"Notwithstanding these striking differences," says the author in the opening of the fifth chapter on the relations of fishes in general, "it is none the less evident to the attentive observer that one single idea has presided over the development of the whole class, and that all the deviations lead back to a primary plan, so that even if the thread seem broken in the present creation, one can reunite it on reaching the domain of fossil ichthyology."* (* Volume 1 chapter 5 pages 92, 93.) Having shown how the present creation has given him the key to past creations, how the complete skeleton of the living fishes has explained the scattered fragments of the ancient ones, especially those of which the soft cartilaginous structure was liable to decay, he presents two modes of studying the type as a whole; either in its comparative anatomy, including in the comparison the whole history of the type, fossil and living, or in its comparative embryology. "The results," he adds, "of these two methods of study complete and control each other." In all his subsequent researches indeed, the history of the individual in its successive phases went hand in hand with the history of the type. He constantly tested his zoological results by his embryological investigations. After a careful description of the dorsal chord in its embryological development, he shows that a certain parallelism exists between the comparative degrees of development of the vertebral column in the different groups of fishes, and the phases of its embryonic development in the higher fishes. Farther on he shows a like coincidence between the development of the system of fins in the different groups of fishes, and the gradual growth and differentiation of the fins in the embryo of the higher living fishes.* (* "Recherches sur les Poissons Fossiles", volume 1 chapter 5 page 102.) "There is, then," he concludes, "as we have said above, a certain analogy, or rather a certain parallelism, to be established between the embryological development of the Cycloids and Ctenoids, and the genetic or paleontological development of the whole class. Considered from this point of view, no one will dispute that the form of the caudal fin is of high importance for zoological and paleontological considerations, since it shows that the same thought, the same plan, which presides to-day over the formation of the embryo, is also manifested in the successive development of the numerous creation which have formerly peopled the earth." Agassiz says himself in his Preface: "I have succeeded in expressing the laws of succession and of the organic development of fishes during all geological epochs; and science may henceforth, in seeing the changes of this class from formation to formation, follow the progress of organization in one great division of the animal kingdom, through a complete series of the ages of the earth." This is not inconsistent with his position as