For the details of the various types of alternations of generations, and their origin, the reader is referred to the body of the work; but a few general remarks on the nature and origin of the process, and on its nomenclature, may conveniently be introduced in this place. The simplest cases are those in which an individual which produces by sexual means gives origin to asexual individuals differently organised to itself, which produce by budding the original sexual form, and so complete a cycle. Instances of this kind are supplied by the Hydrozoa, Annelida and Tunicata. In the case of the Tunicata (Doliolum) two different asexual generations may be interpolated between the sexual generations. In all these cases the origin of the phenomenon is easily understood. It appears, as is most clearly shewn in the case of the Annelida, that the ancestors of the species which now exhibit alternations of generations originally reproduced themselves at the same time both sexually and by budding, though probably the two modes of reproduction did not take place at the same season. Gradually a differentiation became established, by which sexual reproduction was confined to certain individuals, which in most instances did not also reproduce asexually. After the two modes of reproduction became confined to separate individuals, the dissimilarity in habits of life necessitated by their diverse functions caused a difference in their organization; and thus a complete alternation of generations became established. The above is no merely speculative history, since all gradations between complete alternations of generations and simple budding combined with sexual reproduction can be traced in actually existing forms. The alternation of generations as it is found amongst the Entoparasitic Trematodes and most Cestodes, is to be explained in a slightly different way. It appears that in these parasitic forms a complicated metamorphosis first arose from the parasite having to accommodate itself to the different hosts it was compelled to inhabit, owing to the liability of its primitive and subsequent hosts to be devoured[9]. A capacity for asexual multiplication--obviously of immense advantage to a parasite--appears to have been acquired in some of the stages of this metamorphosis, and an alternation of generations thus established. [9] The appearance of Vertebrata on the globe as the forms which most frequently preyed on Invertebrate forms, and were themselves not so liable to be devoured, has no doubt had a great influence on the metamorphosis of internal parasites, and has amongst other things resulted in these parasites usually reaching their sexual state in a vertebrate host.