and even the relative duration of each stage measured 

 from an analysis of the number and persistence of 

 different kinds of pollen grains at various depths in 

 peat bogs. To make such studies, a core of peat is 

 obtained from the deepest part of the bog by means 

 of a special hand auger. The lowest portion of the 

 core is the oldest ; the most recently formed is at the 

 top. Samples of the core at various depths are suit- 

 ably prepared, examined under a microscope, and the 

 pollen grains identified and counted. The predomi- 

 nant kind of pollen at any level of the core represents 

 the probable prevailing species of plants in the vicin- 

 ity at the corresponding period of time, although the 

 proportionality between all kinds of pollen and abun- 

 dance of the various species may not be exact (Davis 

 and Goodlett 1960). Thus, during the last 20,000 

 years, the clisere in eastern North America is repre- 

 sented in simplified form by the following climaxes 

 and climates, reading downward to present time : 



spruce, fir cold, moist 



pine cool, dry 



hemlock, oak, beech warm, moist 



oak, hickory warm, dry 



beech, oak, hemlock cool, moist 



The climate is conjectured from the relation of 

 similar flora to climate at the present time. The com- 

 plete clisere occurs only in regions near the southern 

 limit of the reach of the glacier. The later stages have 

 not developed in more northern localities where the 

 glacier has been gone for a shorter time and where 

 the climate has not warmed up sufficiently. Climatic 

 succession actually occurs at all levels in the biosere, 

 as the serai stages leading to one climatic climax is 

 replaced by the corresponding stages leading to an- 

 other climax (Table 7-(y). 



land and floodplain forests. Stages in the erosion 

 cycle, as it occurs in a stream, may be discerned by 

 examination of habitat and animal life progressively 

 from headwaters to mouth. 



Physiographi 



succession- 



Changes in the earth's surface bring a change 

 of communities. The sea alternately inundated and 

 retreated from the Atlantic coastal plain during the 

 Pleistocene as ice, in which large amounts of water 

 were tied up, alternately melted and formed. In 

 earlier times, the sea inundated much of the conti- 

 nental interior, and on its recession the eosere of 

 plant and animal communities which developed cov- 

 ered vast areas. 



Mountain-building brings the replacement of low- 

 land communities with new ones that invade at higher 

 elevations. As mountains erode, the eosere progresses 

 in the opposite direction, until base-level or pene- 

 planation is attained. The development that the 

 eosere will undergo with continued erosion is some- 

 times locally apparent in the difference between up- 



Geologic 



succession 



The evolution of new forms of life and dis- 

 persal of them through the world entails replacement 

 of pre-existing forms and gradual change in the com- 

 position and character of communities. The first or- 

 ganisms to appear on earth were unicellular forms 

 confined to the sea (Table 3-1). During the Cam- 

 brian and Ordovician periods, the marine animal life 

 differentiated rapidly into a rich variety of inverte- 

 brate types and the anlage of vertebrates. The Si- 

 lurian and Devonian periods are noteworthy for the 

 invasion of fresh water and land by both animals and 

 plants. Fishes became predominant both in fresh 

 water and the sea. During the remainder of the 

 Paleozoic era, a luxuriant flora evolved, especially in 

 swampy areas. Modern conifers, such as spruce, fir, 

 juniper, tamarack, cypress, and yew made their ap- 

 pearance. Amphibians became the predominant ad- 

 vanced animal types, and a diversified invertebrate 

 fauna occurred in all habitats. 



In the Mesozoic era, the existing land flora of 

 giant rushes, tree-ferns, and cycads gave way to for- 

 ests of hardwoods which then spread over the world. 

 Conifers persisted. The earliest woody angiosperms 

 probably originated in the Jurassic and included sas- 

 safras and poplar. The forests soon contained elms, 

 oaks, maples, and magnolias. Herbaceous angio- 

 sperms, such as grasses and sedges, appeared towards 

 the end of the era but did not become important in 

 North America until the drying up of the interior of 

 tiie continent in the middle Cenozoic. Although the 

 Mesozoic is predominantly the age of the giant rep- 

 tiles that lived on the land and in the water and flew 

 through the air, less conspicuous types such as the 

 toothed birds, archaic mammals, and insects were de- 

 veloping rapidly. 



At the beginning of the Cenozoic era, ancient 

 types of animals, including the great reptiles and 

 many types of invertebrates, became extinct and mam- 

 als rose to predominance. It is significant that the 

 rich development of mammals, birds, and insects came 

 after the worldwide establishment of the angiosperms 

 with their rich nutrient seeds, fruits, and grasses. 

 Pleistocene glaciation brought a major change in the 

 habitat of these animals, and some large mammals 

 disappeared. The last stage of the geosere, the Re- 

 cent epoch, brought the dominance of man. Only 

 future ages will determine whetlier this stage is cli- 

 max, or whether new and different types of animal 

 and plant life will someday evolve to replace man and 



24 Background 



