primordial. For it is an established truth of organic evolution that embryos show us, in general ways, the forms of remote ancestors; and that the first changes undergone, indicate, more or less clearly, the first changes which took place in the series of forms through which the existing form has been reached. Describing, in successive groups of plants, the early transformations of these primitive units, Sachs* says of the lowest Alg?? that ???the conjugated protoplasmic body clothes itself with a cell-wall??? (p. 10); that in ???the spores of Mosses and Vascular Cryptogams??? and in ???the pollen of Phanerogams???...???the protoplasmic body of the mother-cell breaks up into four lumps, which quickly round themselves off and contract, and become enveloped by acell-membrane only after complete separation???(p. 13); that in the Equisetace?? ???the young spores, when first separated, are still naked, but they soon become surrounded by a cell-membrane??? (p. 14); and that in higher plants, as in the pollen of many Dicotyledons, ???the contracting daughter-cells secrete cellulose even during their separation??? (p. 14). Here, then, in whatever way we interpret it, the fact is that there quickly arises an outer layer different from the contained matter. But the most significant evidence is furnished by ???the mhies of protoplasm that escapeinto water from the injured sacs of Vaucheria, which often instantly become roundedinto globular bodies,??? and of which the???hyaline protoplasm envelopes the whole as a skin??? (p. 41) which???is denser than the inner and more watery substance??? (p. 42). As in this case the protoplasm is but a fragment, and as it is removed from the influence of the parent-cell, this differentiating process can scarcely be regarded as anything more than the effect of physico-chemical actions: a conclusion which is supported by the statement of Sachs that ???not only every vacuole in a solid protoplasmic body, but also every thread of protoplasm which penetrates the sap-cavity, and finally the inner side of the protoplasm-sac which encloses the sap-cavity, is also bounded by a skin??? (p. 42). If then ???every portion of a protoplasmic body immediately surrounds itself, when it becomes isolated, with such a skin,??? which is shown in all cases to arise at the surface of contact with sap or water, this primary differentiation of outer from inner must be ascribed to the direct action of the medium. Whether the coating thus initiated is secreted by the protoplasm, or whether, as seems more likely, it results from transformation of it, matters not to the argument. Either way the action of the medium causes its formation; and either way the many varied and complex differentiations which developed cell-walls display, must be considered as

originating from those variations of this physically-generated covering which natural selection has taken advantage of. The contained protoplasm of a vegetal cell, which has self-mobility and when liberated sometimes performs am??ba-like motions for a time, may be regarded as an imprisoned am??ba; and when we phi from it to a hi am??ba, which is one of the simplest types of first animals, or Protozoa, we naturally meet with kindred phenomena. The general trait which here concerns us, is that while its plastic or semi-fluid sarcode goes on protruding, in irregular ways, now this and now that part of its periphery, and again withdrawing into its interior first one and then another of these temporary processes, perhaps with some small portion of food attached, .