A paper on 'The Blastopore and Spina Bifida,' and an occasional lecture on 'Old and New Theories of Development,' gave me the opportunity of dealing with Roux's mosaic theory, although that not only shows learning, but apparently is the outcome of experiment. I advocated in its place the theory that 'the embryological development of an organism is no mosaic work. The parts of an organism develop in relation to each other, the development of a part depending upon the development of the whole.' The labours of Roux, as well as the valuable researches of Driesch, induced me to carry out a series of experiments with the object of getting a surer basis for my epigenetic conception of development. The results of these were published recently under the title, _On the Value of the First Cleavage-cells in the Formation of the Organs of Embryos_. In the latter treatise I confined myself advisedly to the exposition and interpretation of the results of my investigations, having in view a subsequent discussion of the more theoretical bearings of my results. It is this that sees the light in the present book. As for many years I have occupied myself with the problem of development, pursuing observation and framing theory, there is due to myself and to others an exposition of the position I have assumed in many of my treatises, but in a more connected and elaborated fashion than has been possible hitherto. This course is the more imperative, as in his recent _magnum opus_ on the germplasm Weismann has propounded a theory of evolution wrought with the greatest care and acuteness, and totally irreconcilable with my conclusions. The chief differences between my views and those of Weismann have now become clearer and more tangible than ever. It is true that in my text-book, _On the Structure and Function of Cells_,[6] published in the autumn of 1892, I gave a short account of my theory of heredity in chapter ix., 'The Cell as the Material Beginning of the Organism.' But in that I could not deal with Weismann's work, which appeared simultaneously, and, moreover, in a text-book it was impossible to do more than sketch my views. My present task is twofold; it has both a positive and a negative side. First, I have to examine the arguments recently alleged in favour of the theory of preformation, testing them to reveal their inherent weaknesses, and to controvert their fallacies. As Weismann unquestionably is the chief of those who have advocated preformation, and has made a closed system of it again, it is necessary for me to take special notice of his conception as it is set forth in _The Germplasm_. Although I am no friend of polemic, the case demands it. For the decision of a question so momentous as the relative scopes of evolution and epigenesis in embryology must have an important bearing on the future of biology, upon its aim and the method of research.