1877.] Critical Periods in the History of the Earth. 555 
Thus, regarding the Tertiary and the Present as consecutive 
eras, and the Quaternary as the transition or critical period be- 
tween, then, if the record of this period had been, lost, corre- 
sponding with the unconformity here found, we should have had 
here an enormous and apparently sudden change of mammalian 
species. Yet this change of fauna, as great as it is, is not to be 
compared with that which occurred between the Archean and 
Paleozoic, or between the Paleozoic and Mesozoic, or even that 
between the Mesozoic and Cenozoic; for the change during the 
Quaternary is mostly confined to species of the higher mammals, 
while the change during previous critical periods extended to 
species of all grades, and not only to species, but to genera, fam- 
ilies, and even orders. We conclude, therefore, that the previous 
critical periods or lost intervals were far longer than the whole 
Quaternary ; or else that the rate of evolution was far more rapid 
in these earlier times. ; 
To sum up, then, in a few words, the general formal laws of 
-eyolution-change throughout the whole history of the earth : — 
(1.) Gradual, very slow changes of form everywhere under 
the influence of all the factors of change, known and unknown: 
for example, pressure of changing physical conditions whether 
modifying the individual (certainly one factor), or selecting the 
fittest offspring (certainly another factor) ; improvement of organs 
by use and the improvement inherited (certainly a third factor), 
and perhaps still other factors yet unknown. This general evo- 
lution by itself considered would produce similar changes every- 
where, and therefore would produce geological faunæ, but not 
geographical diversity. Determination of a geological horizon 
would in this case be easy, because fossil species would be every- 
where identical. 
(2.) Changes in different places and under different physical 
conditions, taking different directions and advancing at different 
tates, give rise to geographical faune. This, if there were noth- 
ng more, would produce far greater geographical diversity and 
more complete localization of faune and floræ than now exists, — 
SO great that the determination of a geological horizon would be 
Impossible, 
(3.) The force of change resisted by heredity, in some species 
and genera more than in others, determines paroxysms of more 
rapid movement of general evolution, affecting sometimes species, 
Sometimes genera or families. The sudden appearance of species, 
‘Senera, families, etc., in quiet times is thus accounted for. 
