ium 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 amoeba-like motions for a time, may
be regarded as an imprisoned amoeba; and when we pass from it to a
free amoeba, 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, there is but an indistinct differentiation of outer from
inner (a fact shown by the frequent coalescence of the pseudopodia in
Rhizopods); but that when it eventually becomes quiescent, the surface
becomes differentiated from the contents: the passing into an encysted
state, doubtless in large measure due to inherited proclivity, being
furthered, and having probably been once initiated, by the action of the
medium. The connexion between constancy of relative position among the
parts of the sarcode, and the rise of a contrast between superficial and
central parts, is perhaps best shown in the minutest and simplest
_Infusoria_, the _Monadinae_. The genus _Monas_ is described by Kent as
"plastic and unstable in form, possessing no distinct cuticular
investment; ... the food-substances incepted at all parts of the
periphery";[45] and the genus _Scytomonas_ he says "differs from _Monas_
only in its persistent shape and accompanying greater rigidity of the
peripheral or ectoplasmic layer."[46] Describing generally such low
forms, some of which are said to have neither nucleus nor vacuole, he
remarks that in types somewhat higher "the outer or peripheral border of
the protoplasmic mass, while not assuming the character of a distinct
cell-wall or so-called cuticle, presents, as compared with the inner
substance of that mass, a slightly more solid type of composition."[47]
And it is added that these forms having so slightly differentiated an
exterior, "while usually exhibiting a more or less characteristic normal
outline, can re