400 ELEMENTS OF BIOLOGY 



of the owner, whether or not the animal was a flesh eater and 

 something of the type of food. The finding of a single bone may be 

 important in completing a record, for it is possible to reconstruct 

 the entire animal from a single bone. Thus a leg bone indicates the 

 probable size of other bones, for parts of a skeleton always have a 

 proportionate relation to each other. In some forms this ratio be- 

 tween the relative sizes of bones has been worked out with mathe- 

 matical accuracy. Similarly, knowing the proportions of the skeleton 

 it is possible to establish the size and proportions of the entire ani- 

 mal. Knowing the size and anatomical characters of the animals, 

 their food supply, and the animals and plants that occur in fossil 

 form in the same layer of rock and in the same region, it is possible 

 to visualize rather clearly a community of living forms that have 

 been extinct for a period of time, the duration of which we can 

 only faintly comprehend. 



Moreover, in spite of the incompleteness of the record, its relia- 

 bility is not open to question, for the preservation of forms that have 

 been discovered has occurred at random and they constitute a fair 

 sample of the variety of types that have existed. Furthermore, the 

 presence of gaps in the record does not in any way invalidate the 

 positive evidence afforded by evolutionary series of animals which 

 are fairly complete. Since evolution has evidently occurred in some 

 groups, it becomes a demonstrated fact. Then, too, experience has 

 been that as time passes new discoveries are made which fill missing 

 portions and complete the picture of evolution within the various 

 groups. 



An extensive account of the evidence for evolution contained in 

 the fossil remains of animals cannot be given here; it will be suffi- 

 cient to illustrate with a general statement and by means of a few 

 examples of forms whose origin in time is fairly clearly understood 

 (Fig. 219). The earliest indications of animal life reveal a con- 

 siderable proportion of the great phyla already represented, the 



