CONSEQUENCES IMPOSED BY GANOID PISHES 435 



this should have been the sequence of the physical events of earth history 

 has depended on obscure conditions in the earth's interior, which appear 

 to have no close correspondence in the two celestial bodies whose surfaces 

 we are able to study — the moon and Mars. The progress of life on the 

 earth has been highly favored, consequently, by the rhythmic pulses of 

 climatic and diastrophic changes which have remorselessly urged forward 

 the troop of living creatures. The progress of organic evolution has de- 

 pended on a series of fortunate physical events, conditioned in the inter- 

 nal nature of sun and earth, rather than dependent on mere life activities 

 as expressed in orthogenesis through long periods of time. Evolution is 

 in no sense an inevitable consequence of life. 



Beyond the peculiarly terrestrial pulsatory forces which have stimu- 

 lated evolution we must keep in the background of the mental vision, 

 furthermore, the fact that the earth had to fulfill certain basal conditions 

 before land life was even possible. To illustrate : the ocean basins exist 

 only by virtue of the fact that certain greater segments of the crust are 

 denser than others, and that in addition a condition of vertical equi- 

 librium prevails for these large segments. Except for these two condi- 

 tions which determine the existence of ocean basins, a nearly universal 

 ocean would prevail, the lands being reduced to new-born mountain 

 ranges or to volcanic islands. 



Continents permanently too wide and high, on the other hand, would 

 be without sufficient rain, or by accelerating erosion would tend to reduce 

 the content of carbon dioxide below that minimum necessary for a vigor- 

 ous growth of plants. Or, if the land surfaces were largely of basaltic 

 rocks, as they were in the earliest known periods of earth history, the 

 content of atmospheric oxygen, so necessary for the activity of the higher 

 life, would be very greatly reduced through oxidation of the high content 

 of ferrous iron and sulphur which these rocks contain. 



These are examples of the many dangers which have beset the progress 

 of life. Happy was the outcome, in that none of these dangers which 

 loomed across the geologic ages were in reality complete barriers to fur- 

 ther progress of the stream of living things. When the difficulties were 

 encountered, there was often a variety of ways by which they could be 

 met. The choices of the ways depended frequently on adventitious and 

 minor causes. The very fact that at many times various organic re- 

 sponses were possible precludes the view that every time the initial ad- 

 vance, while serving the immediate ends, was also the best as measured 

 by the ultimate requirements. Another choice, at the very beginning of 

 our air-breathing life, so far as we can crudely estimate the results, would 

 appear to have made possible a more rapid and competitive advance. 



