108 



CLASSIFICATION, PHYLOGENY, AND EVOLUTION: 



tion that all living phyla likely existed prior to Cam- 

 brian times, or more than one-half billion years ago. 



GEOLOGICAL TIME TABLE 



For convenience, geological time is divided into 

 four time levels (Figure 7.6). The largest category is 

 an era. An era contains a particular major rock group 

 that represents a time of fairly homogeneous topog- 

 raphy and climate and is distinctly separated from 

 rocks of adjacent eras. Eras are subdivided into 

 periods, the largest of the units within eras. Each 

 period is diagnosed by having a single rock system. 

 Further subdivision may provide individual epochs, 

 each having a single rock series. Within epochs there is 

 the small time unit, the age, vkfhich is determined by 

 the basic rock unit, the stage, which locally may be 

 represented by one formation. Unfortunately, forma- 

 tions often vary a great deal in age from locality to 

 locality, so usually conform to no particular time unit. 



Eras prior to the Paleozoic are no longer recognized 

 by many geologists. This entire time often is called 

 the Cryptozoic Eon or simply Precambnan and is 

 contrasted with the named eras grouped into the 

 Phanerozoic Eon. Although one probably should not 



BASIC ROCK UNITS 

 rock stages rock series rock systems 





*tS»^t*wh»^t^%*,^*m 



^♦^**VM*S*>»'** M «»** 



1 



1 



rock 

 group 



(era) 



(ages) (epochs) (periods) 



(BASIC TIME UNITS) 



Figure 7.6 The subdivisions of geological time according to rock 

 loyers. 



subdivide Precambrian time because so little is known 

 about it, its named eras are given. 



Table 7.1 is a geological calendar that barely sum- 

 marizes the greatest trends throughout geological 

 history. The left-hand portion of the table segregates 

 time levels. These levels are arranged to start with 

 the present and go back through time to the origin of 

 the earth and solar system. Therefore, one can start 

 either at the beginning or at the end of the table, trac- 

 ing history backward or forward. The middle column 

 sketches physical events of North America, and the 

 final column summarizes the world-wide history of 

 life. 



EVOLUTION 



Darwin was not the first man to think in terms of 

 one form of life giving rise to another. Thinking about 

 evolution existed in the times of the ancient Greek phi- 

 losophers, at least as far back as the fifth century B.C. 

 when Empedocles proposed a means whereby organ- 

 isms spontaneously arose from the earth. However, 

 as far as is known, it was Aristotle (384-322 B.C.) who 

 first proposed that more complex creatures arose from 

 simpler ones. This was a remarkable step toward the 

 truth, but it was not supplemented until the eight- 

 eenth century when many individuals began to criti- 

 cize the then long-prevalent concept of special crea- 

 tion of each species. In 1802, Jean Baptiste de 

 Lamarck (1744-1829) proposed a mechanism by 

 which simpler creatures evolved into more complex 

 ones. According to Lamarck's hypothesis increased 

 use of structures supposedly would cause their greater 

 development, and disuse would result in their atrophy 

 and loss. Therefore, Lamarckian evolution suggested 

 the inheritance of acquired characteristics, that is, 

 that offspring inherit traits developed by their par- 

 ents. For example, if Lamarckian evolution were true 

 it would mean that if a man exercised and acquired 

 extreme musculature, the man's offspring would have 

 such muscles without any need for exercise. Al- 

 though Lamarck's ideas were a big step toward a 

 theory of evolution, they are not generally accepted at 

 the present time. 



The final significant pre-Darwin contributions to 

 evolution were made by Sir Charles Lyell (1797- 

 1875) and Reverend Thomas R. Malthas (1766- 

 1834). Lyell, an English geologist, related fossils to 

 the geological record in his Principles of Geology, the 



