Previous to the present period there was what is so well known as the Glacial epoch. In the earthy deposits of this age in which fossils are found plants are not uncommon. They are of the same kind as those now growing in the cold regions of the Arctic circle, and on the heights of hills whose temperature is much lower than that of the surrounding lowlands. Glacial epochs occurred in other parts of the world at different times; for example, in South Africa, in the Permo-Carboniferous period, during which time the fossils indicate that the warmth-loving plants were driven much farther north than is now the case. It is largely from the nature of the plant fossils that we know the climate of England at the time preceding the Glacial epoch. Impressions of leaves and stems, and even of fruits, are abundant from the various periods of the Tertiary. Many of them were Angiosperms (see Chap. VIII), and were of the families and even genera which are now living, of which not a few belong to the warm regions of the earth, and are subtropical. It is generally assumed that the fossils related to, or identical with, these plants must therefore have found in Tertiary Northern Europe a much warmer climate than now exists. Not only in Northern Europe, but right up into the Arctic circle, such plants occur in Tertiary rocks, and even if we had not their living representatives with which to compare them, the large size and thin texture of their leaves, their smoothness, and a number of other characteristics would make it certain that the climate was very much milder than it is at present, though the value of some of the evidence has been overestimated. From the Tertiary we are dependent chiefly on impressions of fossils; anatomical structure would doubtless yield more details, but even as it is we have quite enough evidence to throw much light on the physiography of the Tertiary period. The causes for such marked changes of climate must be left for the consideration of geologists and astronomers. Plants are passive, driven before great climatic changes, though they have a considerable influence on rainfall, as has been proved repeatedly in India in recent times. From the more distant periods it is the plants of the Carboniferous, whose structure we know so well, that teach us most. Although there is still very much to be done before knowledge is as complete as we should wish, there are sufficient facts now discovered to correct several popular illusions concerning the Palæozoic period. The “deep, all-enveloping mists, through which the sun’s rays could scarcely penetrate”, which have taken the popular imagination, appear to have no foundation in fact. There is nothing in the actual structure of the plants to indicate that the light intensity of the climate in which they grew was any less than it is in a smoke-free atmosphere to-day.