potato-disease, instructively as its history bears upon that of other epidemics; and I have selected the case of the Peronospora simply because it affords an example of an organism, which, in one stage of its existence, is truly a "monad," indistinguishable by any important character from our Heteromita, and extraordinarily like it in some respects. And yet this "monad" can be traced, step by step, through the series of metamorphoses which I have described, until it assumes the features of an organism, which is as much a plant as an oak or an elm is.
Moreover, it would be possible to pursue the analogy further. Under certain circumstances, a process of conjugation takes place in the Peronospora. Two separate portions of its protoplasm become fused together, surround themselves with a thick coat, and give rise to a sort of vegetable egg called an oöspore. After a period of rest, the contents of the oöspore break up into a number of zoöspores like those already described, each of which, after a period of activity, germinates in the ordinary way. This process obviously corresponds with the conjugation and subsequent setting free of germs in the Heteromita.
But it may be said that the Peronospora is, after all, a questionable sort of plant; that it seems to be wanting in the manufacturing power, selected as the main distinctive character of vegetable life; or, at any rate, that there is no proof that it does not get its proteine matter ready made from the potato-plant.
Let us, therefore, take a case which is not open to these objections.
There are some small plants known to botanists as members of the genus Coleochœte, which, without being truly parasitic, grow upon certain water-weeds, as lichens grow upon trees. The little plant has the form of an elegant green star, the branching arms of which are divided into cells. Its greenness is due to its chlorophyl, and it undoubtedly has the manufacturing power in full degree, decomposing carbonic acid and setting free oxygen under the influence of sunlight.
But the protoplasmic contents of some of the cells of which the plant is made up occasionally divide, by a method similar to that which effects the division of the contents of the Peronospora-spore; and the severed portions are then set free as active monad-like zoospores. Each is oval and is provided at one extremity with two long active cilia. Propelled by these, it swims about for a longer or shorter time, but at length comes to a state of rest, and gradually grows into a Coleochœte.
Moreover, as in the Peronospora, conjugation may take place and result in an oöspore; the contents of which divide and are set free as monadiform germs.
If the whole history of the zoöspores of Peronospora and Coleo-
