At present about half-a-dozen big connections are engaging our attention, leaving as comparatively settled the extent and the duration of such minor "bridges" as that between Africa and Madagascar, Tasmania and Australia, the Antilles and Central America, Europe and North Africa. (Not a few of those who are fascinated by, and satisfied with, the statistical aspect of distribution still have a strong dislike to the use of "bridges" if these lead over deep seas, and they get over present discontinuous occurrences by a former "universal or sub-universal distribution" of their groups. This is indeed an easy method of cutting the knot, but in reality they shunt the question only a stage or two back, never troubling to explain how their groups managed to attain to that sub-universal range; or do they still suppose that the whole world was originally one paradise where everything lived side by side, until sin and strife and glacial epochs left nothing but scattered survivors?
The permanence of the great ocean-basins had become a dogma since it was found that a universal elevation of the land to the extent of 100 fathoms would produce but little changes, and when it was shown that even the 1000fathom-line followed the great masses of land rather closely, and still leaving the great basins (although transgression of the sea to the same extent would change the map of the world beyond recognition), by general consent one mile was allowed as the utmost speculative limit of subsidence.
Naturally two or three miles, the average depth of the oceans, seems enormous, and yet such a difference in level is as nothing in comparison with the size of the Earth. On a clay model globe ten feet in diameter an ocean bed three miles deep would scarcely be detected, and the highest mountains would be smaller than the unavoidable grains in the glazed surface of our model. There are but few countries which have not be submerged at some time or other.)CONNECTION OF SOUTH EASTERN ASIA WITH AUSTRALIA. Neumayr's Sino-Australian continent during mid-Mesozoic times was probably a much changing Archipelago, with final separations subsequent to the Cretaceous period.
Henceforth Australasia was left to its own fate, but for a possible connection with the antarctic continent.
AFRICA, MADAGASCAR, INDIA. The "Lemuria" of Sclater and Haeckel cannot have been more than a broad bridge in Jurassic times; whether it was ever available for the Lemurs themselves must depend upon the time of its duration, the more recent the better, but it is difficult to show that it lasted into the Miocene.
AFRICA AND SOUTH AMERICA. Since the opposite coasts show an entire absence of marine fossils and deposits during the Mesozoic period, whilst further north and south such are known to exist and are mostly identical on either side, Neumayr suggested the existence of a great Afro-Son American mass of land during the Jurassic epoch. Such land is almost a necessity and is supported by many facts; it would easily explain the distribution of numerous groups of terrestrial creatures. Moreover to the north of this hypothetical land, somewhere across from the Antilles and Guiana to North Africa and South Western Europe, existed an almost identical fauna of Corals and Molluscs, indicating either a coast-line or a series of islands interrupted by shallow seas, just as one would expect if, and when, a Brazil-Ethiopian mass of land were breaking up. Lastly from Central America to the Mediterranean stretches one of the Tertiary tectonic lines of the geologists. Here also the great question is how long this continent lasted. Apparently the South Atlantic began to encroach from the south so that by the later Cretaceous epoch the land was reduced to a comparatively narrow Brazil-West Africa, remnants of which persisted certainly into the early Tertiary, until the South Atlantic joined across the equator with the Atlantic portion of the "Thetys," leaving what remained of South America isolated from the rest of the world.
ANTARCTIC CONNECTIONS. Patagonia and Argentina seem to have joined Antartica during the Cretaceous epoch, and this South Georgian bridge had broken down again by mid-Tertiary times when South America became consolidated. The Antarctic continent, presuming that it existed, seems also to have been joined, by way of Tasmania, with Australia, also during the Cretaceous epoch, and it is assumed that the great Australia-Antarctic-Patagonian land was severed first to the south of Tasmania and then at the South Georgian bridge. No connection, and this is important, is indicated between Antarctica and either Africa or Madagascar.
So far we have followed what may be called the vicissitudes of the great Permo-Carboniferous Gondwana land in its fullest imaginary extent, an enormous equatorial and south temperate belt from South America to Africa, South India and Australia, which seems to have provided the foundation of the present Southern continents, two of which temporarily joined Antarctica, of which however we know nothing except that it exists now.
Let us next consider the Arctic and periarctic lands. Unfortunately very little is known about the region within the arctic circle. If it was all land, or more likely great changing archipelagoes, faunistic exchange between North America, Europe and Siberia would present no difficulties, but there is one connection which engages much attention, namely a land where now lies the North temperate and Northern part of the Atlantic ocean.
How far south did it ever extend and what is the latest date of a direct practicable communication, say from North Western Europe to Greenland?