Attention should also be paid to the conditions under which the deposits accumulated, since it is clear that rocks may be formed at the same time in different areas and yet contain many distinct fossils by reason of climatic or bathymetrical differences. Among living marine organisms we find certain forms restricted to sandy or muddy sea-bottoms and others to clear water, and these changes in the conditions of deposition of sediment have played their part in past geological periods in determining differences in the fossil faunas of rocks which were laid down simultaneously. We not infrequently find mudstones passing laterally into limestones, and this lithological change is always accompanied by a more or less notable change in the fossil contents of the two rock types. Such facts emphasize the close connection between stratigraphy and palaeontology, and indicate that the successful tracing out of the geological history of any area is only possible when the evidence of the stratigrapher is reinforced by that provided by the palaeontologist. The fact that species of animals and plants which have been developed in a particular area do not spread all over the world at once but migrate very slowly led Huxley many years ago to put forward his hypothesis of "homotaxis." He agreed that when the order of succession of rocks and fossils has been made out in one area, this order and succession will be found to be generally similar in other areas. The deposits in two such contrasted areas are homotaxial, that is, show a similarity of order, but, he claimed, are not necessarily synchronous in their formation. In whatever parts of the world Carboniferous, Devonian and Silurian fossils may be found, the rocks with Carboniferous fossils will be found to overlie those with Devonian, and these in their turn rest upon those containing Silurian fossils. And yet Huxley maintained that if, say, Africa was the area in which faunas and floras originated, the migration of a Silurian fauna and flora might take place so slowly that by the time it reached Britain the succeeding Devonian forms had developed in Africa, and when it reached North America, Devonian forms had reached Britain and Carboniferous forms had developed in Africa. If this were so a Devonian fauna and flora in Britain may have been contemporaneous with Silurian life in North America and with a Carboniferous fauna and flora in Africa.