At an earlier date Haeckel had made an attempt in this direction in his theory of the “perigenesis of the plastidules.” Peculiar states of oscillation and rhythm in the molecules of the germ-substance, handed on to it from the parent organism and transferable to all the assimilated matter of the offspring, represent, according to this theory, the principle which impels development to follow a particular course corresponding to the type of the parents. This was a _physical_ way of interpreting the matter. Other investigators have given a _chemical_ expression to their theoretical schemes for explaining heredity. Haacke declares both these to be unsatisfactory, and replaces them by morphological formative principles. It is the _structure_ of the otherwise homogeneous living matter that explains morphogenesis and inheritance. Minute “gemmæ,” homogeneous fundamental particles of living substance, not to be compared to or confused with Darwin’s “gemmules,” are aggregated in “Gemmaria,” whose configuration, stability, symmetrical or asymmetrical structure, and so on, are determined by the relative positions of the gemmæ to each other, and these in their turn control the organism and give it a corresponding symmetrical or asymmetrical, a firmly or loosely aggregated structure. The completed organism then forms a system in organic equilibrium, which is constantly exposed to variations and influences due to external causes (St. Hilaire), and to use and disuse of organs (Lamarck). These influences affect the structure of the gemmaria, and as the germ-cells consist of gemmaria, like those of the rest of the organism, the possibility of the transmission of acquired new characters is self-evident. The importance of correlated growth and orthogenesis is explained on a similar basis, and the Darwinian conceptions of the independent variation of individual parts, of the exclusive dominance of utility, of the influence of the struggle for existence in regard to individual selection, and of the omnipotence of natural selection, are energetically denied. Oscar Hertwig,(68) de Vries, Driesch(69) and others attempt to reconcile the preformationist and the epigenetic standpoints, and “to extract what is good and usable out of both.” Hertwig and Driesch, however, can only be mentioned with reservations in this connection. We cannot better sum up the whole tendency of the construction of mechanical theories on these last lines than in the words of Schwann: “There is within the organism no fundamental force working according to a definite idea; it arises in obedience to the blind laws of necessity.”