Baseleveling and Organic Evolution. — Wobdworth. 219 
Ancles and those on the Chilean side, although the climate as 
well as the soil was nearly the same, upon which he comments 
as follows : 
This fact is in perfect accordance with t li<- geological history of the 
Andes; for these mountains have existed us a great barrier, since the 
present races of animals have appeared; and, therefore, unless we sup- 
pose the same species to have been created at two different places, we 
ought not to expect any closer similarity between the organic beings on 
the opposite sides of the Andes than on the opposite sides of the ocean. 
In both cases we must leave out of the question thosekinds which have 
been able to cross the harrier, whether of solid rock or salt water. 
If this divergence of character has developed with the up- 
lift of the mountain barrier, we ought, on the other hand, to 
expect further mutations when the barrier disappears by de- 
nudation; and the tendency of the change will be in part 
towards intermigration and the consequent unification of the 
fauna and flora. Thus on the eastern side of North America, 
on the two slopes of the Appalachian chain, which has been 
once baseleveled, the fauna and flora differ as little as possi- 
ble,* yet, when this mountain system was in an Andesian 
youth and very high, the life of its opposite slopes must have 
been as strongly marked as those of the southern Cordilleras 
at the present time. 
Degradation of uplands. 
Organisms suited to steep slopes and high altitudes with 
low temperaturesf must vary, migrate up the remaining nto- 
nadnocks, or keep their stations at a disadvantage as the 
surface sinks by denudation beneath them. The artificial 
transference of some species from uplands to lowlands is at- 
tended with difficulty. Thus with alpine plants transplanted 
in the plains, "whether from a change of atmospheric pres- 
sure or mean temperature," says Mary Somerville,J "all at- 
tempts to cultivate them at a lower level generally fail ; it is 
much easier to accustom a plant of the plains to a higher 
situation." 
*The chain still acts as a harrier to the I'nioiiid;e. for instance. See 
('has. T. Simpson. Proc. U.S. National Museum, vol. xvi, L893. pp. .">!>]- 
.">: also Am. Naturalist, vol. XXVII, p. 353. 
f-Dr. C. Hart Merriam has shown the importance of temperature con 
trol in the geographic distribution of mammals. Smithsonian Report 
Cor 1 Mi H (pub. L893), p. 100. 
^Physical Geography, second Am. ed.,1850, p. 305. 
