A DEFINITION OF EVOLUTION 



useful, while in other instances they are not. Among vertebrates, the skele- 

 tons are the most commonly preserved parts, and they are also of immense 

 value taxonomically. Skeletal remains may give quite diversified informa- 

 tion about a vertebrate. Obviously, a complete skeleton will indicate the 

 size of the animal; but, in the hands of a competent anatomist, even a 

 single bone, or a mere fragment of a bone, may offer a basis for a reason- 

 able estimate of size. From the scars of muscle attachments on the bones, 

 the sizes and contours of the muscles can be determined. From this it is 

 an easy step to the general appearance of the animal, and its characteristic 

 gait and speed. The skulls give an indication of relative intelligence. The 

 teeth indicate the type of diet eaten. Thus vertebrate skeletons are among 

 the most satisfactory of all fossils. But preservation of hard parts does not 

 always lead to so fortunate a result. Among plants, woody parts are most 

 commonly preserved. But these are of quite secondary importance taxo- 

 nomically. The flowers, which are of great importance for plant taxonomy, 

 are rarely preserved. And for many groups, fossils are very rare because 

 there are no hard parts at all. 



Not all periods in the earth's history have been equally favorable for 

 the formation and preservation of sedimentary rocks. It is well established 

 that the general level of the continents has fluctuated from time to time, 

 with cold ages during which the continental shelves were largely exposed 

 alternating with warm ages, during which the continental shelves and 

 even much of the inland low areas were submerged to form shallow seas. 

 During times of progressive submergence, a newly deposited stratum, 

 together with its included fossils, would be protected by the deposition 

 of additional strata above it. In this way, very thick layers can be formed. 

 But, during periods of elevation, newly formed strata may be quickly 

 raised above the water level, where they become subject to erosion and 

 destruction of fossils by wave action, wind, and rain. The result is that 

 remains from periods of submergence are much richer in fossils than are 

 those from periods of elevation. This is in contrast to the probable relative 

 abundance of species. For elevation of the continents exposes increasing 

 areas and new habitats for colonization. Numbers of species are therefore 

 probably increasing at such times. But during periods of submergence, 

 the habitable world undergoes a shrinkatie, with the attendant extinction 

 of many species. 



Up to the present, this discussion has assumed that the strata, once 

 formed, remain undisturbed imtil struck by a geologists pick. But this is 

 by no means true. The rocks may be profoimdly altered in many ways, 

 together with their contained fossils. Mountains may be thrust up; rivers 

 may cut deep gorges through many successive strata; the rocks may be 

 cracked by tensions and one portion slipped over another; glaciation may 

 carry away superficial rocks and lav bare the deeper ones; erosion by 

 wind and water may wear away the rocks; and inan\' otlier normal geo- 

 logical processes may alter or destroy fossil-bearing strata. All of this is 

 useful in that such processes tend to expose for study deep strata which 

 would be unavailable if left exactly as originally deposited; but all of these 

 processes also result in extensive destruction of fossils. 



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