the vertebrates, for in that case both would be derived from the same stock.
Here then, contrary to all our preconceived ideas, was a new solution of an old and very important problem, probably the most important one before the morphologist since Darwin's time. It was evident that this solution of it, if sustained, would lead to more radical changes in the classification of the animal kingdom than any that have been made since the time of Cuvier and Lamarck. Stated concisely, it was as follows: At some time toward the close of the Cambrian period the sea-scorpions probably gave rise to the ostracoderms, and the latter,
Fig. 2. Semi-diagrammatic Figures of a Sea Scorpion (Eurypterid) and an Ostracoderm (Bothriolepis), showing the sequence in the location of the important functions, and the subdivisions of the body corresponding to the subdivisions of the head and brain in the higher vertebrates.
during the Silurian, to the fishes, or first true vertebrates (Fig. 3). This was an entirely new interpretation in phylogeny, but it was not inherently improbable, or contrary to any established facts; indeed, the first demands of the theory were in full accord with the known facts of anatomy, embryology and paleontology. Let us state it again in this way: In their fundamental structure, living arachnids resemble
