Page:The origin of continents and oceans - Wegener, tr. Skerl - 1924.djvu/116



HIS is not the place to discuss the whole of the problem of the climates of the past. Nevertheless, our reasoning forces us to give it a fleeting consideration, for only thus do the grounds become clear, which, from this standpoint, support the correctness of the displacement theory.

The present very undeveloped state of palæoclimatology is not the result of the absence of data on past climates. Their number is legion, although there are, frankly, many of them that we cannot yet certainly interpret, and unfortunately also many that have been interpreted incorrectly. The petrifactions of plants and animals form a great part of the evidence. The treeless tundra-flora beyond the limit of trees, coinciding with about the 10° C. isotherm of the warmest months, is usually clearly distinguished in the fossil state from the temperate forest flora. The latter can also be separated from the forests in the region of tropical rains by means of the annual rings in the wood, and sometimes also from the evergreen sub-tropical hard-leaved flora, which however only takes up a relatively small space in the present-day climatic system. Palms only occur in present-day climates, the coldest month of which does not have an average temperature below 6° C., and it is very probable that there was a similar temperature-limit for the palms of the past