CHAPTER 35 



The Evidence for Evolution 



The evidence that organic evolution has occurred is so overwhelming 

 that no one who is acquainted with it has any doubt that new species are 

 derived from previously existing ones by descent with modification. 

 The fossil record provides direct evidence of organic evolution and gives 

 the details of the evolutionary relationships of many lines of descent. 

 In addition, there are vast quantities of facts from all of the subdivisions 

 of biological science which acquire significance, and make sense, only 

 when viewed against the background of evolution. 



307. The Fossil Evidence 



The evidence of life in former times is now both abundant and 

 diverse. The science of paleontology, which deals with the finding, cata- 

 loguing and interpretation of fossils, has aided immensely in our under- 

 standing of the lines of descent of many vertebrate and invertebrate 

 stocks. The term "fossil" (Latin jossiliiim, something dug up) refers not 

 only to the bones, shells, teeth and other hard parts of an animal's body 

 which may survive, but to any impression or trace left by previous or- 

 ganisms. Footprints or trails made in soft mud, which subsequently 

 hardened, are a common type of fossil. For example, the tracks of an 

 amphibian from the Pennsylvanian period, discovered in 1948 near Pitts- 

 burgh, revealed that the animal moved by hopping rather than by walk- 

 ing, for the footprints lay opposite each other in pairs. 



The commonest vertebrate fossils are skeletal parts. From the 

 shape of bones, and the position of the bone scars which indicate points 

 of muscle attachment, paleontologists can make inferences about an 

 animal's posture and style of walking, the position and size of its mus- 

 cles, and hence the contours of its body. Careful study of fossil remains 

 has enabled paleontologists to make reconstructions of what the animal 

 must have looked like in life (Fig. 35.1 and Fig. 24.7). 



In some fossils, the original hard parts, or more rarely the soft 

 tissues of the body, have been replaced by minerals, a process called 

 petrifaction. Iron pyrites, silica and calcium carbonate are some of the 

 common petrifying minerals. The petrified muscle of a shark more than 

 300,000,000 years old was so well preserved by petrifaction that not only 

 individual muscle fibers, but even their cross striations, could be ob- 

 served in thin sections under the microscope. A famous example of the 

 process of petrifaction is the Petrified Forest in Arizona. 



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