826 LIFE : OUTLINES OF GENERAL BIOLOGY 



PAL^ONTOLOGICAL SEQUENCE.— After this glance at these 

 general problems, we must pass to what cannot be more than a 

 palaeontological outline of the successive geological periods, indi- 

 cating briefly {a) the outstanding features of the physical 

 environment, including, of course, the climate, {b) the dominant 

 characteristics of the fauna and flora, and (c) any momentous new 

 departure that was distinctive of the time. 



We have shown (Fig. 137), in a scheme which cannot be drawn to 

 scale, the successive geological periods, as represented by the strata 

 building up the earth crust — strata which would form a thickness 

 of about sixty miles if they all occurred in completeness at any one 

 place. 



Arch.^ozoic. — Living creatures were not possible until the earth's 

 crust had cooled into firmness, and until there was abundance of 

 water in a liquid state. In other words, the story begins with a 

 "lithosphere" and a "hydrosphere", and to these must be added 

 a primitive atmosphere. In this there was little free oxygen, and 

 not very much nitrogen; the bulk of the primitive atmosphere 

 consisted of carbon dioxide and water vapour. WTien life began, 

 perhaps under the influence of sunlight radiating through the cloud- 

 curtain, carbon dioxide began to be utilised as a source for the 

 synthesis of carbon-compounds, and free oxygen passed into the 

 atmosphere, which thus became increasingly breathable. The quan- 

 tity of oxygen has steadily increased since the time of the first 

 atmosphere, but though much is used up in oxidation processes in 

 rocks and minerals, and much also in the respiratory processes of 

 animals, this loss is more than met by the liberation of oxygen from 

 green plants. And while carbon dioxide becomes temporarily fixed 

 in plants and more firmly in carbonates and the like, there is a 

 re-supply from volcanoes and hot springs and the breath of animals. 

 Nothing is known of living creatures in this preparatory era, but 

 their presence is indicated by beds of graphite and so forth. It was in 

 all probability the era of Unicellular Life. 



Proterozoic. — The first undeniable remains of organisms occur 

 in the Proterozoic Era, sometimes called the later Pre-Cambrian. As 

 the study of the very early strata becomes more and more pene- 

 trating, the list of Pre-Cambrian fossils grows. They include marine 

 Algae, some Protozoa (Radiolarians), and some not very satisfactory 

 tracks of burrowing worms. It may be thought of as the era of the 

 emergence of the simpler Invertebrates, and it was of long duration. 

 Towards its beginning and ending there were Ice Ages, and its 

 continuance was punctuated by volcanic eruptions and lava flows 

 and vast surface disturbances, such as the "Grand Canyon 

 revolution". 



Cambrian. — ^The upheavals and accumulations which marked the 

 later Proterozoic Period continued in the Lower Cambrian, which 



