264 THE POPULAR SCIENCE MONTHLY. 



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 Cre- 

 taceous 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 con- 

 tains his theory of the origin of sex, which is one of the most impor- 

 tant that he has ever j^ublished. It is that the nuclei of cells are 

 both male and female ; that gradually in some cells one element, per- 

 haps 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 ccelenterates 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 w^hich spring 

 out from the body-cavity of ccelenterates, and Professor Hyatt's the- 

 ory 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 ccelenterates, 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 sci- 

 ence-teaching. His way of teaching is original, and intended to in- 

 spire 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 descrip- 

 tion, saying that it was a fossil petrifaction, etc., and had wliat appeared 



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