190 THE EVOLUTION THEOEY 



say anything more precise and definite than that in the meantime. 

 I should like, however, to say that determinants or groups of deter- 

 minants which had in old ancestral germ-plasms to give rise to a series 

 of quite similar structures by multiplication during the ontogeny, and 

 therefore only needed to 1)6 present singly in the germ-plasm, would, 

 in later descendants, have to shift their nniltiplication back into 

 the germ-plasm itself, if necessity required that the homologous parts 

 which they controlled should become different from each other. Then 

 the previously single group of determinants in the germ-plasm would 

 have to become multiple. But as new determinants can only arise from 

 those which already exist, these new ones must have had their place 

 beside the old, and would therefore probably be exposed to 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 con- 

 stituents 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 

 nuiltiplied 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 there- 

 fore 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 elabora- 

 tion 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 deter- 

 minants 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 



