548 Critical Periods in the History of the Earth. [September, 
form is too slow to keep pace with change of external conditions, _ 
especially in critical periods. If this be so, then an organism 
may be regarded as under the influence of two opposing forces: 
the one conservative, the other progressive; the one tending to 
equilibrium, the other to motion; the one to permanence, the 
other to change of form ; the one static, the other dynamic; the 
one internal, the law of heredity, the other external, the pressure 
of a changing environment. Under the influence of two such 
forces, the one urging, the other resisting, it is evident that even 
with steady changes of external conditions the change of organic 
forms would be more or less paroxysmal. Other kinds of evolu- 
tion, physical and social, evidently advance paroxysmally from 
this cause. As, therefore, in the gradual evolution of earth-feat- 
_ ures there are periods of comparative quiet, during which the 
forces of change are gathering strength but produce little visible 
effect, being resisted by crust-rigidity, and periods when the ac- 
cumulating forces finally overcome resistance and determine com- 
paratively rapid changes; as in social evolution there are peri- 
ods in which forces of 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, that is, pressure of 
changing environment, may accumulate but make little impres- 
sion, being resisted by the law of heredity — of like producing 
like —or type-rigidity, until, finally, the resistance giving way, 
the organic form breaks into fantastic sports which are at once 
seized by natural selection, and rapid change is the result. 
Some persons seem to think that paroxysmal evolution 1s M- 
consistent with the uniformity of nature’s laws. On the con- 
trary, it is in perfect accord.. Laws and forces are indeed unl- 
form, but phenomena are nearly always paroxysmal. The forces 
of volcanoes and earthquakes, of lightning and tempests, are uni- 
form, but the phenomena are paroxysmal. Winds at the earth's 
surface, where the resistance is great, blow in puffs. Atip 
sheet of water over a smooth sloping surface runs in waves. The 
‘law may be illustrated in a thousand ways. In all cases where 
an accumulating force is opposed by a constant resistance, We 
have phenomena in paroxysms. , ‘ t 
But whatever be the cause, the fact of paroxysmal movemen 
of organic evolution is undoubted. All along the course of geo- 
logical history, from beginning to end, even when the times were | 
