Only such homogeneous organisms as are yet not differentiated, and are similar to inorganic crystals in being homogeneously composed of one single substance, could arise by spontaneous generation, and could become the primæval parents of all other organisms. In their further development we have pointed out that the most important process is the formation of a kernel or nucleus in the simple little lump of albumen. We can conceive this to take place in a purely physical manner, by the condensation of the innermost central part of the albumen. The more solid central mass, which at first gradually shaded off into the peripheral plasma, becomes sharply separated from it, and thus forms an independent, round, albuminous corpuscle, the kernel; and by this process the Moneron becomes a cell. Now, it must have become evident from our previous chapters, that the further development of all other organisms out of such a cell presents no difficulty, for every animal and every plant, in the beginning of its individual life, is a simple cell. Man, as well as every other animal, is at first nothing but a simple egg-cell, a single lump of mucus, containing a kernel (p. 297, Fig. 5).
In the same way as the kernel of the organic cell arose in the interior or central mass of the originally homogeneous lump of plasma, by separation, so, too, the first cell-membrane was formed on its surface. This simple, but most important process, as has already been remarked, can likewise be explained in a purely physical manner, either as a chemical deposit, or as a physical condensation in the uppermost stratum of the mass, or as a secretion. One of the first processes of adaptation effected by the Moneron originating by spontaneous generation must have been the condensation of an external crust, which as a protecting covering shut in the softer interior from the hostile influences of the outer world. As soon as, by condensation of the homogeneous Moneron, a cell-kernel arose in the interior and a membrane arose on the surface, all the fundamental parts of the unit were furnished, out of which, by infinitely manifold repetition and combination, as attested by actual observation, the body of higher organisms is constructed.
As has already been mentioned, our whole understanding of an organism rests upon the cell theory established thirty years ago by Schleiden and Schwann. According to it, every organism is either a simple cell or a cell-community, a republic of closely connected cells. All the forms and vital phenomena of every organism are the collective result of the forms and vital phenomena of all the single cells of which it is composed. By the recent progress of the cell theory it has become necessary to give the elementary organisms, that is, the “organic” individuals of the first order, which are usually designated as cells, the more general and more suitable name of form-units, or plastids. Among these form-units we distinguish two main groups, namely, the cytods and the genuine cells. The cytods are, like the Monera, pieces of plasma without a kernel (p. 186, Fig. 1). Cells, on the other hand, are pieces of plasma containing a kernel or nucleus (p. 188, Fig. 2). Each of these two main groups of plastids is again divided into two subordinate groups, according as they possess or do not possess an external covering (skin, shell, or membrane). We may accordingly distinguish the following four grades or species of plastids, namely: 1. Simple cytods (p. 186, Fig. 1 A); 2. Encased cytods; 3. Simple cells (p. 188, Fig. 2 B); 4. Encased cells (p. 188, Fig. 2 A). (Gen. Morph. i. 269-289.)
Concerning the relation of these four forms of plastids to spontaneous generation, the following is the most probable:—1. The simple cytods (Gymnocytoda), naked particles of plasma without kernel, like the still living Monera, are the only plastids which directly come into existence by spontaneous generation. 2. The enclosed cytods (Lepocytoda), particles of plasma without kernel, which are surrounded by a covering (membrane or shell), arose out of the simple cytods either by the condensation of the outer layers of plasma or by the secretion of a covering. 3. The simple cells (Gymnocyta), or naked cells, particles of plasma with kernel, but without covering, arose out of the simple cytods by the condensation of the innermost particles of plasma into a kernel, or nucleus, by differentiation of a central kernel and peripheral cell-substance. 4. The enclosed cells (Lepocyta), or testaceous cells, particles of plasma with kernel and an outer covering (membrane or shell), arose either out of the enclosed cytods by the formation of a kernel, or out of the simple cells by the formation of a membrane. All the other forms of form-units, or plastids, met with, besides these, have only subsequently arisen out of these four fundamental forms by natural selection, by descent with adaptation, by differentiation and transformation.
By this theory of plastids, by deducing all the different forms of plastids, and hence, also, all organisms composed of them, from the Monera, we obtain a simple and natural connection in the whole series of the development of nature. The origin of the first Monera by spontaneous generation appears to us as a simple and necessary event in the process of the development of the earth. We admit that this process, as long as it is not directly observed or repeated by experiment, remains a pure hypothesis. But I must again say that this hypothesis is indispensable for the consistent completion of the non-miraculous history of creation, that it has absolutely nothing forced or miraculous about it, and that certainly it can never be positively refuted. It must be taken into consideration that the process of spontaneous generation, even if it still took place daily and hourly, would in any case be exceedingly difficult to observe and establish with absolute certainty as such. With regard to the Monera, we find ourselves placed before the following alternative: either they are actually directly derived from pre-existing, or “created,” most ancient Monera, and in this case they would have had to propagate themselves unchanged for many millions of years, and to have maintained their original form of simple particles of plasma; or, the present Monera have originated much later in the course of the organic history of the earth, by repeated acts of spontaneous generation, and in this case spontaneous generation may take place now as well as then. The latter supposition has evidently much more probability on its side than the former.
If we do not accept the hypothesis of spontaneous generation, then at this one point of the history of development we must have recourse to the miracle of a supernatural creation. The Creator must have created the first organism, or a few first organisms, from which all others are derived, and as such he must have created the simplest Monera, or primæval cytods, and given them the capability of developing further in a mechanical way. I leave it to each one of my readers to choose between this idea and the hypothesis of spontaneous generation. To me the idea that the Creator should have in this one point arbitrarily interfered with the regular process of development of matter, which in all other cases proceeds entirely without his interposition, seems to be just as unsatisfactory to a believing mind as to a scientific intellect. If, on the other hand, we assume the hypothesis of spontaneous generation for the origin of the first organisms, which in consequence of reasons mentioned above, and especially in consequence of the discovery of the Monera, has lost its former difficulty, then we arrive at the establishment of an uninterrupted natural connection between the development of the earth and the organisms produced on it, and, in this last remaining lurking-place of obscurity, we can proclaim the unity of all Nature, and the unity of her laws of Development (Gen. Morph. i. 164).