DRIFTING CONTINENTS AND LATE PALEOZOIC GEOGRAPHY.

As a result of 19 months spent in Tasmania and eastern Australia and 7 weeks at the southern tip of South America, followed by a year of reading and writing on the biogeography and history of the southern end of the world, I have gradually become convinced that continental drift is not just an intellectual exercise but a probable reality. I was not easily convinced of this. I am a conservative, sixty-year-old biologist and also a professional biogeographer (if there is such a thing), and I have been and still am repelled by the exaggerations and unintentional misrepresentations of biogeographic "evidence" by Wegener and his followers. Nevertheless I am now a Wegenerian, of sorts. I think that some continents probably have drifted, although not in quite the usual Wegenerian pattern. I give this fragment of my own history to justify reviewing a subject which, if there is any reality in it, is of great interest and importance to many persons. Convincing evidence-evidence that has convinced me-of movement of continents comes from the matching shapes of Africa and South America and their relation to the Mid-Atlantic Ridge, from new evidence of convection currents in the earth's mantle that might nmove continents, from the distribution of glaciation and of floras in the late Paleozoic, and from the new (and still very incomplete) record of paleomagnetism. Two questions in the general hypothesis of continental movenment are critical and can perhaps be answered. First, when did Africa and South America separate, if they were united? And second, what were the positions of the southern continents when they were glaciated late in the Paleozoic, 200 million or more years ago? Time of Separation of Africa and South Amnerica.-The fitting of edges of Africa and South Americal is virtual proof that these continents were united, if the hypothesis of continental displacement is accepted at all. Wilson' suggests as a "highly speculative" hypothesis that convection in the earth's mantle, rising along the line of the Mid-Atlantic Ridge, has separated the continents and is pushing them apart, and that the rate of movement calculated from the ages of volcanic islands, which increase in age with distance from the Ridge, indicates that separation occurred about 120 million years ago, about the end of the Jurassic. However, a suboceanic ridge exists in the Pacific too, apparently similar to the MidAtlantic Ridge and similarly bordered by islands of graded ages.2, 3 Convection currents nay be pushing South America from both sides, and both currents (if they exist) may be diving downward under South America rather than moving the continent. Therefore, rate of movement of South America and time of separation from Africa cannot safely be calculated from Wilson's data, although the data may indicate the latest time that separation can have occurred. Paleomagnetism does not show longitudes (see page 1089) and therefore does not show how far apart Africa and South America were in the past. However, if basic assumptions are correct, paleomagnetism does show rotations of continents. If Africa and South America were united, they have not only separated but have also