TEE 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 Helicidse 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 



