810 Comparative Morphology of Chordates 



his needs and desires (technology), and to impress upon it — in written 

 or printed language, painting, or otherwise — a record of his ideas 

 (philosophy, religion, literature, and all the arts). Thus the new in- 

 dividual may arrive at adulthood in full possession of all that human 

 evolution has acquired. He then enters the creative part of his life 

 equipped to make new discoveries and inventions or to impress upon 

 the environment a record of some new idea. 



An important peculiarity of human evolution is that anything 

 created by one individual may be imitated, possessed, or used by his 

 fellow individuals. A newly evolved bodily characteristic can be trans- 

 mitted only to the offspring. But one man invents a telephone and 

 within a few years it is possessed and used by millions of his con- 

 temporaries. Also, there is an acceleration in the rate of human prog- 

 ress. The more complex the man-made environment becomes, the more 

 opportunities it provides for further modification, and, as population 

 steadily increases, there is progressive increase in the number of minds 

 working upon it and they work upon all parts of it at once. 



It has been said that the "higher" vertebrate, in the course of its 

 embryonic development, "climbs its own family tree" ("ontogeny 

 repeats phylogeny "). So far as bodily structure is concerned, they may 

 climb the same tree from its primitive chordate roots upward, genera- 

 tion after generation for thousands or millions of years, never getting 

 any higher. But in the case of human evolution, whose visible and 

 tangible products are environmental, the thing that is climbed is a 

 ladder rather than a tree. Each human generation extends the ladder 

 upward and adds a rung. Each new generation begins at the top rung, 

 where the preceding generation left off, and adds a new rung. Thus a 

 century or even a decade may witness substantial progress in human 

 evolution although no important changes have occurred in the body 

 during thousands of years. Thus human environmental evolution, 

 compared to evolution of bodily structure, proceeds at high velocity, 

 its environmental products are rapidly cumulative, and the process is 

 self-accelerating. 



Epochal Periods in Vertebrate Evolution 



In evolution of the vertebrates may be recognized certain critical 

 events, each fraught with far-reaching possibilities. The first was when 

 animals whose ancestors for millions of years had been fishes emerged 

 from water and acquired an amphibious foothold upon land. The 

 second was when reptiles replaced external water with a fluid-filled 

 amnionic cavity in which its embryo might live, and replaced larval 

 gills by the respiratory allantois, thus completing the emancipation of 



