Modern biologists ridicule the preformation hypothesis as altogether too artificial. And undoubtedly it founders on the facts of embryology, which disclose nothing to suggest the unfolding of a pre-existent miniature model, but show us how the egg-cell divides into two, into four, and so on, with continued multiplication followed by varied arrangements and rearrangements of cells—in short, all the complex changes which constitute development. But a preformation in some sense or other there must be;—some peculiar material predisposition of the germ, which, as such, supplies the directing principle for the development, and the sufficient reason for the repetition of the parental form. This is of such obvious importance from the mechanical point of view that the speculations of to-day tend to move along the old preformationist lines. To these modern preformationists are opposed the modern upholders of epigenesis or gradual differentiation, who attempt to elaborate a mechanical theory of development. And with the contrast between these two schools there is necessarily associated the discussion as to the inheritance or non-inheritance of acquired characters. Darwin’s contribution to the problem of the sixth order was his rather vague theory of “Pangenesis.” The living organism, according to him, forms in its various organs, parts, and cells exceedingly minute particles of living matter (gemmules), which, “in some way or other,” bear within them the special characteristics of the part in which they are produced. These may wander through the organism and meet in the germ-plasm, and then, when a child-organism is produced, they “swarm,” so to speak, in it again “in some way or other,” and in some fashion control the development. This gemmule-theory was too obviously a _quid pro quo_ to hold its ground for long. Various theories were elaborated, and the world of the invisibly minute was flooded with speculations.