108 J. LeConte—Critical Periods in the History of the Earth. 
for the time required to evolve a new genus or a new family is 
of course immensely greater than in the case of a new species. 
We will illustrate the difficulties of the ordinary view by 
one striking example. In the Upper Silurian in the midst of 
a conformable series, where if there be any break—any lost 
record—surely it must be very small, appear suddenly, with- 
out premonition, Mishes : not a connecting link between fishes 
and any form of invertebrates, but perfect, unmistakable fishes. 
Here we have, therefore, the appearance not only of a new class, 
but of a new sub-kingdom or t: of structure, Vertebrata. 
Now to change from any previously existing form of inverte- 
brate, whether worm, crustacean or mollusk, into a vertebrate, 
by a series of imperceptible steps represented by successive 
generations—steps so imperceptible that it would take 100,000 
them to advance from one intermediate species to another— 
would require an amount of time which is inconceivable to the 
human mind, and a number of steps, each be it remembered, 
represented by thousands of individuals, which can scarcely be 
expressed by figures. And yet we must believe that these 
innumerable transitional forms, each represented by innumera- 
ble 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 pre- 
viously 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 incredi- 
ble imperfection of the geological record (imperfect at best)— 
we avoid also the necessity of extending geological time to @ 
degree which cannot be accepted by the physicist—if we admit 
that the derivation of one species from another is not necessa- 
rily by innumerable imperceptible steps, but may sometimes 
be by a few decided steps ; and that the same is true for the orl 
gin of new genera, families, orders, etc.; in a , 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 1s 
often lost. 
Thus, on the a of such rigidity or resistance 10 
i orms, varying in degree in different species 
species re and more rigid. In times of very gradual 
change, the more plastic species change gradually part passt, 
while the more rigid species change paroxysmally, now one, 
