Page:Popular Science Monthly Volume 28.djvu/274

264 orthoceratitic or straight stage, or an approximate form, at an early embryological period. In these shells, too, all varieties, by disease or accident, are clearly shown, and this has led to the well-supported theory brought forth by Professor Hyatt, that much of the evolution of Cephalopods has been directly caused by pathological conditions—accidents or diseases transmitted to successive individuals, until they become firmly established inherited variations, and lead, in the Cretaceous period, to the death of one of the four orders of cephalopods, the Ammonoidea.

Among the subjects upon which Professor Hyatt is at present at work is his "Theory of Cellular Tissues" just published. This contains his theory of the origin of sex, which is one of the most important that he has ever published. It is that the nuclei of cells are both male and female; that gradually in some cells one element, perhaps the male, might predominate, while in others the female would be strongest. Thus we would have the metazoa stage, where the male and female cells are distinct from each other, but still mixed in the same layer. This specialization goes on until in the coelenterates we find special layers of cells especially adapted to perform the function of male or female elements, and later, in the vertebrates, separate animals represent the separate elements. This theory he supports by many interesting facts. In the same paper he endeavors to show that sponges are intermediate between metazoa and protozoa. It is very well established that the body-cavity which immediately surrounds the stomach of all vertebrates is homologous to the sacs which spring out from the body-cavity of coelenterates, and Professor Hyatt's theory is that these sacs are homologous to the branching cavities and sacs which spring from the central cavity in sponges, and further that these fundamental structural modifications originated independently in sponges, in cœlenterates, and probably in vertebrates, from ancestors which never possessed any such characteristic.

The most useful work which Professor Hyatt has done, and that for which be deserves much credit, is in connection with popular science-teaching. His way of teaching is original, and intended to inspire the student with a love for natural history, by teaching him to look about for himself and observe what there is to see. His first interview and study with Louis Agassiz had much to do with shaping his course, and formed the basis of his system of teaching. Since this study has had such an important bearing upon his life, we reproduce the account of it in Professor Hyatt's own words. Professor Hyatt says: "He gave me a Pentacrinite, or stone-lily, a rather complex fossil, and told me to study it. This I thought to be easy work, so I took a stroll in the afternoon and thought little of it. Next morning he came up to my table and asked me what I had found, I had never studied from Nature before, and began giving a very general description, saying that it was a fossil petrifaction, etc., and had what appeared