550 Critical Periods in the History of the Earth. [September, 
sitional forms, each represented by innumerable individuals, are 
all lost, and that this prodigious time shows no evidence in the 
rocky record. If this case were exceptional we might possibly 
admit that fishes appeared in Great Britain by migration (as 
they probably did), but only after having previously existed un- 
told 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 
imperfection of the geological record (imperfect at best); we 
avoid also the necessity 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 movement, then we 
call the period critical, and the record of it is often lost. 
Thus, on the supposition of such rigidity or resistance to 
change in organic forms, varying in degree in different species 
and in different genera, families, orders, etc., a rigidity, also, in- 
creasing by accumulated heredity so long as conditions remain un- 
changed, it is evident that, in times of perfect tranquillity all spe- 
cies 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 resistance is overcome. Finally, in times of revolution 
nearly all forms yield to the pressure of external.conditions and 
change rapidly, only the very exceptionally 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 phys- 
ical geography 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 
Paleozoic types. But the sentence was not immediately execut- 
ed. The Permian represents the time between the sentence 
and the execution, — the time during which the more rigid Palæ- 
ozoic forms continued to linger out a painful existence 1m ni 
of changed and still changing conditions. But the most criti 
time — the time of most rapid change, the time of actual m 
cution — was the lost interval. Only a very few most rigid forms 
pass over this interval into the Trias. 
