Читать книгу The modes of origin of lowest organisms - Bastian Henry Charlton - Страница 3

Bacteria and Torulæ being already in existence, they may, undoubtedly, reproduce organisms similar to themselves by processes of fission and gemmation – in the same way that other low protistic organisms propagate their kind. Although so many reasons rendered this view probable, it was some time before I was able actually to confirm it by personal observations in the case of Bacteria. In the ordinary microscopical examination of portions of an infusion containing these organisms, an observer may watch for hours and never see a single instance of such fission occurring. His attention is apt to be distracted by the number of organisms which are constantly flitting before his view, and he is, moreover, perhaps apt to pay particular attention to those which seem by their movements to be most obviously alive.

I have observed the process most plainly when a few Bacteria have been enclosed in a single drop of fluid, pressed into a very thin stratum, in a “live-box” kept at a temperature of about 90° Fahr. by resting on one of Stricker’s warm-water chambers placed on the stage of the microscope. Under these conditions, I have seen a Bacterium of moderate size divide into two, and each of these into two others somewhat smaller, in the course of fifteen minutes.

It is still more worthy of remark, that in all cases (so far as I have been able to observe), this, the most certain sign of vitality which such organisms are capable of manifesting, is shown by those which, from their stillness, might be considered dead. The Bacteria which are about to divide are generally either motionless,1 or merely present slight oscillating movements. The separation is quickly brought about at the joint, so that the original organism divides into two equal portions; and these, lying close together, soon develop a new construction as they grow, through which a further division may occur.

That the Bacteria which reproduce should be in a comparatively quiescent condition, seems not difficult to understand. Such rudimentary organisms do not appear to possess cilia or other locomotory appendages: their movements are, therefore, in all probability dependent upon the mere molecular changes which are taking place within them, and upon which their life and nutrition depend. The process of fission must, however, be considered as the result of a new effort at equilibrium, which has, perhaps, been necessitated by molecular changes that have occurred during a preceding period of growth. The living matter which is no longer able to exist round a single centre, re-arranges itself around two centres, – as a result of which, fission occurs. It seems only natural, therefore, that whilst this active work of molecular re-arrangement is going on, those other molecular movements which occasion the actual locomotion of the organism from place to place, should be more or less interfered with.

This is the one and only mode of multiplication of Bacteria and of Torulæ which is actually known to occur; and such a limitation is in accordance with the more general fact, that processes of fission or gemmation are the only means of reproduction that are known to occur in the lower kinds of organisms, belonging to the PROTISTIC kingdom.

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 Hallier2 has endeavoured to establish; it is also the doctrine of M. Polotebnow,3 and one, moreover, to which Professor Huxley4 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.

Now, as a matter of fact, worthy of arresting our attention, we do find that some Bacteria are capable of growing into Leptothrix, whilst this is able to develop continuously into a fungus; just as we also know that some Torulæ are capable of growing into other fungi.

Should it be established, therefore, that Bacteria and Torulæ are capable of arising de novo, the facts concerning their mutability are harmonious enough with theoretical indications.

But, as I have before indicated, although it is quite true that some Bacteria develop into fungi, such forms may constitute no necessary links in the life-history of other fungi. I have suggested that in those (occasional) cases in which they do occur as links in the life-history of fungi, there is room for doubt whether these Bacteria are to be considered as normal products, or as abnormal results (heterogeneous offcasts), brought about by some unusual conditions acting upon the parent fungus. That is to say, we may be doubtful whether in such a case their origin ought to be considered Homogenetic or Heterogenetic. It may be that many of the lower fungi are such changeable organisms, and so prone to respond to the various “conditions” acting upon them (which would be almost certainly the case if they had been developed from a Bacterium in two or three days – the Bacterium itself having been evolved de novo) that no very valid distinction can here be drawn between Homogenesis and Heterogenesis. Our whole point of view, in fact, concerning such fungi as are seen to develop through Leptothrix forms from Bacteria must be entirely altered, if it is once conceded that Bacteria may arise de novo. Such simple Mucedineæ would then have to be regarded as mere upstart organisms only a few removes from dead matter, and – in view of the greater molecular mobility of living matter – capable of being modified in shape and form even more than the most changeable crystals under the influence of altering “conditions.” We should have no longer to do with the members of a stable species, which had been reproducing its like through countless geologic ages anterior to the advent of man upon the earth. Indeed, in order to reconcile such a possibility with the seemingly contradictory fact of the known extreme changeability of these lower forms of life, we hear only vague hints thrown out about our imperfect knowledge of the “limits within which species may vary.” As if, in the face of what we do know concerning hereditary transmission, this changeability did not make it almost impossible to conceive that there should have been an unbroken series of such organisms since that remote epoch of the earth’s history, when the first organisms of the kind made their appearance. It does not seem to me that the presumed permanence of a very changeable organism is consistent with, or rendered more explicable by, the supposition that some representatives of the species have constantly been undergoing progressive modifications which have been successively perpetuated by inheritance, in the shape of distinct specific forms. Why should some be presumed to have undergone so much change, whilst others (presenting an equal and an extreme degree of modifiability, even to the present day) are supposed to have preserved the same specific form through a countless series of changing influences?