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Show 102 ISLAND LIFK (PART T. comparatively shallow water formation-that is, one formed at a depth measured by hundreds rather than by thousands of fathoms. The nature of the formations composing all our continents also proves the continuity of those continents. Everywhere we find clearly marked shore and estuarine deposits, showing that every part of the existing land has in turn been on tho sea-shore ; and we also find in all periods lacustrine formations of considerable extent with remains of plants and land animals, proving the existence of continents or extensive lands, in which such lakes or estuaries could be formed. These lacustrine deposits can be traced back through every period, from the newer Tertiary to the Devonian and Cam brian, and in every continent which has been geologically explored; and thus complete the proof that our continents have been in existence under ever changing forms throughout the whole of that enormous lapse of time. On the side of the oceans we have also a great weight of evidence in favour of their permanence and stability. In addition to their enormous depths and great extent, and the circumstance that the deposits now forming in them are distinct from anything found upon the land-surf:1ee, we have the extraordinary fact that the countless islands scattered over their whole area (with one or two exceptions only) never contain any Palreozoic or Secondary rocks-that is, have not preserved any fragments of the supposed ancient continents, nor of the deposits which must have resulted from their denudation during the whole period of their existence ! The exceptions are New Zealand and the Seychelles Islands, both situated near to continents leaving almost the whole of the vast areas of the .Atlantic' Pacific, Indian, and Southern oceans, without a solitary relic of the great islands or continents supposed to have sunk beneath their waves. CHAPTER VII. CHANGES OF CLIMATE WHICH HAVE INFLUENCED THE DISPERSAL OF ORGANISMS: TilE GLACIAL EPOCH. Proofs of the recent occurrence of a Glacial Epoch- Moraines- Travelled Blocks-Glacial deposits of Scotland: the "Till "- Inferences from the g-lacial phenomena of Scotland- Glacial phenomena o£ North America --Effects o£ the Glacial Epoch on animallife~Warm and cold period~:~ - Palreontological evidence o£ alternate cold and warm periods- Evidence of interglacial warm periods on the Continent and in North America-Migrations and extinctions of Organisms caused by the G-lacial Epoch. WE have now to consider another set of physical revolutions which have profoundly affected the whole organic world. Besides the wonderful geological changes to which, as we have seen, all continents have been exposed, and which must, with extreme slowness, have brought about the greater features of the dispersal of animals and plants throughout the world, there have been also a long succession of climatal changes, which, though very slow and gradual when measured by centuries, may have sometimes been rapid as compared with the slow march of geological mutations. These climatal changes may be divided into two classes, which have been thought to be the opposite phases of the same great phenomenon-cold or even glacial epochs in the Temperate zones on the one hand, and mild or even warm periods extending into the .Arctic regions on the other. The evidence for both these changes having occurred is conclusive; and as they must be taken account of whenever we endeavour to explain the past |