all places,
especially when the first rudiments of living things had spread over a
considerable area, there necessarily arose small contrasts between the
degrees and kinds of superficial differentiation effected. As soon as
these became decided, natural selection came into play; for inevitably
the unlikenesses produced among the units had effects on their lives:
there was survival of some among the modified forms rather than others.
Utterly in the dark though we are respecting the causes which set up
that process of fission everywhere occurring among the minutest forms of
life, we must infer that, when established, it furthered the spread of
those which were most favourably differentiated by the medium. Though
natural selection must have become increasingly active when once it had
got a start; yet the differentiating action of the medium never ceased
to be a co-operator in the development of these first animals and
plants. Again taking the lead as there arose the composite forms of
animals and plants, and again losing the lead with that advancing
differentiation of these higher types which gave more scope to natural
selection, it nevertheless continued, and must ever continue, to be a
cause, both direct and indirect, of modifications in structure.

Along with that remarkable process which, beginning in minute forms with
what is called conjugation, developed into sexual generation, there came
into play causes of frequent and marked fortuitous variations. The
mixtures of constitutional proclivities made more or less unlike by
unlikenesses of physical conditions, inevitably led to occasional
concurrences of forces producing deviations of structure. These were of
course mostly suppressed, but sometimes increased, by survival of the
fittest. When, along with the growing multiplication in forms of life,
conflict and competition became continually more active, fortuitous
variations of structure of no account in the converse with the medium,
became of much account in the struggle with enemies and competitors; and
natural selection of such variations became the predominant factor.
Especially throughout the plant-world its action appears to have been
immensely the most important; and throughout that large part of the
animal world characterized by relative inactivity, the survival of
individuals that had varied in favourable ways, must all along have been
the chief cause of the divergence of species and the occasional
production o