1877.] | Critical Periods in the History of the Earth. 549 
quietest, where the record is fullest and apparently without any 
missing leaf, species come and go and others take their place, and 
yet only rarely do we find any transition steps. If this were 
merely once or twice or thrice, or to any extent exceptional, it 
might be explained by loss of record here and there, but it oceurs 
thousands and tens of thousands of times. Now, if evolution 
moves only at uniform rate, if it takes one hundred thousand 
years to transmute one species into another (as it certainly does 
when evolution is moving at its usual rate), if there are at least 
one hundred thousand steps (represented each, of course, by a 
whole generation of many individuals) between every two con- 
secutive species, it is simply incredible that all the individuals 
representing the intermediate steps, so infinitely more numerous 
than the species they connect, should be so generally, almost uni- . 
versally, lost. But the phenomena, as we find them, are easily 
understood if a few generations represent the transition step, and 
many generations the permanent form. 
A similar rapid, almost sudden, appearance and extinction of 
genera, families, and higher groups at certain horizons are also 
common. In these cases the intermediate steps of transition are 
often found, and constitute, in fact, the chief demonstrative evi- 
dence of the truth of evolution. But the difficulty on the as- 
sumption of a uniform rate of evolution is none the less here, 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. 
e will illustrate the difficulties of the ordinary view by one 
striking example. In the Upper Silurian, in the midst of a con- 
formable series, — where if there be any break, any lost record, 
surely it must be very small, — appear suddenly, without premo- 
nition, fishes ; 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 type of structure, Vertebrata. Now, to change 
from any previously existing form of invertebrate, whether worm, 
crustacean, or mollusk, into a vertebrate, by a series of imper- 
ceptible steps represented by successive generations, — steps so 
imperceptible that it would take one hundred thousand of them 
advance from one intermediate species to another, — would re- 
quire 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 tran- 
