6o 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 or 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 the former edition of this book the evidences of evolution were 

 presented in a somewhat arbitrary order, the evidences that seemed to 

 furnish the most direct proof being, for pedagogical reasons, presented 

 first and the more controversial evidences last. Experience, however, 

 has shown that for an appreciation of the data from paleontology and 

 from geographic distribution the student must have a knowledge of 

 the principles of mo^ihology (comparative anatomy) and of classifica- 

 tion. We have, therefore, changed the order of presentation of the 

 evidences to one that has the authority of precedent. The order of 

 treatment will be as follows: 



I. The fundamental assumption underlying all the evidences. 



n. Comparative anatomy {homologies and vestigial structures) : the 

 evidence of the fact that structures in unlike organisms have a com- 

 mon plan and mode of origin; that changes have occurred that are in 

 some way related to changes in habit or environment. 



in. Classification: the evidence that the present groups of animals 

 and plants have arisen by "descent with modification." 



IV. Serology (blood-precipitation tests): the evidence that the 

 chemical specificity of the blood parallels taxonomic specificity. 



V. Embryology (the doctrine of recapitulation) : the evidences that 

 the embryonic development of the individual follows the main out- 

 lines of the evolutionary history of its ancestors. 



VI. Paleontology: the evidences afforded by a study of the distri- 

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

 remains of extinct animals and plants. 



VII. Geographic distribution: the evidences afforded by present 

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

 animals and plants. 



Vni. Genetics (experimental evolution): evidences that heritable 

 variations have occurred under observation in large numbers and in 

 many species of animals and plants, and that new varieties of animals 

 and plants have been produced by processes known to man and to a 

 large extent controlled by him. 



