VERTICAL DISTRIliUTION. 
;]0(i 
eoliriict or perceptible struggle, but because the invading organisms 
are more adaptable to the cliinatal or other vicissitudes of the environ- 
ment and will under comparatively unfavourable conditions prosper 
and increase in numbers. The range of a dominant species is tlius 
geogra])hica]ly continuous, while that of a dispossessed or weaker 
species becomes disconnected or discontinuous, owing to their isolation 
in various more or less undesirable districts, where, how'ever, for a 
period they will be dominant species, disjmssessing in turn the 
relatively still weaker previous occupants, and becoming specialized 
by adajjtation to the more e.xtreme conditions to which they are sub- 
jected and jirobably thus obliterating hereditary ancestral features 
and originating new racial or specific characteristics. 
The Diei-tsion of an imju-oved race, or one tilling some previousl}" 
inade([uately occupieil s})here, whether of man or of more lowly organ- 
isms, from this a.ssumed centre of dis])ersion, is governed by the same 
great laws and ]>riucii)les, the occupation by the stronger races of the 
more desirable regions adjacent to those already inhabited, and the 
eventual more or less comj)lete e.xpulsioii therefrom of the more primi- 
tive and (•omi)etitive species, the dispersal varying in rapidity and 
direction not only according to the vigour of the species already 
occupying the region, which may more or less succes.sfully retard effec- 
tive dis]iersal in certain directions, but also by the nature of the 
]ihysical obstacles to be overcome ; the.se may consist merely of districts 
undesirable to occupy, or may be desert lands, arms of the sea, hroad 
e.xpanses of water, rivers, or mountain ranges, all of which inter- 
pose obstacles to disper.sal wbich can only be overcome by ages of 
time, the more feeble .s])ecies being eventually compelled by the 
])re.ssure of the imiu'oveil races to ad()j)t new habits of life less actively 
(•om])etitive or adapt themselves to the more inhospitable districts, 
rliaracterized by extremes (jf physical or cliinatal condition to which 
they are driven, or be gradually exterminated, and we thus in 
temjierate climes acipiire a mountain, desert, or other i'aiina, which 
is representative of forms of life which may be allied to but is of 
earlier and more generalizi'd ty]ie than those occupying the more 
desirable regions of the plains. 
The Vertical or llypsometrical and Bathymetrical di.stribution of 
the mollusca and other organisms is subject to the same general laws 
as those already described as governing horizontal dispersal. The 
phenomena have been locally studieil by many authors and zones 
