[16] I speak here of determinants, not of groups of determinants, which is the more correct expression, merely for the sake of brevity. It is a matter of course that a whole extremity, such as we have here chosen, cannot be represented in the germ by a single determinant only, but requires a large group of determinants. [17] That this is not so in all cases has recently been shown by Dixey from observations on certain white butterflies of South America which mimic the Heliconids and in which a small, yellowish red streak on the under surface of the hind wing has served as the point of departure and groundwork of the development of a protective resemblance to quite differently colored Heliconids. "On the Relation of Mimetic Characters to the Original Form," in the _Report of the British Association for 1894_. [18] Oscar Hertwig, _Zeit-und Streitfragen der Biologie_, Jena, 1894. It is customary now to look upon the preformation-theory of Bonnet as a discarded monstrosity, and on the epigenesis of K. F. Wolff as the only legitimate view, and to draw a parallel between these two and what might be called to-day "evolution" [i. e. unfoldment] and epigenesis. The evolution, or unfoldment, of Bonnet and Harvey, however, was something totally different from modern doctrines of evolution, and Whitman is quite right when he says that even my theory of determinants would have appeared to the inquirers of the last century as "extravagant epigenesis." Biologists in that day were concerned with quite different questions from what they are at present, and although now we probably all share the conviction of Wolff that new characters do arise in the course of evolution, yet the acceptance of this view is far from settling the question _as to how these new characters are established in the germ-substance_--for in this substance they certainly must have their foundation. When, therefore, O. Hertwig laments over my regarding evolution and not epigenesis as the correct foundation of the theory of development, his sorrow is almost as naive as is the statement of Bourne that epigenesis is a fact and not a theory "a statement of morphological fact," _Science Progress_, April, 1894, page 108), or, as is the latter's unconsciousness that facts originally receive their scientific significance from thought, i. e. from their interpretation and combination, and that thought is theory. And when S. Minot, as the leader of the embryologists, carries his zeal to the pitch of issuing a general pronunciamento against me as a corruptor of youth, in which he declares it to be a "scientific duty to protest in the most positive manner against Weismann's theory," I wonder greatly that he does not suggest the casting of a general ballot in the matter. (See the _Biologisches Centralblatt_ of August 1, 1895.) We see how with these gentlemen the wisdom of the recitation-room regarding the infallibility of epigenesis has grown into a