624 



ORGANIC EVOLUTION 



Throughout this book constant reference 

 has been made to the gradually Increasing 

 complexity of life from Protozoa to mam- 

 mals; in fact, the underlying theme is this 

 basic concept of change from simple to 

 complex. The word evolution, in its broad 

 sense, means unrolling or unfolding and, 

 used in terms of the living world, simply 

 implies that all plants and animals alive to- 

 day have descended by slight modifications 

 from simpler preexisting forms. Our dis- 

 cussion of the various animal groups has 

 been based on this idea and, even though 

 we have accepted it as a basis for our 

 thinking, let us now return to the story of 

 the rise of animal life to examine the theory 

 of organic evolution as a logical explanation 

 of what has taken place. 



EVIDENCE FROM ANCIENT 

 ANIMAL LIFE 



Geologic time 



In tracing life from the beginning, the 

 course of events has afforded us a conven- 

 ient way of reckoning time. The earth's thin 

 crust has provided "clocks" that tell time in 

 millions of years. Geologists are becoming 

 more and more adept at reading these 

 clocks, although a reading with an error of 

 a few millions of years one way or the other 

 is still considered satisfactory. Even though 

 the exact times may not be accurate, the 

 sequence of events and the orders of magni- 

 tude of each event are correct. By reading 

 these geologic time clocks it is possible to 

 approximate dates of the origin of various 

 forms of animal and plant life. 



One of the most accurate methods of 

 reading time which has recently come into 

 prominence employs radioactivity. Radio- 

 active elements disintegrate at a remarkably 

 uniform rate, extending over millions of 

 years in some elements. Uranium, for 

 example, decays or disintegrates at an ex- 

 tremely slow rate into a special kind of lead 

 (atomic weight 206 as compared to 207.2, 

 the atomic weight of ordinary lead) and 



helium gas. It requires 2 billion years 

 for one-quarter of a sample of uranium to 

 decompose into these constituents. In deter- 

 mining time by this method, a rock contain- 

 ing a mineral impregnated with uranium 

 is analyzed for its lead content. Since the 

 mineral was incorporated into the rock 

 when it was formed, the relative propor- 

 tions of lead and uranium would be an indi- 

 cation of the age of the rock. If one-fourth 

 of the uranium had been converted to lead, 

 the rock would be 2 billion years old. It 

 is from such data as these and others that 

 the earth is thought to be at least 2 bil- 

 lion years old. 



In the study of animals that have lived in 

 past ages it is more convenient to use terms 

 which denote sequence of animal life rather 

 than numbers of years. The entire history of 

 the earth's crust is divided into eras, peri- 

 ods, and epochs, each succeeding one being 

 a subdivision of a previous one, as shown 

 in Fig. 25-1. Although the dates are only 

 approximate and may vary one to several 

 million years either way, the sequence of 

 events is rather well established. For ex- 

 ample, it is quite certain that tlie Permian 

 Period precedes the Triassic and follows 

 the Pennsylvanian. Moreover, the relative 

 lengths of the periods are rather well 

 known. Such information is helpful in un- 

 derstanding the relative success of animals 

 as well as the rates at which evolution oc- 

 curred. 



How fossils are formed and preserved 



Under very special conditions parts or 

 even whole animals have been preserved 

 in many different ways. From these remains 

 we can learn not only something about 

 their anatomical features but also how they 

 lived. It must be remembered that fossiliza- 

 tion occurs only under ideal conditions and 

 that only a very small fraction of the ani- 

 mals existing at any one time died under 

 these conditions. Most of them, of course, 

 disintegrated completely, just as they do 

 today, leaving no clue to anyone in the fu- 



