THE EPIC OF EVOLUTION 497 



remains largely meaningless and puzzling. The problems that human 

 anatomy presents are illuminated and largely solved by recourse to 

 comparative anatomy. 



The Key to Comparative Anatomy Is Organic Evolution 



A mere description of the structure of different kinds of animals and 

 plants would in itself be monotonous and colorless, an uninterpreted 

 mass of isolated facts, were it not that a thread of relationship, con- 

 necting these forms of life with each other, gives significance to the 

 whole. Identity of plan, based upon derivation from a common stock, 

 with adaptive variation in the working out of that plan to meet changing 

 environments or new functions, is the creed of the comparative anato- 

 mist, by which he makes sense out of what he observes. 



Goethe (1749-1832), in whom superlative excellence as a poet 

 overshadowed his real greatness as a pioneer biologist, pointed out 

 that the sepals, petals, stamens, and pistils of flowers are to be inter- 

 preted as modified leaves, crowded together on a short stem, to meet 

 the requirements of a different function. Again, an examination of 

 the hearts of different vertebrates, for example, shows an evolving 

 series of structures as illustrated on page 306. The fish heart is a 

 single pump, with a thin-walled atrium receiving the returning blood, 

 and a muscular ventricle for pumping it over the body. In the 

 amphibian the single pump begins to become double by the introduc- 

 tion of two atria, thus making a sort of heart-and-a-half arrangement, 

 while in most reptiles, by the formation of a partial ventricular sep- 

 tum, the organ is advanced to become a heart-and-three-quarters. 

 Finally, in the crocodiles, birds, and mammals, the heart becomes a 

 double pump with two auricles and two ventricles. This continuous 

 series of modifications the comparative anatomist interprets as due 

 to progressive evolution based upon relationship. 



Inspired by Darwin's "Origin of Species," the eminent German 

 anatomist, Robert Wiedersheim, has written a remarkable book^ in 

 which he describes in charming and scholarly manner a long array of 

 human anatomical structures that in every instance are matched by 

 corresponding details exhibited by other vertebrates. He concludes 

 that there is nothing unique or original in the "structure of man." 

 Even such differences as appear in the distinctive human brain as 

 compared with the brain of his "poor relations" in the animal king- 

 dom, are quantitative rather than qualitative. 



' Der Bau des Menschen. 



