AS if in atonement for a prolonged neglect, the study of the organisation of fossil plants is now receiving wide-spread attention. The task first undertaken by Henry Withan has now been shared by many observers. The result is that we already possess a much more complete acquaintance with the ancient vegetation of the globe than we did even a few years ago. But whilst this is undoubtedly true, it is equally so that wide differences of opinion on important points still exist amongst those who have taken a leading part in this investigation. Thus, M. Brongniart and Dr. Dawson believe that the Sigillariæ were Gymnospermous Exogens; whilst Mr. Carruthers and myself are convinced that they were Lepidodendroid Cryptogams. In common with Prof. Schimper and Mr. Carruthers, I regard the whole of the Calamites as Cryptogamic plants, having Equisetaceous affinities; whilst M. Brongniart, M. Grand-Eury, and, perhaps partially, Dr. Dawson, deem some of them to be Equisetaceans, and others Gymnospermous Exogens. Mr. Carruthers and Mr. Binney regard the fruits known by the name of Volkmannia Binneyi to be the cones of Calamites. On the other hand, whilst I do not deny that such may possibly be their nature, I contend that we have neither proof nor even probable evidence sustaining this idea. Dr. Dawson says that Asterophyllites and Annularia are very distinct plants. Mr. Carruthers affirms that they are not. M. Grand-Eury and myself contend that Asterophyllites is wholly distinct, both in type and organisation, from Calamites. Mr. Carruthers believes that Asterophyllites and Annularia are alike the foliage of Calamites. It would be easy to multiply illustrations proving the existence of these opposite conclusions on important points amongst those observers who have enjoyed the best opportunities of forming reasonable opinions on such subjects. It is sufficiently obvious that some of us must be in error on these questions; possibly each of us is so in a greater or less degree; but when we regard the scientific status of the observers to whom I have referred, leaving myself out of the question, I ask, are they men whom we can accuse lightly of carelessness or ignorance? Must we not rather infer that each man has observed special facts leading him to his own conclusions, and that what we want is a careful comparison of such facts with those which have led our fellow-labourers in an opposite direction to ourselves? Whatever may be the explanation of these discrepant opinions, surely our mutual duties are clear. If any of us thinks that his fellow-labourer has made mistakes (and who has not) let him say so openly, and not suggest the idea by indulging in deprecatory insinuations. Let his opposing argumentam be ad rem, and not ad hominem. Further, let it be ad rem and not ad alteram rem. Let it not rest upon mere analogies, which may or may not be sound. Let us not reject a conclusion before we know all the facts from which it is drawn, merely because we think we have reason to deem it an impossible one. We have all lived to see many such conclusions take their places as undoubted truths.