106 J. LeConte—Critical Periods in the History of the Earth. 
preferred, thus far, to represent the organic kingdom as lying, 
as it were, passive and plastic under the moulding hands of the 
environment. I have done so because it is in accordance with 
true method to exhaust the more obvious causes of evolution 
or of resistance to change. Of these, however, the latter seems 
to be most certain. There may be in the organic kingdom an 
‘“anherent tendency” to change in special directions, similar to 
that which directs the course of embryonic evolution—a tend- 
ency, in the case of the organic kingdom, inherited from phys- 
ical nature from which it sprung, as in the case of the embryo 
it is inherited from the organic kingdom through the line of 
ancestry. This cause, however, is too obscure, and I therefore 
pass it b 
y: 
But whether or not there be any such inherent tendency to 
e 
