Progress 



A DRAMA OF EVOLUTION IX FIVE ACT.S 

 By T. L). A. COCKE RE LL 



Professor of Zoology, University of Colorado 



Argument. — E\olutionai-y progress 

 has not flowed in a single continuous 

 stream from amceba to man; it has 

 branched and l^ranched again, so that 

 the ramifications are more numerous 

 than the mind can follow. The most 

 significant new branches have not arisen 

 from the ends of the old ones, but as 

 entirely new departures from the main 

 trunk of the tree. Thus each great in- 

 novation, full of meaning for the future, 

 has at first appeared to contradict the 

 teachings of the past. The new types 

 have usually been feeble and insignifi- 

 cant, never robust and dominant; and 

 if we permit ourselves to imagine an 

 attitude of the other creatures toward 

 them, it must be one of contempt. In 

 -the first act, the forerunners of the 

 vertebrates are represented b\' the 

 modern Prochordates, to enable us to 

 visualize the types, although the actual 

 actors in the drama are of course extinct 

 and unknown. For similar reasons, the 

 invertebrates are represented by living 

 species. The adoption of a new posi- 

 tion, whereby the main ner\'e cord is 

 dorsal, contradicts all invertebrate usage 

 from the earliest times; the notochord 

 is an entirely new development. In the 

 course of development, the tunicate 

 loses all the characters suggesting an 

 approach to the vertebrate types and 

 becomes a degenerate, sedentary sac. 

 The Balanoglossus resembles a worm; 

 l)ut the Aviphioxus retains its fishlike 

 form, its well-developed nerve cord and 

 notochord. 



The vertebrate type having duly 



de\eloped in the water, the second act 

 records the disco^'er^' of the land by 

 some primitive amphibian, here per- 

 sonified by the frog. The frog cele- 

 brates his passover every spring; no 

 wonder he sings aloud in the marshes! 

 The ability to live on land opened up a 

 great new field for growth and de\-elop- 

 ment, with the accompanying modifica- 

 tion of the paired fins into digitate 

 limbs, the fundamental change of struc- 

 ture making possible all future progress. 



The vertebrate type on land developed 

 into mighty but cold-blooded beasts, such 

 as the giant Diplodocus, named after 

 Mr. Carnegie, to be seen in the Carnegie 

 Museum, Pittsburg, and in the American 

 Museum. These vast dinosaurs were 

 contemporaneous with early forms of 

 mammals, small but warm-blooded. In 

 time the great reptiles perished, and the 

 mammals came to their own. 



After a long course of mammalian 

 evolution, a creature appeared, erect 

 upon its hinder legs, with hands free 

 to use tools. Much earlier, the birds 

 had ceased to walk upon the anterior 

 limbs, but had missed the possibility of 

 human-like change through developing 

 wdngs. Now comes man, relatively 

 feeble, ugly from the standpoint of the 

 other animals (even we regard with 

 disgust a hairless Mexican dog), appar- 

 ently a sort of developmental joke, but 

 destined to l)ecome the topmost branch 

 of the evolutionary tree. Conscious of 

 his own weakness, he nevertheless puts 

 on a bold front. 



In these modern days, teachers, pro- 



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