Leuckart finds such a great resemblance between the newly born young of _Distoma_ and the Orthonectides, that he is inclined to believe that the latter are Trematodes, ‘which in spite of sexual maturity have not developed further than the embryonic condition of the _Distoma_’ (‘Zur Entwicklungsgeschichte des Leberegels,’ Zool. Anzeiger, 1881, No. 99). In reference to the Dicyemidae, which resemble the Orthonectides in their manner of living and in their structure, Gegenbaur has stated his opinion that they belong to a ‘stage in the development of Platyhelminthes’ (Grundriss d. vergleich. Anatomie). Giard includes both in the ‘phylum Vermes,’ and regards them as much degenerated by parasitism; and Whitman—the latest investigator of the Dicyemids—speaks of them in a similar manner in his excellent work ‘Contributions to the Life-history and Classification of Dicyemids’ (Leipzig, 1882). Footnote 79: ‘Dauer des Lebens;’ translated as the first essay in this volume. Footnote 80: See the first essay upon ‘The Duration of Life,’ p. 22 et seq. Footnote 81: ‘Ursprung des Todes,’ p. 29. Footnote 82: l. c., p. 5. Footnote 83: See the preceding essay ‘On Heredity.’ Footnote 84: The problem is very easily solved if we seek assistance from the principle of panmixia developed in the second essay ‘On Heredity.’ As soon as natural selection ceases to operate upon any character, structural or functional, it begins to disappear. As soon, therefore, as the immortality of somatic cells became useless they would begin to lose this attribute. The process would take place more quickly, as the histological differentiation of the somatic cells became more useful and complete, and thus became less compatible with their everlasting duration.—A. W. 1888. Footnote 85: See the preceding essay ‘On Heredity.’ Footnote 86: See the first essay on ‘The Duration of Life.’ Footnote 87: See the first essay on ‘The Duration of Life.’ Footnote 88: These assumptions can be authenticated among the Infusoria. The encysted _Colpoda cucullus_, Ehrbg. divides into two, four, eight, or sixteen parts; _Otostoma Carteri_, into two, four, or eight; _Tillina magna_, Gruber, into four or five; _Lagynus_ sp. Gruber, into two; _Amphileptus meleagris_, Ehrbg. into two or four. The last two species and many others frequently do not divide at all during the encysted condition. But while any further increase in the number of divisions within the cyst does not occur in free-swimming Infusoria, the interesting case of _Ichthyophthirius multifiliis_, Fouquet, shows that parasitic habits call forth a remarkable increase in the number of divisions. This animal divides into at least a thousand daughter individuals. Footnote 89: