900 J. BAHRELL MEASUREJMENTS OF GEOLOGIC TIME 



marvelously like the parent; the wonder is not that there is so much 

 variation, but that there is so little. Yet this development of the meta- 

 zoan is only a higher expression of powers already evolved in the proto- 

 zoan. The evolution of the habits of conjugation, of the exchange and 

 combination of Mendelian factors, of the habits of precisely similar 

 growth generation after generation, are all parts of a protoplasmic ma- 

 chinery which had to become established before even protozoan progress 

 was possible. This machinery could not rapidly evolve. Like all later 

 advances, it must have been the result of numberless variations, the effi- 

 cient sifted out by the elimination of the unfit. It would seem that the 

 vast length of the early Precambrian ages was none too long for the ac- 

 complishment of this half of evolution of which we know so little. 



Turning to a better known side of the subject, the evolution of the 

 vertebrates; the geologic record shows that the great advances coincide 

 with changes in environment. The advances represent modifications of 

 older structures and their combination to new ends and with increased 

 efficiency. The variations must come from within by modifications of the 

 germ plasm, and these, once established, are carried forward and per- 

 petuated through the stability of inheritance. But the efficient varia- 

 tions, representing improved adaptations to a changing environment, 

 must be sifted out from inefficient and degenerative variations by natural 

 selection leading to the survival of the fittest. The same principles doubt- 

 less led to the long upward journey in the organization of the protozoa. 



If evolution were due only to internal changes, to the mere sloughing 

 off of inhibiting Mendelian factors, it might proceed with rapid pace ; but 

 if, as the geologic record testifies, it waits on environmental change and 

 requires a transformation of unrelated organs with mutual support and 

 efficiency, as seen in the organization of lungs, circulation, and limbs — 

 needed to transform the fish into the amphibian — then evolution must 

 proceed with surpassing slowness. Millions of generations must cross 

 the stage of life to furnish again and again the chance combinations 

 which are seized on by nature at times when the environment exerts a 

 critical and selective pressure. These forces which make for evolution 

 work intermittently, not continuously. Degeneration rather than evolu- 

 tion is the result where nature ceases to drive and organisms are left to 

 procreate freely. 



Viewed in this light, it seems impossible to compress the evolution and 

 deployment of Cenozoic mammals, gained through the march of many 

 successive faunas, into any snch limited time as 3,000,000 yesus. Ten 

 times this seems none too long. In fact, fifty or sixty million years, giv- 



