On the whole it is safe to conclude from the above and other publications that the exceptional conditions of the tropics have produced many distinctive lichens, but that a greater abundance both of species and individuals is now to be found in temperate and cold climates. III. FOSSIL LICHENS In pronouncing on the great antiquity of lichens, proof has been adduced from physiological rather than from phytogeological evidence. It would have been of surpassing interest to trace back these plants through the ages, even if it were never possible to assign to any definite period the first symbiosis of the fungus and alga; but among fossil plants there are only scanty records of lichens and even these few are of doubtful determination. The reason for this is fairly obvious: not only are the primitive thalline forms too indistinct for recognizable preservation, but all lichens are characterized by the gelatinous nature of the hyphal or of the algal membranes which readily imbibe water. They thus become soft and flaccid and unfit to leave any impress on sedimentary rocks. It has also been pointed out by Schimper[1103] that while deciduous leaves with fungi on them are abundant in fossil beds, lichens are entirely wanting. These latter are so firmly attached to the rocks or trees on which they grow that they are rarely dislodged, and form no part of wind- or autumn-fall. Trunks and branches of trees lose their bark by decay long before they become fossilized and thus all trace of their lichen covering disappears. The few records that have been made are here tabulated in chronological order: 1. PALAEOZOIC. Schimper decides that there are no records of lichens in the earlier epochs. Any allusions[1104] to their occurrence are held to be extremely vague and speculative. 2. MESOZOIC. Braun[1105] has recorded a _Ramalinites lacerus_ from the Keuper sandstone at Eckersdorf, though later[1106] he seemed to be doubtful as to his determination. One other lichen, an _Opegrapha_, has been described[1107] from the chalk at Aix. 3. CAINOZOIC. In the brown-coal formations of Saxony Engelhardt[1108] finds two lichens: _Ramalina tertiaria_, a much branched plant, the fronds being flat and not channelled “and of further interest that it is attached to a carbonized stem.” The second form, _Lichen dichotomus_, has a dichotomously branching strap-shaped frond. “There is sufficient evidence that these fronds were cylindrical and that the width is due to pressure. In one place a channel is visible, filled with an ochraceous yellow substance.” Other records on brown coal or lignite are: _Verrucarites geanthricis_[1109] Goepp., somewhat similar to _Pyrenula nitida_, found at Muskau in Silesia; _Opegrapha Thomasiana_[1110] Goepp., near to _Opegrapha varia_, and _Graphis scripta succinea_ Goepp.[1111] on a piece of lignite in amber beds, all of them doubtful.