Le Conte] 



Critical Periods. 



33' 



would still farther increase the change of organic forms by compel- 

 ling a new adjustment of the whole organic kingdom. This is 

 especially true of the effect of the appearance of man. The effect 

 of all this would be to hasten the steps of evolution and increase 

 organic diversity, but to diminish geographical diversity. These 

 critical periods form the lines which delimit the primary divisions 

 of history. 



Re-isolation. — Lastly there come re-isolations of the mixed faunas 

 and floras in their new positions; and geographical diversity again 

 commences and continues to increase without limit in proportion 

 to the time and degree of isolation. 



Thus in the history of the earth there have been alternate 

 periods of minglings and re-isolations of faunas and floras; of efface- 

 ment, and reestablishment of geographical diversity. Every period 

 of mingling has increased the force and quickened the rate of evo- 

 lution and the diversity of organic forms. 



As already shown, the glacial period is the last of these critical 

 periods of migrations, minglings, struggles, and the present a period 

 of re-isolations and increasing geographical diversity (except in so 

 far as interfered with by man) and of readjustment of the whole 

 organic kingdom to the dominant type, man. Therefore the glacial 

 period furnishes the key to the present geographical distribution of 

 species, especially among the higher forms, and conversely the pres- 

 ent geographical distribution of species furnishes a key to the 

 changes in physical geography and the directions of migrations of 

 species during that critical period. 



SUDDENNESS OF CHANGES AND RARITY OF TRANSITIONAL FORMS. 



There are many important questions suggested by the foregoing 

 discussion. Chief among these and the only one I now touch, is 

 the question of the cause of the suddenness of the appearance of new 

 organic forms, and the extreme rarity of transitional forms or con- 

 necting links at all times, but especially during the critical periods. 



As already said, the greatest and most rapid of all these changes 

 is that which occurred in the critical period between the Palaeozoic 

 and Mesozoic. We may therefore take this as the type in this 

 regard. It is almost impossible to exaggerate the greatness of the 



