He found in Kovalevsky a young man with shy but cordial manners and the clear sweet eyes of a pure child, obviously an idealist. He had for science an absolute cult, the sacred fire of the worshipper; no sacrifice was too great, no difficulty too repellent for his ardour. On a closer acquaintance, the small, timid young man proved to be a hard fighter where science was concerned. The two young men formed an excellent impression of each other, and a friendship was started between them which was to last a lifetime. Though very different from each other, they met on common ground, a passion for science. They worked with the greatest energy, going together on zoological excursions, exchanging their ideas, discussing their aspirations; a similarity of tastes lent great attraction to their friendship. At Giessen, Elie had read Fritz Müller's _For Darwin_, a book which had a decisive influence on the future direction of his researches. Fritz Müller, in his embryological works on certain crustaceans, had been the first to confirm in a concrete manner Darwin's evolutionist theories; he had thus demonstrated that it was chiefly in embryology that precious indications were to be found concerning the genealogy of organisms.[9] Under the influence of this work, Elie, who until now had limited himself to introductory researches, resolved to concentrate all his efforts on the comparative embryology of animals. He started to work in that direction, and his researches confirmed him more and more in the opinion that the key of animal evolution and genealogy was to be sought for in the most primitive stages, in those simple phases of development where no secondary element has yet been introduced from external conditions. In those primordial stages, essential characters, common to all, reveal the analogy and connections between animals from different groups. [9] In later years Metchnikoff often dwelt on the fact that Fritz Müller was not fully appreciated and that it was he who had most efficaciously contributed to the confirmation of Darwinian theories.