246 GENERAL BIOLOGY 



by the first section of this chapter, and the figure on page 237 

 is a map of evolutionary progress. This, however, is 

 hypothetical. It is a conception of what may have hap- 

 pened since life appeared on the earth. How shall we know 

 what actually has happened ? 



Palaeontology offers the actual record of the past history 

 of organisms. This record consists in their fossil remains, 

 buried and preserved in the crust of the earth. It is a very 

 fragmentary and incomplete record; for only the hard 

 parts of organisms are capable of preservation in the rocks. 

 Therefore, the more soft-bodied and primitive forms dis- 

 appear, and leave no trace. The parts preserved as fossils 

 are fragments, merely, of organisms; shells of molluscs; 

 teeth and bones and armor plates of vertebrates; wings 

 and legs of insects, leaf and stem prints of the larger plants. 

 The uncovering of such specimens demands the greatest 

 care, and the study of them demands the greatest knowl- 

 edge of corresponding parts in living forms. Yet, notwith- 

 standing the necessary defects of the material, the best 

 specimens, and more especially, the best series of specimens, 

 are of the highest scientific value. The degree of perfection 

 seen in the preservation of structures even so delicate as the 

 venation of an insect's wing, is truly remarkable when one 

 considers the long processes of fossilization through embed- 

 ding in sedimentary rocks. Figure 1 50 , for example, is from 

 a photograph of a fossil dragonfly. It is a mere impression 

 upon the surface of a slab of lithographic stone from a 

 Bavarian quarry, but how completely are most details of 

 the venation preserved. Even a novice would have no 

 difficulty in determining with which of the two living forms 

 whose wings are figured beside it (fig. 151) the fossil form is 

 allied. 



Although paleontology has only the hard parts to deal 

 with, every trace- of nerve and muscle and every other vital 



