THE PALEONTOLOGIC RECORD 339 
riers, we are coming to a point where the efficiency of this factor can 
be safely taken into account. The outcome of free interbreeding, as 
Jordan has pointed out, is to unify species and obliterate variations. 
Per contra, isolation checks this process and gives freer play to tend- 
encies arising from other factors in variation. The effect is thus, as 
a general rule, negative, but expresses itself freely enough in geo- 
graphic provinces severed by some barrier or condition which has the 
effect of a barrier. Among existing species the formative effects of 
segregation have been very largely illustrated from restricted areas 
such as the subdivisional valleys and forests of Hawaii with its dis- 
tinctive forms of the Helicidz and other terrestrial snails—a case that 
is paralleled in paleontology by the snails of Steinheim. But the effect 
is to be reckoned with in larger or continental areas between which 
there has been at one time opportunity of interchange, especially in 
the case of marine species, with which we chiefly deal, along the epi- 
continents. 
I have particularly in mind phenomena which have been brought 
to my notice by a somewhat extended study of the Devonian faunas 
of the southern hemisphere and the broader application of the factor 
is best enforced and illustrated by this instance. I may say that this 
broader notion seems to be that entertained by Darwin so far as he 
specified the conception of geographic segregation as an element in 
natural selection and it was his work in South America that formed 
the basis of his conclusions. 
With other students we recognize the existence during the Devon- 
ian of austral continental lands which have been variously designated 
and variously outlined. By some this land has been posited as a north 
and south Atlantis lying in the meridional axis of the present ocean, 
by others a broken land mass partly crossing the southern Atlantic 
from east to west. But now we begin to see its continuity and the ex- 
tent of its strands, with something of its changes in outline during 
its early history. It was the precursor and the nucleus of Gondwana- 
land. With it began, so far as we now know, the long history of that 
continental land and the successive records of life developing under 
continued conditions of geographic isolation from the northern strands. 
From Argentina, Bolivia and northern Brazil we have very lucid 
evidence, on the basis of paleontology, that in the late Silurian the 
shore lines were continuous with those of the north. We have no de- 
pendable knowledge of these earlier faunas at the east and indeed 
their entire absence is indicated by stratigraphy; but with the sub- 
mergence of the Silurian at the west, there entered from the African 
east upon this south Atlantic field, a positive diastrophism whose axis 
was well nigh normal to that of the present Atlantis, and along the 
shores of this growing land bridge entered an invasion of marine life 
