Resemblances and differences between the forms of the leaves alone must naturally appear to the botanist of the present day to be a very inadequate basis for a general system of classification. Nevertheless Mathias de l’Obel worked out a scheme on these lines which had great merit, and was a considerable advance on previous efforts. He put forward his system in his ‘Stirpium adversaria’ (1570-71) and used it also in his later work. It was thus published much earlier than the very primitive schemes of d’Aléchamps and Dodoens to which we have just referred. The best point of his system is that, by reason of their characteristic differences of leaf structure, he distinguishes the classes now known to us as Monocotyledons and Dicotyledons. He introduces a useful feature in the shape of a synoptic table of species which precedes each more or less natural group of plants. The superiority of his classification to the other arrangements in the field at the time was immediately realised. We have evidence of this in the fact that, after his ‘Kruydtbœck’ was published, Plantin brought out an album of the wood-engravings used in the book, which, although they had also appeared as illustrations to the works of Dodoens and de l’Écluse, were now arranged as in the scheme put forward by de l’Obel, “according to their genus and mutual relationship[32].” There seems little doubt that de l’Obel made a more conscious effort than any of his predecessors to arrive at a natural classification, and that he realised that such a classification would reveal a unity in all living beings. In the preface to his ‘Stirpium adversaria nova’ of 1570 he writes—“For thus in an order, than which nothing more beautiful exists in the heavens or in the mind of a wise man, things which are far and widely different become, as it were, one thing.” De l’Obel’s scheme is not expressed in the clear manner to which we have become accustomed in more modern systems, because, in common with other botanists of his time, he did not, as a rule, give names to the groups which we now call _orders_, or draw any sharp line of distinction between them.