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, Paramoecium, than it is to a monad.  But Paramoecium 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 Coleochaete 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 where 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 Psorospermioe and the Panhistophyton, which are as much animal as vegetable in structure, but are animal in their dependence on other organisms for their food.