CRITICAL PERIODS IN THE BISTORT OF THE EARTH 523 



It is jjossible that general movements affecting alike all classes may be 

 accounted for in this way alone. But there arc maoy facts in the evolution 

 of the organic kingdom, especially the sudden appearance of new forms in 

 the quietest times, which can hardly be thus explained. There seem to be 

 internal as well as external factors of evolution. Again, the internal factors 

 may be either in the form of tendencies to change or of resistance to change. 

 Of these, however, the latter seems to be most certain. There may be in 

 the organic kingdom an " inherent tendency" to change in special directions, 

 similar to that which directs the course of embryonic evolution, — a tend- 

 ency, in the case of the organic kingdom, inherited from physical nature 

 from which it sprang, as in the case of the embryo it is inherited from the 

 organic kingdom through the line of ancestry. This cause, however, is too 

 obscure, and 1 therefore pass it by. 



But whether or not there be any such inherent tendency to change, there 

 certainly is an inherent tendency to stability, — to persistence of organic 

 form, .If there be no inherent force of progress, there certainly is an in,- 

 herent force of conservation greater in some species than in others. It seems 

 probable that in many of the more rigid types this stability is so great, and 

 therefore variation of offspring so slight, that progressive change of 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 pro- 

 gressive ; 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 urg- 

 ing, the other resisting, it is evident that even with steady changes of ex- 

 ternal conditions the chang-e of organic forms would be more or less par- 

 oxysmal. Other kinds of evolution, physical and social, evidently advance 

 paroxysmally from- this cause. As. therefore, in the gradual evolution of 

 earth-features there are periods of comparative quiet, during which the 

 forces of change are gathering strength but produce little visible effect, be- 

 ing resisted by crust-rigidity, and periods when the accumulating forces 

 finally overcome resistance and determine comparatively rapid changes ; 

 as in social evolution there are periods in which forces of social change are 

 gathering strength but make no visible sign, being resisted by social con- 

 servatism, — 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 impression, 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 is inconsistent 

 with the uniformity of nature's laws. On the contrary, it is in perfect accord. 



