He obtained a much larger series of Planorbis than Hilgendorf bad, and was obliged to prove that, although there was a general gradation from the flattened species to the spiral, through many intermediate forms, it was not true that the series of species succeeded each other in time, as claimed by Hilgendorf. All the species, in all their curious modifications, were found together in the lower stratum. Theoretically a graded series was traceable; and no doubt the flattened spiral forms were the ancestors of the more conical spiral forms, lie also pointed out the marked resemblance between diseased and wounded individuals of a species, and the degraded form and the correlations of these with the transformations taking place in the old age of other and healthier species of the same group. He attributes his result to the use of mechanical methods. The shells were gathered in bags, carefully labeled, from each stratum, taken home, sifted through graded sieves constructed for the purpose, and every specimen, to the number of several hundreds in each bag, was thus necessarily passed through his hands. Professor Richard Owen, the eminent anatomist. Director of the British Museum, has said of this memoir, "It is a model of the mode in which such researches should be conducted." Besides these, Professor Hyatt is the author of many smaller papers upon nearly all subjects relating to natural history, and he has described many new genera of cephalopods.
Professor Hyatt has discovered that evolutionary changes in general were much more rapid in earlier ages than now, and could be compared closely with the isolated cases of very rapid evolution of forms in such limited localities occurring in later times, as at Steinheim. For instance, in the Silurian period there was a continual struggle for better adaptation to the environment. In other Paleozoic ages, also, evolution must have been rapid to have accounted for the observed changes. It must have been particularly rapid immediately after the groups or individuals originated, and thus should be represented as expanding suddenly from their point of origin, like the spokes of an expanded fan. He further believes that evolution of Cephalopoda has taken place both by progression and retrogression, in four branches. From the straight orthoceratic forms all fossil and modern Cephalopoda have descended. To use his own words: "The efforts of the Orthoceratite to adapt itself fully to the requirements of a mixed habitat gave the world the Nautiloidea; the efforts of the same type to become completely a littoral crawler developed the Animonoidea. The successive forms of the Belemnoidea arose in the same way; but here the ground-swimming habitat and complete fitness, for that was the object, whereas the Sepoidea represent the highest aims, as well as the highest attainments, of the Orthoceratitcs, in their surface-swimming and rapacious forms." No better group for the study of evolution is found in fossiliferous beds, for in the shells every step of growth can be traced, and it can be seen that the coiled forms all go through the
