1877.] Critical Periods in the History of the Earth. 547 
the interval was a continental period, and therefore probably a 
period of broad land connections between Nearctic and Palæarc- 
tie regions. The complete examination of the uppermost Creta- 
ceous of different portions of Asia will probably reveal the im- 
mediate progenitors of the early Tertiary mammals of Europe — 
and America. ‘This introduces us to a most important element of- 
rapid local faunal change, especially in higher animals, namely, 
migrations. If we do not dwell longer now on this, it is only 
because we shall have to recur to it again. 
I have preferred, thus far, to speak of general evolution- 
changes of organisms, whether slow or rapid, as produced by 
varying pressure of external conditions, and of the most striking 
local changes by migrations from other regions, where the appar- 
ently suddenly-appearing species had previously existed, having , 
originated there by evolution in the usual way. I have chosen, 
thus far, to represent the organic kingdom as lying, as it were, 
passive and plastic under the molding hands of the environment. 
I have done so because it is in accordance with true method to 
exhaust the more obvious causes of evolution before appealing to 
the more obscure and doubtful. 
It is possible that general movements affecting alike all classes 
may be accounted for in this way alone. But there are many 
facts in the evolution of the organic kingdom, especially the sud- 
den appearance of new forms in the quietest times, which can 
hardly be thus explained. There seem to be internal as well as 
external factors of evolution. Again, the internal factors may be 
either in the form of tendencies to change or of resistance to 
change. Of these, however, the latter seems to be most certain. 
There may be in the organic kingdom an “ inherent tendency” 
to change in special directions, similar to that which directs the 
course of embryonic evolution, — a tendency, in the case of the 
organic kingdom, inherited from physical nature from which it 
Sprang, as in the case of the embryo it is inherited from the or- 
ganic kingdom through the line of ancestry. This cause, how- 
ever, is too obscure, and I therefore pass it by. 
But whether or not there be any such inherent tendency to 
change, there certainly is an inherent tendency to stability, — to 
persistence of organic form. If there be no inherent force of 
progress, there certainly is an inherent force of conservation 
_ Steater in some species than in others. It seems probable that in 
many of the more rigid types this stability is so great, and there- 
fore variation of offspring so slight, that progressive change of 
