CHAPTER III 

 THE RELATIONSHIP OF EXISTING ORGANISMS 



1. CLASSIFICATION 



In any primary study of the organic world such as we have 

 assumed to be the starting point of evohitionary thought, the 

 things which an individual might see about him or the largcn- aggre- 

 gate which might be collected through the efforts of many ob- 

 servers in different places must necessarily^ have been the whole 

 source of facts. The accumulated knowledge of modern science 

 has extended this field and added to it records of extinct organisms 

 whose fossilized remains are the material of palaeontology. To- 

 gether with the information worked out by geologists concerning 

 the relative ages of the rock deposits in which fossils occur, palae- 

 ontology gives us in many cases an accurate idea of the past 

 histories of existing plants and animals, and shows within certain 

 limits of accuracy from what forms they spring and through what 

 changes they have proceeded to their present state. The periods 

 covered by this natural record are often unbelievably vast. They 

 show us conclusively that all of our written records together are 

 no more than a page out of the history of the world of nature, and 

 that in the field of evolution, all of our observations of living things 

 are of the present. The few centuries during which we have been 

 making and setting down exact scientific observations are so in- 

 significant in relation to all time that they are but as a moment, 

 and all of the records so slowly and laboriously accumulated are 

 little more than a fairly complete catalogue of the things present 

 in the world at any moment. 



Species. In the examination of living things in this brief span 

 of human experience a resemblance is noted at once between cer- 

 tain individual organisms. We see birds in the trees and call them 

 robins or bluel^irds or crows. The individuals thus grouped to- 

 gether have rather definite features in common which enable us 

 to associate them easily in the smallest groups commonly used in 

 scientific classification, the species. In spite of the fact that such 



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