Le Conte.] 



Critical Periods. 



333 



Migrations and Consequent Unconformity of Faunas. — Last of all 

 among the causes, we must not forget migrations, which, aside from 

 the rapid changes produced by severer struggles, produce also 

 unconformity of faunas and floras, and therefore an appearance of 

 suddenness of change in any one place. Such changes of species 

 by migrations, although most conspicuous in critical periods, take 

 place locally even in the quietest times and the most conformable 

 strata, by changes of bottom from sand to clay or lime, or vice versa, 

 produced by a change of currents, or slight changes of shore line. 



Effect of Specialized and Generalized Forms. — Again, as to the 

 nature of the process of change in the changing organism itself: 

 Long continued prosperous times with unchanging conditions give 

 rise to highly specialized forms, perfectly adapted to these condi- 

 tions, but correspondingly unadopted to other conditions, in other 

 words, to rigid forms, rigid through the accumulation of heredity 

 for many successive generations on the same point. Generalized 

 forms, on the contrary, are less perfectly adapted to any one set of 

 conditions, but more easily adaptable to many kinds of conditions. 

 They are a sort of Jack-of-all-trades and not very good at any. 

 These are therefore more plastic to the moulding influence of a 

 changing environment. Therefore long continued periods of quiet 

 give rise to many specialized dominant forms, perfectly and rigidly 

 adapted to these conditions. During the following critical period, 

 these dominant forms are unable to adapt themselves to new condi- 

 tions. Too rigid to change with the changing times, they are 

 destroyed, while only the few and less conspicuous generalized, and 

 therefore plastic forms save themselves by modifications in many 

 directions adapted to the new conditions. Thus all the old forms 

 may quickly disappear, some by extinction, and some by modifica- 

 tion into new forms, but all the new forms come only by modifica- 

 tion of the old. 



Periodic Law Universal. — But again, and lastly: Even in most 

 prosperous times, and in the most continuous strata, with every 

 evidence of great fullness of life, and when, therefore, the changes 

 of species were slow and continuous, even under these most favor- 

 able conditions, still the links between successive species are rare. 

 In nearly all cases progressive changes seem to take place by sub- 



