'2\S The A mi ricu a Geologist,. October, 1891 
elusion that "whenever the physical or organic conditions 
change to however small an extent some corresponding change 
will be produced in the flora and fauna, since, considering the 
severe struggle for existence and the complex relations of the 
various organisms, it is hardly possible that the change 
should not he beneficial to some species and hurtful to oth- 
ers." 
In any land which has undergone degradation from a moun- 
tainous topography to a peneplain, we ought to find a marked 
change in the organisms at the close of the cycle of denuda- 
tion. In the first stages of change from original construc- 
tional topography, effects will be discernable. Sculptured 
slopes with ravines, sharp divides and peaks, cradle species 
and varieties by barriers which oppose ingress and egre>- 
With the development of the umbrella-shaped topography of 
the island of Oahu. the land snails have varied from a com- 
mon ancestral, coastal type to valley-cradled, differentiated 
varieties, in the upper and disjointed valleys of this disman- 
tled, volcanic island cone.* 
In the progress toward final baseleveling.the repeated diver- 
sion of streams or the reversals of drainage are a constant 
cause of changed conditions. The cycle begins in a moun- 
tainous tract with the least facility for migration of species, 
and ends in broad lowlands which favor the easy migration 
and wide distribution of plants and animals. 
FtnUiHj dirni/ of divides. 
If we follow the surface of the lands downward from an 
original high relief to the completed baseleveled plain, we 
note first that the fading away of divides will throw animals 
and plants before separated by barriers into one field, and 
that thus new adaptations between species will be enforced. 
Leveling off a mountain system and throwing the life of its 
opposite slopes into the same field would now in many parts 
of the world bring together species which must contest for 
survival. 
Darwinf states that he was struck with the remarkable dif- 
ference between the vegetation, the quadrupeds, and in a less 
degree the birds and insects, of the eastern valle} T s of the 
*.T. T. Gulick: Proc. Boston Soc. Nat. Hist., vol. xxiv, 1890. pp. 166-7. 
{-Journal of Researches, etc., chap. xv. Am. ed., pp. 326 and 327. 
