780 EVOLUTION AND CONSERVATION Part VI 



lutionary advancement of life, such as Pre-cambrian, Paleozoic (primitive 

 life), Mesozoic (intermediate life), and Cenozoic (modern life); even the 

 Cenozoic Era extends back millions of years. The eras are divided into 

 periods or epochs named for the locality where the rocks formed in that period 

 were found or are best developed. Thus Cambrian, Ordovician, Silurian, and 

 Devonian take their names from ancient inhabitants of England or Wales. 

 Jurassic refers to the Jura Mountains in Switzerland and Cretaceous to the 

 chalk deposits in western Europe. The limits of all these eras, periods, and so 

 on are due to changes of conditions especially of climate caused by that pro- 

 found shifting of the earth's crust that gave rise to mountains, moved the lines 

 between sea and land, and caused destruction or changes in the inhabitants. 



Increase of Life 



From its beginning, life increased. It began spreading over the earth and 

 has never stopped. There were animals in the sea during the Cambrian Period 

 but none on land or in the air. Now, through great expanses of the earth every 

 handful of soil is alive with organisms, microscopic or otherwise; the tropics 

 and all summer airs resound with songs of birds and the hum of insects. The 

 history of plants and animals is the story of increase, and the invasion and 

 filling of habitable space (Fig. 38.2). New occupants opened as well as closed 

 the way to others. Wherever plants grew on land the plant-eating animals fol- 

 lowed, and where the plant eaters were the carnivores came and preyed upon 

 them. Little animals lived in the spaces between large ones. Insignificantly 

 small mammals hid among the giant dinosaurs of the Late Reptilian Age. Long 

 time residents of the water, such as the protozoans, moved into the pools where 

 the newly come large animals fed and lounged. Certain of the protozoans 

 moved into the larger animals and finally became parasites. Animals took the 

 places left by other animals through desertion or death. As mammals over- 

 spread the earth, porpoises and dolphins took possession of the seas in which 

 the great swimming reptiles (ichthyosaurs) had lived before them. Replace- 

 ments were not exact for environments changed. 



All living plants and animals have behind them unbroken streams of life 

 that come from beginnings which we may surmise but do not know. 



The Environment and the Organism 



Adaptations. An adaptive structure or characteristic of an organism is one 

 that is useful to it under the conditions in which it lives. Two mechanisms by 

 which adaptations become established are inheritance and natural selection 

 or the selective action by the environment. This question of how living things 

 come to be the way they are is far from answered although many facts are 

 known. Adaptation is characteristic of all living organisms and is one of the 

 key puzzles of nature. 



