“Whilst comparative anatomy,” said Johannes Müller, in 1844, in his lectures upon this science (and the opinions of my memorable teacher were for many years my own), “whilst comparative anatomy shows us the infinitely multifarious formation of the same organ in the Animal Kingdom, it furnishes us at the same time with the means, by the comparison of these various forms, of recognising the truly essential, the type of these organs, and separating therefrom everything unessential. In this, developmental history serves it as a check or test. Thus, as the idea of development is not that of mere increase of size, but that of progress from what is not yet distinguished, but which potentially contains the distinction in itself, to the actually distinct,—it is clear, that the less an organ is developed, so much the more does it approach the type, and that, during its development, it more and more acquires peculiarities. The types discovered by comparative anatomy and developmental history must therefore agree.” Then, after Johannes Müller has combated the idea of a graduated scale of animals, and of the passage through several animal grades during development, he continues:—“What is true in this idea is, that every embryo at first bears only the type of its section, from which the type of the Class, Order, etc., is only afterwards developed.” In 1856, in an elementary work,[3] in which it is usual to admit only what are regarded as the assured acquisitions of science, Agassiz expresses himself as follows:—