The third mode of origin is what is called Heterogenesis; whilst the first and second modes are the representatives of more familiar processes, included under the head of Homogenesis. Thus, in accordance with the first view, Bacteria may be regarded as low organisms having a distinct individuality of their own and multiplying by a process of fission—thus affording instances of what I propose to term direct Homogenesis. Whilst, in accordance with the second view, Bacteria are supposed to represent merely one stage in the life-history of higher organisms, which are therefore reproduced by an indirect or cyclical process of Homogenesis.
The possible modes of origin of Bacteria and Torulæ may, therefore, be tabulated as follows:—
| Modes of origin of Bacteria and Torulæ. |  | 1. Homogenesis. |  | a. Direct. |
| b. Indirect. | ||||
| 2. Heterogenesis. | ||||
| 3. Archebiosis. |
I. Homogenetic Mode of Origin of Bacteria and Torulæ.
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.