 million years, and leave the point 

 which is of little importance for some future genera- 

 tion to determine. 



It is not necessary for our purpose to study the 

 succeeding geological periods in detail, but one or two 

 broad changes must be noted. In the secondary epoch 

 (which embraces the Triassic, Jurassic, and Cretaceous 

 strata) the climate slowly rises once more and the ice- 

 sheets melt away. In Europe especially, the crust sinks 

 lower and lower, until Europe becomes little more than 

 a widely scattered group of islands (the peaks of its 

 mountains) peeping out of a semi-tropical sea. In the 

 third part of the period the ocean that overlies the whole 



