any intra-germinal causes of variation in common with them--that is to say, they will tend to vary even later in a similar manner. For instance, we might think of the segments of primitive Annelids, which in form and contents are for the most part alike, as arising from one germ-rudiment, from which, when, in the higher Annelids, the various regions of the body had to take a different form, several primary constituents of the germ-plasm separated themselves off; and in a similar way the much higher and more complex differentiation of the somatic segments in the Crustaceans must have been brought about. Thus we understand how the determinant groups of the germ-plasm multiplied according to the need for increasing differentiation, but remained in intimate relation, which exposed them in some measure to a common fate, that is, to common modifying influences, and in many cases determined them to similar variation. But we cannot see directly into the germ-plasm, and are therefore thrown back on the inductions we can make from the facts presented to us by the phenomena of visible living organisms. As yet the material for such inductions is scanty, because it has been got together haphazard, and not collected on a definite plan. I therefore refrain for the present from attempting any further elaboration of my germ-plasm theory. It is only when an abundance of observation material, collected according to a definite plan, lies at our disposal that anything more in regard to the intimate structure of the germ-plasm, or the mutual influences and relations of its determinants and its modification in the course of phylogeny can be deduced with any certainty. Meanwhile, we must content ourselves with having, through the hypothesis of determinants, made intelligible at least the one fundamental fact, how it is possible that in the course of the phylogeny single parts and single stages can be thrown out or interpolated, or even only caused to vary, without giving rise to variation in all the rest of the parts and stages of the animal. A theory of epigenesis cannot do this, for, if no representative particles were contained in the germ-plasm, then every variation of it would affect the whole course of development and every part of the organism, and variations of individual parts arising from the germ would be impossible. LECTURE XXVIII THE GENERAL SIGNIFICANCE OF AMPHIMIXIS