Science and Culture, and Other Essays

by Thomas Henry Huxley

If the whole history of the zoospores of Peronospora, and of Coleochæte were unknown, they would undoubtedly be classed among “Monads” with the same right as Heteromita; why then may not Heteromita be a plant, even though the cycle of forms through which it passes shows no terms quite so complex as those which occur in Peronospora and Coleochæte? And, in fact, there are some green organisms, in every respect characteristically plants, such as Chlamydomonas, and the common Volvox, or so-called “Globe animalcule,” which run through a cycle of forms of just the same simple character as those of Heteromita.

The name of Chlamydomonas is applied to certain microscopic green bodies, each of which consists of a protoplasmic central substance invested by a structureless sac. The latter contains cellulose, as in ordinary plants; and the chlorophyll which gives the green colour enables the Chlamydomonas to decompose carbonic acid and fix carbon as they do. Two long cilia protrude through the cell-wall, and effect the rapid locomotion of this “monad,” which, in all respects except its mobility, is characteristically a plant. Under ordinary circumstances, the Chlamydomonas multiplies by simple fission, each splitting into two or into four parts, which separate and become independent organisms. Sometimes, however, the Chlamydomonas divides into eight parts, each of which is provided with four instead of two cilia. These “zoospores” conjugate in pairs, and give rise to quiescent bodies, which multiply by division, and eventually pass into the active state.

Thus, so far as outward form and the general character of the cycle of modifications, through which the organism passes in the course of its life, are concerned, the resemblance between Chlamydomonas and Heteromita is of the closest description. And on the face of the matter there is no ground for refusing to admit that Heteromita may be related to Chlamydomonas, as the colourless fungus is to the green alga. Volvox may be compared to a hollow sphere, the wall of which is made up of coherent Chlamydomonads; and which progresses with a rotating motion effected by the paddling of the multitudinous pairs of cilia which project from its surface. Each Volvoxmonad, moreover, possesses a red pigment spot, like the simplest form of eye known among animals. The methods of fissive multiplication and of conjugation observed in the monads of this locomotive globe are essentially similar to those observed in Chlamydomonas; and, though a hard battle has been fought over it, Volvox is now finally surrendered to the Botanists.

Thus there is really no reason why Heteromita may not be a plant; and this conclusion would be very satisfactory, if it were not equally easy to show that there is really no reason why it should not be an animal. For there are numerous organisms presenting the closest resemblance to Heteromita, and, like it, grouped under the general name of “Monads,” which, nevertheless, can be observed to take in solid nutriment, and which, therefore, have a virtual, if not an actual, mouth and digestive cavity, and thus come under Cuvier’s definition of an animal. Numerous forms of such animals have been described by Ehrenberg, Dujardin, H. James Clark, and other writers on the Infusoria. Indeed, in another infusion of hay in which my Heteromita lens occurred, there were innumerable infusorial animalcules belonging to the well-known species Colpoda cucullus.[35]

Full-sized specimens of this animalcule attain a length of between 1/300 or 1/400 of an inch, so that it may have ten times the length and a thousand times the mass of a Heteromita. In shape, it is not altogether unlike Heteromita. The small end, however, is not produced into one long cilium, but the general surface of the body is covered with small actively vibrating ciliary organs, which are only longest at the small end. At the point which answers to that from which the two cilia arise in Heteromita, there is a conical depression, the mouth; and, in young specimens, a tapering filament, which reminds one of the posterior cilium of Heteromita, projects from this region.

The body consists of a soft granular protoplasmic substance, the middle of which is occupied by a large oval mass called the “nucleus;” while, at its hinder end, is a “contractile vacuole,” conspicuous by its regular rhythmic appearances and disappearances. Obviously, although the Colpoda is not a monad, it differs from one only in subordinate details. Moreover, under certain conditions, it becomes quiescent, incloses itself in a delicate case or cyst, and then divides into two, four, or more portions, which are eventually set free and swim about as active Colpodæ.

But this creature is an unmistakable animal, and full-sized Colpodæ may be fed as easily as one feeds chickens. It is only needful to diffuse very finely ground carmine through the water in which they live, and, in a very short time, the bodies of the Colpodæ are stuffed with the deeply-coloured granules of the pigment.

And if this were not sufficient evidence of the animality of Colpoda, there comes the fact that it is even more similar to another well-known animalcule, Paramæcium, than it is to a monad. But Paramæcium is so huge a creature compared with those hitherto discussed—it reaches 1/120 of an inch or more in length—that there is no difficulty in making out its organisation in detail; and in proving that it is not only an animal, but that it is an animal which possesses a somewhat complicated organisation. For example, the surface layer of its body is different in structure from the deeper parts. There are two contractile vacuoles, from each of which radiates a system of vessel-like canals; and not only is there a conical depression continuous with a tube, which serve as mouth and gullet, but the food ingested takes a definite course, and refuse is rejected from a definite region. Nothing is easier than to feed these animals, and to watch the particles of indigo or carmine accumulate at the lower end of the gullet. From this they gradually project, surrounded by a ball of water, which at length passes with a jerk, oddly simulating a gulp, into the pulpy central substance of the body, there to circulate up one side and down the other, until its contents are digested and assimilated. Nevertheless, this complex animal multiplies by division, as the monad does, and, like the monad, undergoes conjugation. It stands in the same relation to Heteromita on the animal side, as Coleochæte does on the plant side. Start from either, and such an insensible series of gradations leads to the monad that it is impossible to say at any stage of the progress—here the line between the animal and the plant must be drawn.

There is reason to think that certain organisms which pass through a monad stage of existence, such as the Myxomycetes, are, at one time of their lives, dependent upon external sources for their protein matter, or are animals; and, at another period, manufacture it, or are plants. And seeing that the whole progress of modern investigation is in favour of the doctrine of continuity, it is a fair and probable speculation—though only a speculation—that, as there are some plants which can manufacture protein out of such apparently intractable mineral matters as carbonic acid, water, nitrate of ammonia, metallic and earthy salts; while others need to be supplied with their carbon and nitrogen in the somewhat less raw form of tartrate of ammonia and allied compounds; so there may be yet others, as is possibly the case with the true parasitic plants, which can only manage to put together materials still better prepared—still more nearly approximated to protein—until we arrive at such organisms as the Psorospermiæ and the Panhistophyton, which are as much animal as vegetable in structure, but are animal in their dependence on other organisms for their food.