384 Comparative Anatomy — Its History, Aim, and Method 



gestive organs are adapted to various kinds of food. Vertebrates may 

 breathe by gills only, by lungs only, by skin only (some adult amphibi- 

 ans), and in some fishes the tail-fin and the rectum serve as accessory 

 respiratory organs. Blood-vessels are notably variable. Even the main 

 channels, especially the veins, differ greatly in the several classes of 

 vertebrates. 



It is evident, therefore, that discovery of long-distance genetic 

 relationship depends on recognition of the homologies in organs which 

 are of the highest degree of conservatism. The homologies of organs of 

 less conservative character serve for the recognition of comparatively 

 short-range relationships. 



Final judgment as to an animal's place in a genealogic scheme re- 

 quires that the numerous homologies between the organs of that animal 

 and those of other animals be somehow summed up. Close similarity 

 revealed in one homology may be offset by striking differences in some 

 other. The Australian spiny anteater (Fig. 294) is like a mammal in 

 having milk-glands and hair, and its body bristles with stiff, sharp 

 spines; but the animal reproduces like a reptile. Is it, therefore, to be 

 classified as a reptile or as some sort of porcupine? How can the 

 similarities and differences be evaluated? In the present state of our 

 knowledge, it is impossible to do this describing, comparing, and sum- 

 ming up in any mathematically exact way. Final judgment is largely 

 a matter of personal opinion. Therefore, the smaller branches and 

 twigs of our genealogic trees are in a more or less unstable condition. 



Whether or not the Theory of Evolution is true may be debatable, 

 but that the theory exists and has played an important part in science 

 is a plain matter of fact. A science must not try to prove a theory. It 

 must test theories, but without bias as to whether they stand or fall. 

 It has been the main business of a century of Comparative Anatomy 

 to test the Theory of Evolution. At the outset, it was logically possible 

 that Comparative Anatomy might eventually prove that the idea of 

 Evolution must be abandoned. To say that nearly all present-day 

 scientists regard Evolution as fully proved is merely a statement of 

 fact. 



The following chapters are devoted mainly to presenting facts of 

 comparative anatomy. Occasionally, attention will be directed to the 

 bearing of facts on theory. In the reading of the remaining chapters, 

 the theories and concepts which have been stated and defined in the 

 present chapter should be kept in mind, in order that the reader may 

 form his own judgment as to how well the Theory of Evolution fits 

 the facts. 



