EVOLUTIONARY DEVELOPMENT AND PAST HISTORY I29 



sedimentary rocks). At least in the latter time, relatively simple 

 Algae and Sponges and apparently also Fungi and Bacteria were 

 widespread. The pre-Cambrian was followed by the Palaeozoic era, 

 of invertebrates and Fishes and large Pteridophytes. It extended 

 for about 360,000,000 years from the Cambrian period through the 

 Ordovician, Silurian, Devonian, and Carboniferous (Mississippian 

 and Pennsylvanian) periods to the Permian period, which ended 

 about 180,000,000 years ago. Next came the Mesozoic era, which 

 extended for some 130,000,000 years through the Triassic, Jurassic, 

 and Cretaceous periods and was the great era of Reptiles and Gymno- 

 sperms. Finally, extending over the last 60,000,000 or so years, has 

 been the Cainozoic (Cenozoic) era, of Angiosperms and Mammals. 

 This is commonly considered to consist of two periods, the Tertiary 

 (made up of the Paleocene, Eocene, Oligocene, Miocene, and Pliocene 

 epochs) and the Quaternary. In the higher latitudes and altitudes 

 this last period consisted of alternating glacial and interglacial times 

 and may be referred to as the Pleistocene epoch, the * recent ' being 

 the time since the last ice recession took place, although some con- 

 sider this a mere interglacial. The Quaternary period has extended 

 over perhaps the last 1,000,000 or so years^ and has seen the advent 

 and ascendancy of Man. A chart showing the eras and main periods 

 etc. is given in Fig. 37 (p. 145). 



Groups of Fossil Lower Plants 



We can only guess at the form of the first living organisms, which 

 were probably not distinct as either plants or animals, but must have 

 possessed the powers of deriving energy from outside sources and of 

 sustaining themselves. Presumably they were microscopic bits of 

 naked protoplasm far simpler than any organisms of which fossils 

 are known. From such a source sprang the ' tree of life ', near the 

 bottom of which the Bacteria appear to remain. These organisms 

 play such essential roles as agents of decomposition that it is difficult 

 to conceive of the balance of nature being maintained without them, 

 and indeed there is evidence that they were in existence in very 



^ According to F. E. Zeuner's Dating the Past : an Introduction to Geochronology, 

 third edition (Methuen, London, pp. xx + 495 and 24 additional plates, 1952). 

 Other modern estimates range from one-half to double this total, a difficulty being 

 to decide at what point in time the Quaternary began. Very recently, Professor 

 Zeuner has suggested {in lift. 1957) that ' an estimate of 600,000 years for the 

 period from the First [Pleistocene] Glaciation onwards is a reasonable one '. 



