60 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.) 
