True development also takes place in the above-mentioned _Ichthyophthirius_. While in other Infusoria the products of fission exactly resemble the parent, in _Ichthyophthirius_ they have a different form; the sucking mouth is wanting while provisional clasping cilia are at first present. In this case therefore the word germ may be rightly applied, and _Ichthyophthirius_ affords an interesting example of the phyletic origin of germs among the lower Flagellata and Gregarines. Cf. Fouquet, ‘Arch. Zool. Expérimentale,’ Tom. V. p. 159. 1876. Footnote 90: Bütschli, long ago, doubted the application of the fundamental law of biogenesis to the Protozoa (cf. ‘Ueber die Entstehung der Schwärmsprösslings der Podophrya quadripartita,’ Jen. Zeit. f. Med. u. Naturw. Bd. X. p. 19, Note). Gruber has more recently expressed similar views, and in fact denies the presence of development in the Protozoa, and only recognizes growth (‘Dimorpha mutans, Z. f. W. Z.’ Bd. XXXVII. p. 445). This proposition must however be restricted, inasmuch as a development certainly occurs, although one which is coenogenetic and not palingenetic. Footnote 91: See the first essay on ‘The Duration of Life,’ p. 23 _et seq._ Footnote 92: See Appendix to the first essay on ‘The Duration of Life,’ pp. 43-46. Footnote 93: See the first essay on ‘The Duration of Life,’ p. 21. ------------------------------------------------------------------------ IV. THE CONTINUITY OF THE GERM-PLASM AS THE FOUNDATION OF A THEORY OF HEREDITY. 1885. ------------------------------------------------------------------------ CONTINUITY OF THE GERM-PLASM, &c. PREFACE.