We must not forget that this presence of rain throughout geological time implies, as we have seen in our last chapter, a constant and universal distribution of atmospheric dust. The two chief sources of this dust--the total quantity of which in the atmosphere must be enormous--are volcanoes and deserts, and we are therefore sure that these two great natural phenomena have always been present. Of volcanoes we have ample independent evidence in the presence of lavas and volcanic ashes, as well as actual stumps or cores of old volcanoes, through all geological formations; and we can have little doubt that deserts also were present, though perhaps not always so extensive as they are now. It is a very suggestive fact that these two phenomena, usually held to be blots on the fair face of nature, and even to be opposed to belief in a beneficent Creator, should now be proved to be really essential to the earth's habitability.
Notwithstanding this prevalence of warm and uniform conditions, there is also evidence of considerable changes of climate; and at two periods--in the Eocene and in the remote Permian--there are even indications of ice-action, so that some geologists believe that there were then actual glacial epochs. But it seems more probable that they imply only local glaciation, owing to there having been high land and other suitable conditions for the production of glaciers in certain areas.
The whole bearing of the geological evidence indicates the wonderful continuity of conditions favourable for life, and for the most part of climatal conditions more favourable than those now prevailing, since a larger extent of land towards the North Pole was available for an abundant vegetation, and in all probability for an equally abundant animal life. We know, too, that there was never any total break in life-development; no epoch of such lowering or raising of temperature as to destroy all life; no such general subsidence as to submerge the whole land-surface. Although the geological record is in parts very imperfect, yet it is, on the whole, wonderfully complete; and it presents to our view a continuous progress, from simple to complex, from lower to higher. Type after type becomes highly specialised in adaptation to local or climatal conditions, and then dies out, giving room for some other type to arise and be specialised in harmony with the changed conditions. The general character of the inorganic change appears to have been from more insular to more continental conditions, accompanied by a change from more uniform to less uniform climates, from an almost sub-tropical warmth and moisture, extending up to the Arctic Circle, to that diversity of tropical, temperate, and cold areas, capable of supporting the greatest possible variety in the forms of life, and which seems especially adapted to stimulate mankind to civilisation and social development by means of the necessary struggle against, and utilisation of, the various forces of nature.