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. The singular circumstance observed by Meyer, that the _Torula_ of yeast, though an indubitable plant, still flourishes most vigorously when supplied with the complex nitrogenous substance, pepsin; the probability that the _Peronospora_ is nourished directly by the protoplasm of the potato-plant; and the wonderful facts which have recently been brought to light respecting insectivorous plants, all favour this view; and tend to the conclusion that the difference between animal and plant is one of degree rather than of kind; and that the problem whether, in a given case, an organism is an animal or a plant, may be essentially insoluble. VIII. ON CERTAIN ERRORS RESPECTING THE STRUCTURE OF THE HEART ATTRIBUTED TO ARISTOTLE. In all the commentaries upon the “Historia Animalium” which I have met with, Aristotle’s express and repeated statement, that the heart of man and the largest animals contains only three cavities, is noted as a remarkable error. Even Cuvier, who had a great advantage over most of the commentators in his familiarity with the subject of Aristotle’s description, and whose habitual caution and moderation seem to desert him when the opportunity of panegyrising the philosopher presents itself, is betrayed into something like a sneer on this topic. “Du reste il n’attribue à cet organe que trois cavités, erreur qui prouve au moins qu’il en avait regardé la structure.”[36] To which remark, what follows will, I think, justify the reply, that it “prouve au moins” that Cuvier had not given ordinary attention, to say nothing of the careful study which they deserve, to sundry passages in the first and the third books of the “Historia” which I proceed to lay before the reader. For convenience of reference these passages are marked _A_, _B_, _C_, etc.[37]