To understand the problem from the Earth’s point of view, let us review the facts with which geology presents us. The flora of paleologic times, as we see both at their advent in the Devonian and from their superb development in the Carboniferous era, consisted wholly of forms whose descendants now seek the shade.[18] Tree ferns, sigillaria, equisetæ, and other gloom-seeking plants composed it. That some tree fern survivals to-day can bear the light does not invalidate the racial tendency. We have plenty of instances in nature of such adaptability to changed conditions. In fact, the dying out and deterioration of most of the order shows that the conditions have changed. And these plants, grown to the dimensions of trees, inhabited equally the tropic, the temperate, and the frigid zones as we know them now. Lastly, no annual rings of growth are to be found on them.[19] In other words, they grew right on, day in, day out. The climate, then, was as continuous as it was widespread. [18] De Lapparent, Dana, Geikie, _passim_. [19] De Lapparent. On the other hand, astronomy and geology both assert that the seas were warm.[20] From this it follows that a vastly greater evaporation must have gone on then than now, and that a welkin of cloud must thus inevitably have been formed. [20] De Lapparent, Dana, Geikie, _passim_. Now put the two facts together, and you have the solution. The climate was warm and equable over the whole globe because a thick cloud envelope shut off the Sun’s heat, the heat being wholly supplied from the steamy seas. At the same time, by the same means the light was necessarily so tempered as to produce exactly that half-light the ferns so dearly love. One and the same cause thus answers the double riddle of greater warmth and less light in those old days than is now the case.