In order to prove, however, that the _Bacteria_ which happen to go through this development into _Leptothrix_ and thence into fungi, are strictly to be considered as _necessary_ links in the life-history of fungi, it would be essential for the person holding such views, to show that _Bacteria_ could not arise independently--or at least that no independently evolved _Bacteria_ could develop through _Leptothrix_-forms into a fungus. And, similarly, for the other kinds of organisms: in order to establish that the _Torula_ cell is a necessary link in the life-history of certain fungi, or the gonidial cell a necessary link in the life-history of lichens, it would be necessary to show that _Torulæ_ or gonidial cells could not originate _de novo_--that no independently evolved _Torula_ or gonidial cell could develop into a fungus or a lichen. An easier position to establish would be, that the _Bacterium_ or the _Torula_ were _occasionally_ links in the life-history of fungi, or that the gonidial cell was an occasional link in the life-history of a lichen. This doctrine would leave the other more difficult problems,--as to the possible existence of supplementary modes of origin for such organisms by Heterogenesis or by Archebiosis--perfectly open questions. To establish the position that _Bacteria_ are occasional links in the life-history of fungi, it would be only necessary to show that some of the _Bacteria_ which develop into fungi through _Leptothrix_ have derived their origin from pre-existing fungi. This is the view which Hallier[2] has endeavoured to establish; it is also the doctrine of M. Polotebnow,[3] and one, moreover, to which Professor Huxley[4] inclines. Even this mode of origin for _Bacteria_, however, has not been so decisively established as might be desired. With regard to _Torulæ_, we do possess sufficient evidence tending to show that some of them may arise from pre-existing fungi, and we are equally certain that some gonidial cells are thrown off from lichens. The analogical evidence is, therefore, in favour of the view that minute particles which are budded off from the mycelium of certain fungi, may subsequently lead an independent existence, and multiply in the form of _Bacteria_--although many of the cases in which such buds _seem_ to be given off, may be merely cases in which co-existing _Bacteria_ have become adherent to fungus filaments or to _Torulæ_.[5] [2] Phytopathologie, 1867. Hallier seems, however, strongly inclined to disbelieve in the origin of these organisms by Heterogenesis or by Archebiosis. [3] Sitzungsber. der K. Akad. zu Wien, 1870, Band lx., Heft iv. [4] Quarterly Journal of Microscopical Science, Oct., 1870.