However well this process of fission may have been established, as a frequent mode of reproduction of Bacteria, such a fact does not lend any support to the notion that these are necessarily distinct and independent organisms. Torulæ (of which beer-yeast is the most familiar example) may similarly undergo this process of mere vegetative repetition to an indefinite extent, whilst only some of the products develop into fungi. The gonidia of lichens may also reproduce indefinitely in this fashion, and only some of the products of multiplication may go on to the production of lichens similar to that from which the gonidia had been derived.
It is a fact, however, admitted by many, and which any patient microscopist is capable of verifying for himself, that some Bacteria do develop into Leptothrix filaments, and that these are capable of passing into a dissepimented mycelial structure of larger size and undoubtedly fungus nature—from which fructification of various kinds may be produced. Some Bacteria may therefore develop into some fungi, just as certainly as some Torulæ may develop into other fungi, or, just as surely as some multiplying gonidia may develop into lichens.
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]
But, with reference to these supposed cases of budding, and also to those others in which the contents of a spore or sporangium break up into what Professor Hallier calls “micrococci” (which are generally incipient Bacteria), it would be difficult for us to decide whether such processes are normal or abnormal. When we have to do with such organisms, in fact, there may be the nicest transitions between what is called Homogenesis, and what, when occurring in other organisms, we term Heterogenesis. It may be that the production of such “micrococci” from the spore or sporangium of the fungus is not an invariable incident in the life-history of the species, but rather an occasional result of the influence of unusual conditions, or of failing vigour on the part of the organism. In this latter case we should have to do with a process of Heterogenesis; although, as I have just stated, in respect to such low and changeable organisms, scarcely any distinct line of demarcation can be drawn between Homogenesis and Heterogenesis.
The evidence seems, therefore, against the notion that Bacteria or Torulæ are ordinary, independent living things, which merely reproduce their like.
That some Bacteria are produced from pre-existing Bacteria, just as some Torulæ are derived from pre-existing Torulæ, may, it is true, be considered as settled. But, so far as we have yet considered the subject, there may be just as good evidence to show that Bacteria and Torulæ are capable of arising de novo, as there is that some of them are capable of developing into fungi.
If this were the case, such types could only be regarded as the most common forms assumed by new-born specks of living matter; and, by reason of their origin—which would entail an absence of all hereditary predisposition—they might be supposed to be capable of assuming higher developmental forms.