_Lepidodendron_ and _Sigillaria_ can hardly be claimed as the direct ancestors of any existing type of Lycopodiales, but while exhibiting points of contact with _Lycopodium_, _Selaginella_, and _Psilotum_ they are perhaps more closely allied to _Isoetes_. Lady Isabel Browne[661], who has recently published an excellent summary of the evidence on the relation of the Lepidodendreae to _Isoetes_, concludes her examination of the arguments by expressing the opinion that there is a strong probability of the correctness of the view that _Isoetes_ may be derived “from the Lepidodendraceae in the widest sense of the word.” This decision seems to me to accord best with the facts. The further question as to the relation of these Palaeozoic genera to plants higher in the scale must be reserved for fuller consideration in another volume. An attempt will also be made to consider how far anatomical structure may be used as a guide to the conditions under which _Lepidodendron_ and _Sigillaria_ as well as other members of the Permo-Carboniferous floras passed their lives. The secondary xylem of _Lepidodendron_ and _Sigillaria_ affords a striking example of water-conducting tissue of homogeneous structure comparable with the wood of Conifers rather than with that of Angiosperms. It was presumably formed, for the most part, under uniform climatic conditions: the absence of rings of growth points to uninterrupted supply to evergreen shoots exposed to no alternation of activity and arrested growth. Attention has already been called to the absence of any tissue corresponding to secondary phloem. Even in young shoots of _Lepidodendron_, no tissue has been found external to the meristematic zone agreeing in the form of its elements with the channels through which the elaborated food is conveyed from the leaves of recent plants to the regions of cell-building. That the ‘secretory zone’ may have served this purpose, at least in young stems, is not improbable. On the other hand, it is difficult to understand why older Lepidodendron stems show no indication of additions to the secretory zone. If this tissue served for the transport of proteids we should expect to find provision made for its constant renewal _pari passu_ with the secondary growth of the xylem. The conclusion seems to me inevitable that the supply of building-material was otherwise provided for than in recent vascular plants. The physiological division of labour may have been less complete in the tissue-systems of the Palaeozoic Lycopods than in the more highly specialised organs of such an extinct genus as _Lyginodendron_ or than in recent plants. Our knowledge of the anatomical structure of many extinct types has already reached a stage when we should take greater heed of the _modus operandi_ of the complex machinery revealed by a study of petrified stems. From the known we proceed to interpret the unknown; but there is a danger of neglecting