THE EVIDENCE FOR ORGANIC EVOLUTION 507 



beings there is a natural classification, the basis of which is structure. 

 When one classifies animals on this basis, one does no more than 

 recognize and apply the principles of Comparative Anatomy and 

 Embryology. Such a classification merely expresses the degree 

 of structural resemblance, as when a certain number of animals 

 are placed together in the Phylum Chordata, the Class Amphibia 

 (cf. p. 12), the Order Anura, the Family Ranidce, or the Genus 

 Rana. A complete classification of the animal kingdom would 

 be a family tree of animal life after the manner explained in the 

 chapter on Classification (cf. pp. 237-243). Indeed, classification 

 summarizes all the evidence for evolution that is derived from 

 structure. This summation need not be introduced into the 

 present discussion, since it is only an extension of the principles 

 of Comparative Anatomy and Embryology. 



Evidence from Distribution 



Geological Distribution. — Paleontology. — The study of fossils 

 constitutes the science of Paleontology. This may be regarded 

 either as a biological or as a geological subject, although it is more 

 commonly associated with Geology. What is called geological 

 distribution is a study of the distribution of fossils and is included 

 in Paleontology. It may be contrasted with the distribution of 

 animals over the earth at the present time, which is the subject 

 matter of zoogeography, or geographical distribution. These two 

 kinds of distribution cannot be separated, since geologic distribu- 

 tion represents only the geographic distribution that occurred in 

 by-gone times. In like manner the structure of fossil animals 

 cannot be separated from that of existing forms, if the animals 

 now li\dng are believed to have descended from those of the past. 

 It is convenient, however, to discuss separately these two phases 

 of distribution before explaining the correlations between them. 



Fossils. — The remains or traces of animals and plants that 

 occur in rocks of periods before the present are known as fossils. 

 Shells and other hard parts are buried in the sands and muds of 

 river flats, of lake beds, and ocean bottoms, by the deposit of sedi- 

 ments. In time they may become " petrified," or hardened into 

 stony material, as water dissolves the original substances and 

 replaces them with others. Shells usually become filled with mud 

 during burying, and later the shell itself may be dissolved by 



