J. LeConte—Critical Periods in the History of the Earth. 118 
influence of all the factors of change, known’ and un nown— 
€. g., pressure of changing physical conditions whether modi- 
Sying the individual, certainly one factor, or selecting the fittest 
everywhere, and therefore would produce geological faune, 
be impossible. 
8. The forces of change resisted by heredity, in some species 
and genera more than others, determines paroxysms of more 
Tapid movement of general evolution affecting sometimes spe- 
cles, Sometimes genera or families. The sudden appearance of 
— genera, families, ete., in quiet times is thus accounted 
4 Du 
changes ph: ; 
rapid rate of change in all forms, Ist, by greater pressure 
Physical conditions, and, 2d, by migrations partly enforced by 
