56 EVOLUTION, GENETICS, AND EUGENICS 



forever a fixed set of structural patterns without variation or improve- 

 ment. Since, then, both special creationist and evolutionist find it 

 equally necessary to assume the principle of heredity, there should be 

 little argument on this score. But let the reader beware at this point 

 in the discussion, for if he admits the postulates already presented — 

 and how can he help but admit them? — he cannot avoid the inevit- 

 able conclusion that the theory of descent with modification is the only 

 reasonable explanation of organic resemblances and differences. 



HOMOLOGY VERSUS ANALOGY 



Much difficulty in connection with the study of resemblances and 

 differences in animals and plants is occasioned by a failure to under- 

 stand the fact that there are two kinds of resemblances and differences. 

 Structures that are similar in anatomical detail and in their mode of 

 embryonic origin, irrespective of whether they perform the same or 

 different functions, are known as homologous. The test of homological 

 equivalence is a study of the anatomical details of the adult structure 

 followed by a study of the developmental history of the part in ques- 

 tion. If the part under examination be a bone, for example, this bone 

 must have a certain relation to the other bones, must occur in a certain 

 part of the body, must be supplied with certain muscle attachments, in 

 order to be considered homologous with another bone that has the 

 same relations. If two structures have the same anatomical relations 

 and arise from equivalent embryonic rudiments they are said to be 

 homologous, in spite of small or great differences in relative size, ap- 

 pearance, or function. If structures are homologous it is believed that 

 they represent the same hereditary units and that these equivalent 

 hereditary units have been derived from the same or similar ancestors. 



Analogous structures are of an entirely different sort. They may 

 be more or less superficially alike in form or in function, usually in 

 both, though anatomically quite different. As an example of analo- 

 gous structures let us examine the three types of aquatic vertebrates 

 shown in Figure 80. These three kinds of vertebrates, one a fish, one 

 a reptile, and the third a mammal, might be mistaken by the casual 

 observer to be all fishes of different kinds. All have the same fusiform 

 body with lines best adapted for swift locomotion in the water; all 

 have median, paired, and caudal fins; all swim in about the same way. 

 Yet the resemblance is only skin-deep, as it were, for beneath the sur- 

 face the one is all fish, the second all reptile, and the third all mammal. 

 The structures that look alike and function alike are, from the stand- 



