ON BIOLOGY. 35 



When such comparisons receive our further reflections 

 it becomes apparent that eras, ages, periods, etc., in all 

 history insensibly graduate into each other, "though 

 sometimes the change is more rapid and revolutionary." 

 We may even make it comparable with individual 

 history, where those ages we call infancy, childhood, 

 youth, and manhood imperceptibly merge into each 

 other. And here, not infrequently, some deeply felt ex- 

 perience may hasten on the characteristics of the suc- 

 ceeding stages. Certain vicissitudes make some in- 

 dividuals men of affairs before they are twenty, yet they 

 may retain some of those characters which pertain to 

 youth. 



"In social and political life, too, successive phases of 

 civilization, embodying successive dominant principles, 

 usually graduate into each other; yet great events have 

 sometimes determined exceptionally rapid changes in the 

 direction or the rate of movement. So also is it in geo- 

 logical history. The eras, periods, etc., usually shade 

 more or less insensibly into each other; yet there have 

 been times of comparatively rapid or revolutionary 

 change. In all history there are periods of comparative 

 quiet, during which forces of change are gathering 

 streneth, separated by periods of more rapid change, 

 during which the accumulated forces produce conspicu- 

 ous effects." 



Geologic history teaches us that early in the earth's 

 growth there was a time when those organisms we call 

 mollusks weie the dominant types throughout nature. It 

 was an age of mollusks. They appeared gradually and 

 gradually culminated, then declined, then imperceptibly 

 became less and less numerous, to exist in diminishing 

 numbers up to the present time. As they began to de- 

 cline a higher group, or the fishes appeared, which passed 

 through a similar rise, culmination and declination, and 

 again carried up in a similar manner to the present age. 

 Corresponding ages of reptiles and mammals, successively 

 follow overlapping and merging with each other in pre- 

 cisely the same manner and each in turn successively 

 dominating, ihus giving us an age of mollusks, an age of 

 fishes, and age of reptiles and so on. Here, as in individ- 

 ual, or in social history, or the history of civilization, we 

 find each overlapping age foreshadowing the previous 

 one and the dominating characteristics of each arising 

 in the successive ages of their predecessors. When 

 these phases are made comparable with individual his- 

 tory, Le Conte reminds us; "In youth, the characteristic 



