favorable for the growth of certain microscopic organisms, whose shells gave rise to extensive 

 beds of chalk. 



5. CENOZOIC ERA. ''Recent life.'" Further advances in the continents and seas, in their 

 plant and animal life brought the earth still nearer to that of to-day. In the west there was 

 great volcanic and mountain making activity during the early half of the era, while in the 

 latter half the ice-sheets formed in Canada and invaded the United States. Similar events 

 were occurring in Europe and elsewhere in the world. The modern genera of trees w r ere in- 

 troduced early and some of those now growing in our southern states extended northward 

 even to northern Greenland. It was the age of higher flowering plants; the dicotyls, includ- 

 ing the deciduous trees. The two highest groups of vertebrates, the birds and mammals, 

 reached their fullest development simultaneously. Scientists believe that they were each 

 derived independentl)' from certain groups of Mesozoic reptiles, as the result of their adjust- 

 ment to new environments. They attained great size and numbers under the favorable con- 

 ditions that prevailed before the glacial period. Man slipped in quietly, as the highest type of 

 mammal, and, in Europe if not in America, saw the great ice-sheets slowly invade his native 

 forests. 



6. PSYCHOZOIC ERA. This, the "age of mind'' may well be s -parated from the preceding, 

 although no line of demarcation can be drawn between the two. Human reason has grad- 

 ually displaced brute force and man is gaining the ascendency over Nature and compelling 

 her to do his bidding. Dangerous plants and animals are being exterminated, useful ones 

 have been preserved through domestication and their usefulness still further increased through 

 artificial selection. The waste places of the earth are made to blossom as the rose, the 

 energies of Nature are being garnered and time and distance annihilated. 



C. Present Day Changes. Through the operation of the forces described in chapter I 

 constant changes in the earth, plants and animals are in progress to-day. The rapidity and 

 amount of change depends upon the rate of activity of these various forces. At great depths 

 in the ocean, the conditions of which may be readily imagined, changes occur with inconceiv- 

 able slowness, while upon the land, elevated more or less, relatively rapid changes are con- 

 stantly in progress. In both land and water the plants and animals are directly and strongly 

 affected by their immediate surroundings {environment}. Some of the more important 

 changes will be briefly outlined. 



i. LAND. Through the internal energies of the earth changes in elevation of the 

 surface are in constant progress, the crust of the earth being subject to upheaval and sub- 

 sidence. The activity of the wind, water, ice and plants leads to a constant readjustment of 

 levels, cutting here and filling there. These forces are all engaged in continually changing 

 the character of the soil. For reasons not so clearly understood slow, progressive changes in 

 climate occur. In a great variety of ways plants and animals must adjust themselves to their 



10 



