6o READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



to confirm other facts, and, when reinforced by other evidences, are 

 themselves strongly substantiated. Perhaps the crowning evidence 

 of the truth of evolution is that all of these diverse bodies of phenomena 

 invariably support one another and all point in the same direction and 

 to the same conclusion, viz., that organic evolution is a fact. 



In presenting the evidences of evolution, those evidences that are 

 believed to furnish the most direct proof are discussed first and those 

 whose evidence is subsidiary and confirmatory are dealt with later. 

 The order of treatment, therefore, will be as follows: 



I. Palaeontology — the evidence afforded by a study of the dis- 

 tribution in time (vertical distribution in the earth's strata) of the 

 fossil remains of extinct animals and plants. 



II. Geographic distribution — the evidence afforded by present 

 (also, to some extent, past) horizontal distribution of contemporaneous 

 animals and plants. 



III. Classification — the evidence that the present groups of 

 animals and plants have arisen by "descent with modification," 

 which is an evolutionary conception. 



IV. Comparative anatomy {homologies and vestigial structures) — 

 the evidence derived from the fact that structures in unlike organisms 

 have a common plan and mode of origin ; that changes have occurred 

 which are in some way related to changes of habit or of environment. 



V. Serology (blood-transfusion tests) — the evidence that the 

 chemical specificity of the blood parallels taxonomic specificity. 



VI. Embryology (the doctrine of recapitulation) — the evidence that 

 the embryonic development of the individual follows the main outlines 

 of the evolutionary history of its ancestors. 



VII. Experimental evolution (genetics) — evidences that heritable 

 variations can be produced experimentally and that these are of the 

 same general character as those which occur spontaneously in Nature. 

 (This material will be presented in some detail in Part IV of this 

 book.) 



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