BIRTH-PLACE OF LIFE-TYPES. 
.S86 
The pliysioal obstacles to uniform dispersal are mountain chains, 
deserts, marshes, rivers, arms of the sea, or any natural features dis- 
similar to those to which the particular .species is more especially 
adapted. Some of these barriers to dispersion have apparently been 
permanent through vast geological periods, hut iu other cases effective 
barriers have been formetl after considerable diffusion of the earlier 
types of life has taken j)lace. 
Islands, especially in the warm and tropical zones, are often exceed- 
ingly rich iu mollusca, and sometimes exhibit faume which are 
remarkably distinct, owing to the survival and external modification 
therein of archaic forms of life which were formerly much more 
uuifurmly dispensed over the globe, hut which have long ago been 
exterminated or driven off from the neighbonring continental lands in 
which comi)etition is more active: the British Isles, however, do not 
possess a special and pecnliar fauna, their isolation being too recent to 
show any but the slightest ditfereutiatiou from the coutiuental forms. 
The classical researches of Pilshry, Semper ami others into the 
organization of the Ilelicidm, in connection with information derived 
from other .sources, enable us to iudicate the probably true place of 
origin of the chief types of .structure, not only of Helicidic but of 
other more imjxutaut groups, ami to sketch out a i)rol)able route by 
which the earth has become ])opulated, for although evolution in a 
lesser degree is a characteristic of every region, the theatre of the 
evolution of the great groups of all forms of life appears to have been 
much more restricted, and a consideration of the circumstances in- 
clines one to the belief in a cbief evolutionary area, in which have arisen 
the more important types of structure at present inhabiting the globe. 
'I'lie Place of Origin of the chief types of terrestrial organized life 
is the Eurasian tract, which is the largest land mass u])on the globe, 
embraiug all the cool and temi)erate parts of the old world and having 
hut few absolutely insuperable barriers, there is a freedom of commu- 
nicati(m with a conse(pient rivalry and struggle for existence and 
supremacy of such intensity that those forms of life able to maintain 
themselves upon tliis extensive continental region exhibit a superiority 
of adaptability and organization with an ability to i)rosper and increase 
under adverse conditions, which enables them easily to overcome the 
more archaic species inhabiting restricted or insular areas with which 
they come in competition and also to dominate the life of all other 
l)arts of the globe. 
