PART IV 

 EVOLUTIONARY MORPHOLOGY 



CHAPTER XXXVI 



THE BEARING OF PHYSICAL AND CLIMATIC FACTORS ON CHORD ATES 



To understand their evolution and life it is essential to consider 

 animals in relation to their environment. During the time 

 since chordate animals first appeared, the environment has 

 changed very considerably at one time or another. Of the 

 most primitive forms there is no record preserved, for the 

 simple reason that these animals did not possess structures 

 capable of preservation by fossilisation. The earliest known 

 vertebrates are from the Silurian period, and they were fish. 

 The earth was at this time covered with shallow seas containing 

 coral-reefs which are indicative of a mild climate. In the 

 ensuing Devonian period, shallow lagoons and enclosed basins 

 were in abundance, and the land which had emerged enjoyed 

 desert conditions with little rainfall. It is towards the end of 

 this period that the first land-vertebrates (Stegocephalian 

 amphibia) appeared. The next or Carboniferous period was 

 one of tropical climates, during which luxuriant forests covered 

 the land. The trees had no rings of growth, which fact proves 

 that there were no seasons. True reptiles first appeared here. 

 In the late Carboniferous and Permian period the climate 

 became colder as the continents rose and mountain chains 

 were formed, resulting in an ice-age or glacial period. In the 

 following Trias, warm conditions returned, without seasonal 

 variation. The earliest known mammals belong to this period. 

 Warm conditions persisted throughout the Jurassic period, in 

 which the first birds are found, but this period is pre-eminently 



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