J. LeConte— Critical Periods in the History of the Earth, 107 
social change are gathering strength, but make no visible sign, 
being resisted by social conservatism—rigidity of the social 
crust—and periods in which resistance gives way and rapid 
changes occur ; so also in the evolution of the organic kingdom 
the forces of change, i. e. pressure of changing environment, 
_ Some persons seem to think that saad evolution is 
inconsistent with the uniformity of Nature’s laws. n th 
without any missing leaf, species come and go an others take 
their place, and yet only rarely do we find any transition-steps. 
If this were only once or twice or thrice, or to any extent 
genera, families and higher prone at certain horizons, are 
n. these cases the i 
sail ence of the truth of evolution. But the difficulty on the 
ssumption of a uniform rate of evolution is none the less here, 
