FOUNDATION OF A THEOKY OF HEEEDITY. 201 



of the idioplasm of this cell into germ-plasm. But this latter 

 transformation is just the point which most needs proof upon any 

 theory except the one which assumes that the primitive germ-cell 

 still contains unchanged germ-plasm. Every attempt to render 

 probable such a re-transformation of somatic nucleoplasm into germ- 

 plasm breaks down before the facts known of the Hydroids, in 

 which only certain cells in the body, out of the numerous so-called 

 embryonic cells, are capable of becoming primitive germ-cells, while 

 the rest do not possess this power. 



I must therefore consider as erroneous the hypothesis which 

 assumes that the somatic nucleoplasm may be transformed into 

 germ-plasm. Such a view may be called ' the hypothesis of the 

 cyclical development of the germ-plasm.' 



Nageli has tried to support such an hypothesis on phyletic 

 grounds. He believes that phyletic development follows from an 

 extremely slow but steady change in the idioplasm, in the direction 

 of greater complexity, and that such changes only become visible 

 periodically. He believes that the passage from one phyletic stage 

 to another is chiefly due to the fact that ' in any ontogeny, the 

 very last structural change upon which the separation of germs 

 depends, takes place in a higher stage, one or more cell-generations 

 later ' than it occurred in a lower stage. ' The last structural 

 change itself remains the same, while the series of structural 

 changes immediately preceding it is increased.' I believe that 

 Nageli, being a botanist, has been too greatly influenced by the 

 phenomena of plant-life. It is certainly true that in plants, and 

 especially in the higher forms, the germ-cells only make their 

 appearance, as it were, at the end of ontogeny; but facts such 

 as these do not hold jn the animal kingdom: at any rate they 

 are not true in the great majority of cases. In animals, as I have 

 already mentioned several times, the germ-cells are separated from 

 the somatic cells during embryonic development, sometimes even at 

 its very commencement ; and it is obvious that this latter is the 

 original, phyletically oldest, mode of formation. The facts at our 

 disposal indicate that the germ-cells only appear, for the first time, 

 after embryological development, in those cases where the forma- 

 tion of asexually produced colonies takes place, either with or with- 

 out alternation of generations ; or in cases where alternation of 

 generations occurs without the formation of such colonies. In 



