vert at will to a pseud-amoeboid and repent state."[48]
Here, then, we have several indications of the truth that the permanent
externality of a certain part of the substance, is followed by
transformation of it into a coating unlike the substance it contains.
Indefinite and structureless in the simplest of these forms, as instance
again the _Gregarina_,[49] the limiting membrane becomes, in higher
_Infusoria_, definite and often complex: showing that the selection of
favourable variations has had largely to do with its formation. In such
types as the _Foraminifera_, which, almost structureless internally
though they are, secrete calcareous shells, it is clear that the nature
of this outer layer is determined by inherited constitution. But
recognition of this consists with the belief that the action of the
medium initiated the outer layer, specialized though it now is; and that
even still, contact with the medium excites secretion of it.

A remarkable analogy remains to be named. When we study the action of
the medium in an inorganic mass, we are led to see that between the
outer changed layer and the inner unchanged mass, comes a surface where
active change is going on. Here we have to note that, alike in the
plant-cell and in the animal-cell, there is a similar relation of parts.
Immediately inside the envelope comes the primordial utricle in the
one case, and in the other case the layer of active sarcode. In either
case the living protoplasm, placed in the position of a lining to the
cuticle of the cell, is shielded from the direct action of the medium,
and yet is not beyond the reach of its influences.

* * * * *

Limited, as thus far drawn, to a certain common trait of those minute
organisms which are mostly below the reach of unaided vision, the
foregoing conclusion appears trivial enough. But it ceases to appear
trivial on passing into a wider field, and observing the implications,
direct and indirect, as they concern plants and animals of sensible
sizes.

Popular expositions of science have so far familiarized many readers
with a certain fundamental trait of living things around, that they have
ceased to perceive how marvellous a trait it is, and, until interpreted
by the Theory of Evolution, how utterly mysterious. In past times, the
conception of an ordinary plant or animal which prevailed, not
throughout the world at large only but among the most instructed, was
that it is a