Lk Conte. ] 



Critical Periods. 



335 



from every realm of nature, but it is w ell recognized that all phenom- 

 ena are more or less periodic or paroxysmal. Now in the case 

 here in hand, the resistance or conservative force is heredity and the 

 force tending to change or the progressive force is the pressure of 

 a changing environment, physical and organic. Under these condi- 

 tions, progressive movement or evolution must be more or less par- 

 oxysmal. By heredity the species resist change until at last the 

 want of harmony with the environment becomes so great that the 

 .species must either change or die. Some accept the one alternative 

 and some the other; the generalized forms change, the specialized 

 die. When the change begins, I imagine, it goes on rapidly until 

 equilibrium is again restored. The change being comparatively 

 rapid is completed in a few generations. The steps of change are 

 therefore represented by comparatively few individuals on that 

 account. But again the times of change would have all the pecu- 

 liarities of hard times. Not, indeed, hard times for all species as are 

 critical periods, but for some particular species. Therefore the steps 

 of change are represented not only by a few generations but also by 

 comparatively few individuals in each generation. For both of 

 these reasons, therefore, there would be a comparative poverty of 

 fossils of intermediate forms until the necessary change is completed, 

 adaptation is restored, and life becomes abundant again. 



In a word it seems to me impossible to account for the pheno- 

 mena of the evolution of organic kingdom, and especially for the 

 rarity of transitional forms, unless we recognize a law of paroxysms 

 in evolution, the greatest ones constituting critical periods and mark- 

 ing the primary divisions of time, the lesser ones the subdivisions; 

 but the law entering into even the minutest details of succession of 

 species in the most tranquil and prosperous times. 



CONCLUDING REFLECTIONS. 



In conclusion, permit me to say, I have tried to give what seems 

 to me, under the present lights, the most rational view concerning 

 the primary and secondary divisions of geological history. I can- 

 not for a moment hope that the view here presented is final or even 

 nearly so. There must continue -to- be evolution of thought on this 



