This idea of cosmic cycles, or Great Years, appears to have originated in the Orient, possibly with the Chaldeans.[15] It was firmly established among the Ionian Greeks[16] and Pythagoreans,[17] from whom Plato adopted it. Many and various opinions prevailed about the violence and character of the changes produced by the celestial cycles. The Chaldeans had thought that whenever all the planets come into conjunction on one straight line in the sign of the zodiac Cancer, the entire universe is destroyed by fire but destroyed only to be born again; similarly the world is destroyed by water when the same phenomenon occurs in Capricorn.[18] The theory of a complete and universal birth and rebirth (_palingenesis_) was held by some of the Greek philosophers.[19] Plato and Aristotle, however, seem to have restricted the destructive effects of the celestial influence to the sublunar sphere and maintained that the realms above the moon were eternal.[20] On the whole, belief in periodically recurrent destructions of the earth by water was more widespread and was given greater definition than belief in corresponding destructions by fire.[21] The main reason for this is probably to be looked for in the dissemination among nearly all peoples of legends of a great flood, but it also in no small measure may be attributed to rudimentary geological observations (notably of the presence of shells on high ground) which showed that portions of the earth’s surface had at one time lain beneath the waters.[22] GEOGRAPHIC APPLICATION OF THE THEORY OF THE GREAT YEARS The theory of the Great Years was invoked to explain changes in geographic and climatic conditions on the earth’s surface.[23] When the various planets and stars bear a certain relation to one another, a period of dryness and heat, or a Great Summer, is experienced; conversely, when other stellar relationships prevail, there is a period of cold and wetness, or a Great Winter. Even land and sea gradually change places under stellar control. Certain parts of the land, Aristotle observed, had once been covered by the sea, and what is now sea had once been land: like plants and animals, land and sea grow to maturity and old age. If the causes adduced for these changes were not so utterly different from those that are now accepted, we might almost be tempted to think that Aristotle had some conception of climatic cycles and cycles of erosion.