CRITICAL PERIODS IN TEE HISTORY OF THE EARTH. 525. 



ures. And yet we miast believe that these innumerable transitional forms, 

 each represented by innumerable individuals, are all lost, and that this pro- 

 digious time shows no evidence in the rocky record. If this case were ex- 

 ceptional we might possibly admit that fishes appeared in Great Britain by 

 migration (as they probably did), but only after having previously existed 

 untold millions of ages somewhere else; but similar cases are too common 

 to be explained in this way. 



Now the whole difficulty disappears, — we avoid the incredible imper- 

 fection of the geological record (imperfect at best); we avoid also the neces- 

 sity of extending geological time to a degree which cannot be accepted by 

 the physicist, — if we admit that the derivation of one species from another is 

 not necessarily by innumerable imperceptible steps, but may sometimes be by 

 a few decided steps ; and that the same is true for the origin of new genera, 

 families, orders, etc. ; in a word, that there are in the history of evolution of 

 species genera, families, orders, etc., and of the organic kingdom periods of 

 rapid movement. When the whole organic kingdom is involved in the move- 

 ment, then we call the period critical, and the record of it is often lost. 



Thus, on the supposition of sucii rigidity or resistance to change in or- 

 ganic forms, varying in degree in different species and in different genera, 

 families, orders, etc., a rigidity, also, increasing by accumulated heredity so 

 long as conditions remain unchanged, it is evident that, in times of perfect 

 tranquillity all species grow more and more rigid. In times of very gradual 

 change the more plastic species change gradually pari passu, while the more 

 rigid species change paroxysmally, now one, now another, as their resist- 

 ance is overcome. Finally, in times of revolution nearly all forms yield to 

 the pressure of external conditions and change rapidly, only the very ex- 

 ceptionally rigid being able to pass over the interval to the next period of 

 readjusted equilibrium. 



Thus, for example, the great and wide-spread changes of jDhysical geog- 

 raphy which occurred at the end of the Carboniferous, appropriately called 

 in this country the Appalachian revolution, were the death-sentence of the 

 long continuing and therefore rigid Palaeozoic types. But the sentence was 

 not immediately executed. The Permian represents the time between the 

 sentence and the execution, — the time during which the more rigid Palaeo- 

 zoic forms continue to linger out a painful existence in spite of changed 

 and still changing conditions. But the most critical time — the time of 

 the most rapid change, the time of actual execution — was the lost interval. 

 Only a very few most rigid forms pass over this interval into the Trias. 



The Quaternary, a Critical Period. We have given examples of several 

 general unconformities, the signs of wide-spread oscillations of the earth- 

 crust, attended with incriease and decrease of land, and therefore with great 

 and wide-spread changes of clirnate and other physical conditions, and also 

 with great and rapid changes of organic species. These times of general 

 oscillation are therefore the natural boundaries of the Eras or primary di- 

 visions of time. We have called them critical periods, transition periods, 



